EFFECT OF LENGTH OF CUT AND KERNEL PROCESSING ON NUTRIENT

UTILIZATION OF CORN SILAGE BY LACTATING DAIRY COWS

by

KELLY MICHELLE COOKE

(Under the Direction of JOHN K. BERNARD)

ABSTRACT

Mechanical processing of corn silage has been shown to improve starchconcentrations and digestibility and milk yield in dairy cattle as chop length increases.The objective of this research was to determine the degree of mechanical processing ofcorn silage needed to improve animal production as chop length increases. Corn silagewas harvested at 3/4 milk line stage of maturity and at two theoretical lengths of cut of1.90-cm and 2.54-cm. At each chop length, silage was processed at two degrees ofkernel processing of 2 and 8-mm roll clearance. Milk yield tended to be higher for cowsfed diets containing processed corn silage compared to unprocessed corn silage. Drymatter, crude protein, starch, neutral detergent fiber and acid detergent fiber digestibilitieswere all improved with mechanical processing. These results indicate that processingcorn silage as chop length increases can improve starch digestibility and milk yield inlactating dairy cows.

INDEX WORDS: Corn silage, theoretical length of cut, mechanical processing.

EFFECT OF LENGTH OF CUT AND KERNEL PROCESSING ON NUTRIENT

UTILIZATION OF CORN SILAGE BY LACTATING DAIRY COWS

by

KELLY MICHELLE COOKE

B.S., North Carolina State University, 1997

A Thesis Submitted to the Graduate Faculty of The University of Georgia in Partial

Fulfillment of the Requirements for the Degree

MASTER OF SCIENCE

ATHENS, GEORGIA

2003

© 2003

Kelly M. Cooke

All Rights Reserved

EFFECT OF LENGTH OF CUT AND KERNEL PROCESSING ON NUTRIENT

UTILIZATION OF CORN SILAGE BY LACTATING DAIRY COWS

by

KELLY MICHELLE COOKE

Major Professor: John K.Bernard

Committee: Mark A. FroetschelJoe W. West

Electronic Version Approved:

Maureen GrassoDean of the Graduate SchoolThe University of GeorgiaDecember 2003

iv

ACKNOWLEDGEMENTS

My deepest appreciation goes to Dr. John Bernard for serving as my Major

Professor and for all of his guidance, support, and encouragement throughout this degree

program. I would also like to thank Dr. Mark Froetschel and Dr. Joe West for their

willingness to serve as committee members and for their commitment of time and support

during my time in the program.

I would like to express sincere appreciation to Heath Cross for all of his hard

work and guidance at the dairy, and to Melissa Tawzer for her assistance in the

laboratory.

My greatest appreciation goes to my family for all of their love and support.

Thanks to my husband Chris, for agreeing to upend his whole life in support of my goal

of furthering my education. His unconditional love and support during this time will

always be appreciated. My sincerest appreciation goes to my parents John and Gail

Cooke, my sister Jennifer Cooke, and my mother-in-law Yvonne Lemons for all their

love and understanding during this time.

v

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS.................................................................................................v

LIST OF TABLES............................................................................................................ vii

CHAPTER

1 INTRODUCTION .............................................................................................1

2 LITERATURE REVIEW ..................................................................................3

Utilization of corn silage as a feedstuff.........................................................3

Fiber requirements for lactating dairy cattle .................................................4

Theoretical length of cut, particle size, and fiber ..........................................6

Packing density and silage fermentation characteristics ...............................6

Particle size distribution ................................................................................7

Nutrient composition and TLC......................................................................7

Nutrient digestibility and TLC ......................................................................8

Chewing activity, rumination times, and ruminal fermentation response.....9

DMI and chop length...................................................................................10

Production responses and TLC....................................................................11

Current recommendations for TLC .............................................................12

Mechanical processing of corn silage .........................................................12

Chemical composition changes in processed corn silage............................13

vi

Changes in physical characteristics with corn silage processing……….....14

Silage fermentation characteristics and pack density..................................14

Changes in intake and digestibility of nutrients with mechanical

processing……………………………………………………………..15

Ruminal fermentation and pH .....................................................................16

Production responses...................................................................................17

Mechanical processing systems...................................................................18

Stationary roller mills ..................................................................................18

Recutter screens...........................................................................................19

Pull-type and self-propelled forage harvesters ............................................21

Conclusions .................................................................................................22

3 EFFECT OF LENGTH OF CUT AND KERNEL PROCESSING ON

NUTRIENT UTILIZATION OF CORN SILAGE BY LACTATING

DAIRY COWS............................................................................................24

Abstract .......................................................................................................25

Introduction .................................................................................................27

Materials and Methods ................................................................................28

Results and Discussion ................................................................................33

Conclusions..................................................................................................51

4 CONCLUSIONS...............................................................................................52

REFERENCES ..................................................................................................................53

vii

LIST OF TABLES

Page

Table 3.1: Ingredient composition of diets containing unprocessed and processed corn

silage cut at different theoretical lengths of cut (DM basis)........................................31

Table 3.2: Chemical composition of diets containing unprocessed or processed corn

silage cut at different theoretical lengths of cut (DM basis).........................................35

Table 3.3: Chemical composition of corn silage cut at 2 different TLC and 3 different

degrees of kernel processing..........................................................................................41

Table 3.4: Particle size distribution of corn silage, diets, and orts of diets containing corn

silage cut at 2 different TLC and 3 different degrees of kernel processing..................42

Table 3.5: Nutrient intake and digestibility of diets containing corn silage cut at 2

different TLC and 3 different degrees of kernel processing .........................................46

Table 3.6: Least square means of performance parameters for lactating dairy cows fed

corn silage cut at 2 different TLC and 3 different degrees of kernel processing..........50

CHAPTER 1

INTRODUCTION

Recently, there has been increased interest in processing whole plant corn silage using

forage harvesters equipped with onboard processing rolls. Although processing corn

silage has been a common practice in Europe for years, it has only recently been explored

in the United States. Lack of interest in silage processing was due to several reasons.

Previous research in the U.S. showed no improvement in animal performance with kernel

processing (Buck et al., 1969, Miller et al., 1969, Rojas-Bourillon, 1987). However,

these studies used corn silage with a relatively short theoretical length of cut (TLC, 3-10

mm), thereby reducing particle size and depressing fiber digestion to such a point that it

may have offset any improvement in starch digestibility associated with kernel

processing. Furthermore, there was a decade long decline in production of corn silage in

the U.S. Until recently, the availability of processing systems on pull type forage

harvesters has been limited. These processing systems were only available on the more

expensive self-propelled forage harvesters. Finally, producers typically cut corn silage at

a short TLC to reduce whole cob sections and to be compatible with upright silo loaders.

By cutting silage at a shorter TLC, kernels were often broken and there was little concern

about incomplete kernel utilization (Shinners et al, 2000).

New interest in corn silage processing in the U.S. has come about for several reasons.

In the late 1990's, more affordable pull-type forage harvesters with onboard kernel

processors were made available to producers. Also, producers are now growing more

corn for silage because with a limited land base, corn silage allows maximum dry matter

2

yield per acre and expansion of herd size. Also recent research has shown that an

increased TLC is necessary to maintain adequate effective fiber in the ration. With an

increase in TLC, less kernel breakage occurs and processing is necessary to nick and

break kernels to allow for greater starch digestibility, thereby improving animal

performance. Processing corn silage is a good way to increase kernel breakage and

starch digestibility while maintaining particle length and coarse dietary fiber.

Furthermore, corn silage cut at a longer TLC is compatible with bunker and bag silos that

are typically unloaded with front-end loaders (Shinners, 2000).

Currently there is no data indicating what degree of kernel processing is adequate for

current TLC standards. The objective of this experiment is to evaluate the effects of

theoretical length of cut and degree of kernel processing on starch and fiber digestibility,

milk yield, and milk composition when fed to lactating dairy cows.

CHAPTER 2

LITERATURE REVIEW

Utilization of corn silage as a feedstuff

Whole plant corn silage is the primary forage used in dairy rations in the southeastern

United States. In a time when cropland is often limited, corn silage provides producers

with a high-energy forage crop with high dry matter yield per hectare while providing

animals with a highly digestible and palatable feed. Corn silage harvested to maintain

adequate effective fiber can be fed as the sole source of roughage without sacrificing milk

production (Brown et al., 1965). In areas where hay curing is difficult and curing losses

are high, corn silage offers an alternative that alleviates these issues. Silage is easily

automated into feeding systems compared to hay feeding (Hemken and Vandersall,

1967). Corn silage requires less labor per ton to produce than many other forages and

allows for an extended harvest period for the entire corn acreage. This provides an

opportunity for salvage of stressed or damaged corn fields. Corn silage can efficiently

recycle plant nutrients, especially large quantities of nitrogen and potassium (Roth and

Adams, 2001).

The nutritive value of whole plant corn silage is affected by several factors.

Proportions of grain and stalk content, neutral detergent fiber (NDF), cob and leaves,

concentrations and digestibility of NDF, starch digestibility, starch content of the grain,

oil content, and protein content all affect the nutritive value of corn silage (Bal et al.,

2000b). Yearly variations and plant maturity at harvest are also factors that can influence

4

nutritive value (Johnson et al., 1999). Management practices including hybrid selection,

optimal harvesting date to ensure proper grain and stover digestibilities and maturity, and

adequate TLC to produce physically effective fiber are important in producing high

quality corn silage.

Fiber requirements for lactating dairy cattle

Fiber is defined as the slowly digestible or indigestible fraction of feeds that occupies

space in the gastrointestinal tract of animals (Mertens, 1997). Dairy cattle require

adequate amounts of fiber in their diets to maintain a healthy and stable rumen

environment. It has been recommended that diets be formulated based on neutral

detergent fiber (NDF) concentrations of the ingredients because of the positive

relationships between NDF and rumen fill and the negative relationships between NDF

and energy density (Mertens, 1997; Tjardes et al., 2002). According to the NRC (2001),

the recommended dietary NDF requirement ranges from a minimum of 25 up to 33% of

DM, with 75% of NDF supplied by forages. However, these recommendations do not

take into account the effectiveness of fiber as it relates to forage particle size. Chemical

measures of diet alone are inadequate to balance diets for dairy cows. Because fiber

varies in it effectiveness to stimulate chewing and therefore buffering capacity of the

rumen (Allen, 1997), forage particle size must be considered when balancing diets for

dairy cows. Furthermore, effectiveness of fiber influences the animal's ability to

maintain milk fat percentage, ruminal fermentation, and overall animal health and

metabolism (Mertens, 1997).

5

When diets contain excess amounts of fiber, dry matter intake (DMI) decreases,

energy density is low, and productivity is decreased. Alternatively, with too little fiber in

the diet, metabolic disorders can occur, regardless if all nutrient requirements are met

within diet formulation. The inability of the finely chopped forage to stimulate chewing

leads to decreased buffering capacity and reduced ruminal pH. As ruminal pH decreases,

ruminal fermentation patterns are altered, resulting in lowered acetate to propionate

ratios. Because acetate is an important precursor to milk fat synthesis, reduced acetate

concentration leads to reduced milk fat production (Mertens, 1997).

Metabolic disorders related to reduced ruminal pH include abomasal ulcers (Bide and

Dorward, 1975), displaced abomasum (Breukink and deRuyter, 1976), laminitis (Brent,

1976), and acidosis (Brent, 1976). The lack of coarse material in the diet reduces rumen

motility (Colvin et al., 1978; Nocek and Kesler, 1980) leading to reduced muscle tone

and displaced abomasums (Julien and Conrad, 1977). Diets with inadequate effective

fiber are generally higher in readily fermentable carbohydrates, which tend to create an

acidic environment in the rumen. This acidic environment reduces feed efficiency, and

increases the incidence of subclinical or clinical acidosis and other secondary disorders

related to decreased pH, including laminitis (Sudweeks et al., 1980). While severe acute

lactic acidosis in dairy cattle has a negative economic impact, the greatest economic

impact on dairy production is more likely mild or borderline subclinical acidosis, which

effects ruminal digestive efficiency, intake and metabolism, milk fat production, and the

long-term health of the animal (Mertens, 1997).

6

Theoretical length of cut, particle size and fiber

The relationship between particle size and fiber digestibility in the maintenance of

rumen function and milk production is well documented. Longer particles are needed in

the diet to stimulate chewing, salivation, and rumination. However, with increased

particle size, DMI decreases due to rumen fill effects, which may affect production

levels. Reduction of chop length may improve digestibility because of increased

microbial attachment sites, but if cut too short, it may decrease digestibility due to

increased rate of passage of digesta. Short TLC may also reduce chewing time and

ruminal pH. An optimal TLC must be determined to maintain time spent chewing and

ruminating without decreasing DMI and affecting production levels and animal health.

Packing density and silage fermentation characteristics

Early research examining TLC for corn silage recommended a fine chop length of 3-

10 mm (Buck et al., 1969; Miller et al., 1969) to ensure adequate packing density in the

silo to exclude air and ensure an anaerobic environment. When silage is chopped at such

a short chop length and fed to the animals, production and health is negatively effected.

Recent research has indicated that adequate packing density can be achieved with longer

TLC without sacrificing silage fermentation or nutrient content of the silage. Current

standards recommend a TLC of 0.95 cm (Shaver, 2000) for unprocessed corn silage to

maintain good animal production and health. Harrison et al., (1998) examined the effects

chop length and mechanical processing on packing density of corn silage. Unprocessed

corn silage chopped at a long chop length had the same dry pack density as medium

length corn silage (45 ± 19 vs. 43 ±21 kg DM/m3).

7

Three experiments conducted by Johnson et al. (2002a) examined the DM recovery

and pack density of processed or unprocessed corn silage harvested at different maturities

and chop lengths of 0.64 cm and 1.27 cm. While dry pack density increased numerically

for long chopped unprocessed corn silage compared to unprocessed silage chopped at

0.64 cm (average 165 vs. 146 kgAF/m3), DM recovery did not greatly differ between the

two chop lengths (average 99.7 vs. 96.0 %). Lactate concentrations were greater for TLC

1.27 cm compared to 0.64 cm at 1/3 milk line (3.04 vs. 2.81 % of DM). Acetate

concentrations were greater at the shorter chop length (1.35 vs. 1.15% of DM). Similar

results have been reported by Bal et al. (2000a).

Particle size distribution

Theoretical length of cut affects particle size distribution. As chop length increases,

there is less kernel and cob breakage, reducing surface area for rumen microbial attack

(Kuehn et al, 1997, Clark and Armentano, 1999). Decreasing TLC reduces particle size

and increases surface area for microbial attack, thereby increasing digestibility and

production response. Shinners et al. (2000) compared unprocessed whole plant corn

silage cut at 0.95 and 1.90 cm TLC. Mean particle size was greater for 1.90 cm cut silage

compared to 0.95 cm (19.6 vs.11.4 mm). The percent of broken and cracked kernels in

the 0.95 cm silage was 47.2% of total kernel mass compared to 38.7% at 1.90 cm TLC.

Percent of whole intact cob fraction was reduced by half at the shorter TLC (4 vs. 8 %)

and coarse fiber fraction was 55.1 % of total mass of 1.90 cm silage compared to 15.1%

with 0.95 cm corn silage.

8

Nutrient composition and TLC

There is little to no effect of TLC on nutrient composition of corn silage. Schwab et

al. (2002) compared nutrient composition of unprocessed corn silage cut at 1.3 cm and

1.9 cm TLC. There were no differences between organic matter (OM), crude protein

(CP), or acid detergent fiber (ADF). Concentrations of NDF increased slightly as particle

length increased, from 25.5 ± 1.5 to 26.6 ± 1.5 % of DM. Also, starch concentrations

decreased slightly with an increase in particle length (26.2 ± 0.8 to 24.2 ± 0.7 % of DM).

Clark and Armentano (1999) observed similar results. Bal et al. (2000a) noted that these

differences in NDF and starch concentrations may be due to more uniform sampling of

finely chopped whole plant corn silage.

Nutrient Digestibility and TLC

Results regarding the effect of chop length of corn silage on total tract digestibilities

of DM and OM are variable. Several studies have reported no differences in DM and

OM digestibilities with increased TLC (Sudweeks et al, 1979; Stockdale and Beavis,

1994; Bal et al., 2000a; Schwab et al., 2002). Johnson et al. (2003a) observed that as

chop length increased from 1.11 cm to 2.78 cm, DM and OM digestibilities tended to

increase. Geasler and Henderson (1970) also reported decreased DM digestibility with

reduced chop length.

With a short TLC, more of the kernel fraction of the corn silage is broken or damaged.

Breaking the hard waxy pericarp of the kernel allows greater microbial attack of the

starch fraction of the inner kernel and, theoretically, starch digestion increases in the

animal. With more energy supplied to the animal in the form of starch, production

should increase. However, research examining starch digestibility at different TLC is

9

variable. Johnson et al. (2003a) reported that as chop length increased from 2.78 cm to

3.97 cm, total tract digestibility decreased. Others have reported no change in total tract

starch digestibility due to chop length of corn silage (Bal et al., 2000a; Schwab et al.,

2002).

Digestibility of NDF is not affected by chop length (Kuehn et al., 1997; Schwab et al.,

2002; Krause and Combs, 2003; Johnson et al., 2003a). Schwab et al. (2002) compared

digestibilities of unprocessed corn silage cut at 1.3 cm and 1.9 cm and reported there

were no differences in NDF digestibilities among treatments (50.7 vs. 51.0%,

respectively).

Chewing activity, rumination times and ruminal fermentation response

As TLC of corn silage decreases, total chewing time decreases (Grant et al., 1990; De

Boever et al., 1993). Because particle size in the diet is reduced at short chop lengths,

there is less need for the animal to chew feed to the point at which it can be swallowed.

The reduction in chewing time leads to less saliva production (Latham et al., 1974;

Sudweeks et al., 1981; Krause et al., 2002a). Saliva production is crucial to the buffering

capacity of the rumen and to maintaining overall animal health. With decreased

buffering of the rumen, pH drops make the animal more susceptible to disorders such as

metabolic acidosis and laminitis.

As forage particle size decreases, time spent ruminating also decreases (Santini et al.,

1983; Grant et al., 1990; Couderc et al., 2002; Krause and Combs, 2003). Rate of

passage of silages cut at shorter TLC is much greater than rates associated with longer cut

silages (Krause et al., 2002a). Krause and Combs (2003) hypothesized that the greater

10

rumination activity for coarse cut silage suggests that particle size was greater for these

silages even after initial chewing and swallowing.

The TLC affects ruminal fermentation characteristics by altering chewing activity and

rumination times. When TLC is reduced, rumen pH drops due to the inability of

inadequate effective fiber to stimulate chewing and rumination (Gregorini et al., 2002;

Grant et al., 1990; Mertens, 1997). As chop length increases, chewing and rumination

times increase, stimulating saliva production (Latham et al., 1974; Sudweeks et al., 1981;

Krause and Combs, 2003). As a result, rumen pH is maintained and particulate and fluid

passage rate is enhanced. Kuehn et al. (1997) examined the effects of particle length of

corn silage on rumen function. Silages cut at 0.87 cm (long) and 0.32 cm (short) were

fed to 3 mid-lactation fistulated Holstein cows and rumen fluid samples collected hourly

over an 8-h interval. Researchers observed that rumen pH declined during the first 2-h

post feeding for cows in both groups. However, while the short chop length group

continued to drop in rumen pH up to 4-h post feeding, the long group had recovered and

stabilized by 2-h post feeding. Similar results were observed by Johnson et al. (2003a).

Ruminal volatile fatty acid (VFA) production is affected by chop length. Acetate

concentrations are reduced with decreased chop length (Gregorini et al., 2002) whereas

propionic and butyric acid concentrations increase (Sudweeks et al., 1979; Johnson et al.,

2003a). However, total VFA concentrations are not affected (Johnson et al., 2003a) due

to the shift in acetate: propionate ratio.

DMI and chop length

In general, reducing TLC increases DMI (Gregorini et al., 2002b; Schwab et al., 2002).

However, with these studies, short chopped forage composed 60 % of the total mixed

11

ration (TMR). Recent studies have shown little increase in DMI when forage particle

size is reduced provided the forage contributes 40% or less of the total forage DM and is

combined with other forages of adequate particle size (West, 1998; Clark and Armentano,

1999). Studies conducted with dairy cattle have shown no change in DMI with reduction

of chop length (De Boever et al., 1993, Clark and Armentano, 1999; Bal et al., 2000a).

Production responses and TLC

Increasing TLC up to 1.27 cm in corn silage has no effect on milk yield in dairy cattle

(Grant et al., 1990; Kuehn et al., 1997; Clark and Armentano, 1999; Krause et al., 2002a;

Krause and Combs, 2003; Johnson et al., 2003a). This is due to maintenance of similar

DMI with increasing TLC of corn silage (Krause and Combs, 2003). Kuehn et al. (1997)

observed no differences in milk yield between silages cut at 0.87 cm and 0.32 cm.

Johnson et al., (2003a) reported no differences in milk yield for silages cut at 1.11 cm,

2.78 cm and 3.97 cm with similar DMI.

Milk fat yield and percentage is reduced as TLC decreases (Woodford and Murphy,

1988; Grant et al., 1993; Johnson et al, 2003a). This reduction is generally associated

with lack of physically effective fiber in finely chopped forages (Mertens, 1997; Krause

et al., 2002b). When a ration contains inadequate effective fiber, chewing activity is

reduced, leading to less saliva production and buffering. This further decreases ruminal

pH and results in ruminal fermentation patterns that favor propionate production,

reducing acetate concentrations. Acetate plays a crucial role as a precursor in milk fat

synthesis. Decreased levels of acetate will therefore decrease milk fat synthesis

(Mertens, 1997).

12

Body weight may be affected by changes in TLC. However, results of recent research

are variable. Johnson et al. (2003a) observed a decrease in body weight as corn silage

chop length increased from 2.78 cm to 3.97 cm (P<0.03). Others (Clark and Armentano,

1999; Bal et al., 2000a) have reported no change in body weight with increased TLC.

Current recommendations for TLC

Previous standards recommend a TLC of 0.95 cm (Shaver, 2000). This recommended

TLC varies depending on whole plant and kernel moisture, hybrid, and crop processing.

Corn silage chopped at a later stage of maturity (>one-half milk line) and greater DM

content may need to be chopped at a TLC of 0.64 cm if kernel processing is not used.

Immature or wet silages and hybrids that exhibit soft kernel texture should be chopped at

a TLC of at least 1.27 cm. Recent research recommends a chop length of 1.91 cm for

silages harvested with a crop processor (Shaver, 2000). An increased TLC of corn silage

offsets the increase in power requirements due to mechanical processing (Johnson et al.,

2003a). However, excessive equipment wear has been reported at chop lengths of 2.54

cm or greater (Shaver, 2000).

Mechanical processing of corn silage

With researchers recommending an increased TLC of corn silage of up to 1.95 cm to

maintain adequate effective fiber, kernel breakage at these longer TLC is reduced

compared to damage at shorter chop lengths. At shorter chop lengths, the knives of the

silage cutterhead are close enough together that kernels are often nicked and damaged

and cob material is quartered, making kernel processing unnecessary. As chop length

increases, less kernel damage and cob size reduction occurs. Mechanical processing of

13

corn silage can provide the kernel breakage and cob size reduction necessary to provide

better nutrient composition of corn silage. The mechanical processing of corn silage nicks

and breaks the kernel fraction of the silage while further shearing the stover and cob

portion, increasing surface area of the silage for increased microbial attack. Because the

waxy outer coating of the kernel is damaged, starch digestion is greater for processed

kernels. Fewer whole kernels end up in the feces and instead are digested and used by

the animal. Pull-type forage harvesters with onboard kernel processors have recently

become available in the United States, making it economically feasible for producers to

process corn silage in the field.

Chemical composition changes in processed corn silage

The chemical composition of whole plant corn silage differs between unprocessed

and processed silage. Primarily, NDF and ADF concentrations are greater for

unprocessed silage compared to processed silage (Bal et al., 2000b; Dhiman et al., 2000;

Johnson, 1996; Johnson et al., 2002a; Johnson et al., 2002b; Johnson et al., 2003a; Rojas-

Bourrillon et al., 1987). In a study by Johnson et al. (2002b) two varieties of whole plant

corn silage were harvested at one-third milk line, two-third milk line, and physiological

maturity at a TLC of 1.27 cm and either processed through at 1mm roll clearance

(processed) or 15.9 mm roll clearance (unprocessed). Regardless of hybrid, at

physiological maturity, the concentrations of NDF and ADF were higher for unprocessed

corn silage compared to processed corn silage.

Mechanical processing of corn silage increases starch concentrations in the silage as

compared to unprocessed silage. Rojas-Bourrillon (1987) reported that processed corn

silage cut at 0.95 cm TLC had a higher starch concentration than unprocessed corn silage

14

(40.4 vs. 38.6 %DM). In the study by Johnson et al. (2003a), corn silage was harvested

at TLC of 2.78 cm and 3.97 cm with (1mm roll clearance) and without kernel processing.

Regardless of TLC, processing numerically increased starch concentration in the corn

silage. At 2.78 cm TLC, the processed silage contained 32.6% starch, while the

unprocessed corn silage contained 29.2% starch. At the 3.97 cm TLC, starch

concentrations for the processed and unprocessed corn silage was 32.6% and 29.5%

respectively.

Changes in physical characteristics with corn silage processing

The crushing and shearing action of mechanical processors changes the physical

characteristics of corn silage. At any given TLC, mechanical processing of corn silage

reduces particle length by 15 to 30% (Schurig and Rodel, 1993). Shinners et al. (2000)

observed that corn silage cut at a TLC of 1.95 cm and processed at 1mm roll clearance

and 42% speed differential had similar whole plant geometric mean particle size to that of

unprocessed corn silage cut at 0.95 cm (11.5 mm and 11.4 mm respectively).

Unprocessed corn silage cut at 19 mm TLC had a mean particle size of 19.6 mm. In

processed corn silage, as roll clearance increased from 1 to 3 to 5 mm, geometric mean

particle size increased, regardless of speed differential (average 11.7, 12.4, and 14.0 mm

respectively). However, the whole intact cob fraction was 0% of total mass for all

processed corn silage, regardless of roll clearance.

Silage fermentation characteristics and pack density

Mechanical processing of corn silage affects silage fermentation characteristics and

reduces DM losses during ensiling. Rojas-Bourrillon et al. (1987) reported that lactic

acid concentration was higher and butyric acid was lower in rolled silage as compared to

15

control silage. Johnson et al. (2002a) also reported increased lactate levels in processed

corn silage. Pack density of corn silage increased with mechanically processed corn

silage due to a reduction in particle size (Harrison et al, 1998). The pH of processed corn

silage is lower when compared to unprocessed silage (Rojas-Bourrillon et al., 1987;

Johnson et al., 1997).

Changes in intake and digestibility of nutrients with mechanical processing

Dry matter intake increases with mechanical processing of corn silage cut at adequate

chop length (Bal et al., 2000a; Dhiman et al., 2000; Ferreira et al., 2002; Andrae et al.,

2001). Intake of NDF is not affected by mechanical processing of corn silage (Johnson

et al., 2002c; Schwab et al., 2002; Johnson et al, 2003a). Starch intake increases with

mechanical processing (Johnson et al., 2002d; Schwab et al., 2002).

Neutral detergent fiber digestibility is reduced with mechanical processing of corn

silage (Dhiman et al., 2000; Andrae et al., 2001; Johnson et al., 2003a). Andrae et al.

(2001) explained that decreased NDF digestibility may be due to several reasons.

Increased starch digestion with processed corn silage may cause a competitive

disadvantage for fibrolytic bacteria in the rumen by decreasing rumen pH below optimal

level for these bacteria. However, research regarding the effect of processing on rumen

pH is variable. Increased passage rates with decreased particle size of processed corn

silage could also explain the decreased fiber digestion. Finally, animals consuming

unprocessed corn silage tend to sort more than those animals fed processed corn silage.

Cob refusal is greater with unprocessed corn silage and animals fed unprocessed corn

silage. Therefore animals consuming these diets consume more digestible NDF. Cows

16

fed processed corn silage consume a larger portion of cobs, a poorly digestible NDF

source.

Starch digestibility is greatly increased with mechanical processing of corn silage

(Rojas Bourrillon et al., 1987; Bal et al., 2000a; Dhiman et al., 2000; Weiss and Wyatt,

2000; Andrae et al., 2001; Fanning et al., 2002; Johnson et al., 2003a). Mechanically

processing corn silage disrupts the pericarp of the kernel fraction of the corn, allowing for

greater microbial attack and digestion of the starch fraction of the kernel in the rumen.

The digestibility of the starch fraction that reaches the small intestine is also increased

with mechanical processing (Dhiman et al., 2002).

Bal et al., (2000a) examined the differences in starch digestibility among corn silages

at TLC of 0.95, 1.45, and 1.95 cm that were processed at 1mm roll clearance and 0.95 cm

unprocessed corn silage. Total tract starch digestibility increased by 4.2 percentage units

on average for processed whole plant corn silage versus unprocessed silage.

Ruminal fermentation and pH

The changes in ruminal fermentation response with processed corn silage are due

primarily to increased starch digestibility. With mechanical processing, acetate

concentrations decrease and propionate concentrations increase in the rumen (Doggett et

al.,1998; Dhiman et al., 2000; Johnson et al., 2002d; Zobell et al., 2002). Propionate is

an important precursor to gluconeogenesis and increased propionate concentrations in the

rumen lead to increased glucose synthesis. Increased glucose concentrations provide

increased energy to the animal, thereby increasing production response.

Results regarding rumen pH levels vary. Longuski et al (2002) observed no changes

in ruminal pH with mechanical processing of corn silage. This observation was

17

supported by Stockdale and Beavis (1993) and Bal et al. (2002a). However, Weiss and

Wyatt (2002) noted an increase in rumen pH with processing of corn silage (3.92

processed vs. 3.78 unprocessed).

Production responses

Mechanical processing of corn silage increases milk yield in dairy cattle (Johnson et

al., 1996; Satter, 2001; Bal et al., 2000a). With the increased starch digestibility

associated with mechanical processing, the amount of available energy is increased,

thereby increasing production. Shinners et al. (2000) compared corn silage chopped at

different TLC and processed at 1 mm roll clearance to unprocessed corn silage.

Regardless of chop length, milk yield was greater in processed silage compared to

unprocessed corn silage.

Milk fat percentage also increases in cattle fed mechanically processed corn silage

(Johnson et al., 1996; Dhiman et al., 2000; Satter, 2001). This response seems

contradictory because observed ruminal fermentation response notes a decrease in acetate

concentrations with mechanical processing. Because acetate is an important precursor to

milk fat synthesis, a reduction in concentrations would theoretically lead to decreased

milk fat synthesis. However, the increase in milk fat percentage may be explained by

greater NDF intake resulting from the reduction of particle size associated with

processing whole plant corn silage. Processing corn silage reduces the particle size of the

cob fraction of the whole plant. With reduced particle size of the cob, less sorting and

cob refusal occurs. This increased cob intake can offset the decrease in acetate

concentrations to increase milk fat percentage (Dhiman et al., 2000).

18

Body weight gain increases in cows fed processed corn silage (Bal et al., 2000a;

Ferreira et al., 2002). The increased energy content from greater starch availability in

processed corn silage provides additional energy which is converted to either increased

milk yield or increase body weight gain.

Mechanical Processing Systems

Whole corn kernels are virtually indigestible in the rumen, especially as the kernel

matures (McAllister et al., 1990). Kernel processing nicks or crushes the whole kernel,

rupturing the waxy coating of the kernel. The proportion of intact corn kernels present in

corn silage tends to be greater in unprocessed than processed silages (Johnson et al.,

1999, Shinners et al., 2000, Zobell et al., 2002). The primary goal in processing corn

silage is to rupture the kernel (pericarp) to expose the starch granule and increase starch

digestibility in the rumen. Mechanical processing minimizes the physical barriers to

rumen microbial attack of both the stover and grain fractions of corn silage, allowing

more surface area for enzymatic activity, thereby allowing increased digestion of NDF

and starch (Johnson et al., 1999).

Various mechanical processing systems have been developed to aid in rupture of the

corn kernel. These include stationary roller mills, recutter screens and processing rollers.

Stationary Roller mills

Stationary roller mills consist of two corrugated rollers which compress and crush the

corn grain into fine particles ranging from flattened kernels to a fine powder. Stationary

roller mills can be integrated into the silo blower and activated when filling the silo or

can be used prior to feeding to process corn silage. Unlike current mechanical

19

processors, there may be no speed differential between the processing rolls of the mill.

While these roller mills adequately disrupt the kernel fraction of the silage, the stover

fraction of the silage remains intact with little to no shredding, reducing the surface area

available for microbial attack (Johnson et al., 1999). Furthermore, when processing prior

to ensiling, there is an additional power requirement to operate the processor with the

blower. Straub et al. (1996) observed that the average specific energy required to operate

the unit was 1.43 kWh/metric ton of fresh material when a grinder-blower unit is used to

process silage prior to ensiling. Also there is a decreased throughput of material when

the roller mill is engaged. In the same study by Straub et al. (1996), researchers were

able to process an average of 28.1 metric ton/h at 60% moisture content (i.e. 11.2 metric

ton DM/h). For these reasons, the use of on farm processors has become somewhat

impractical and no longer cost effective for producers. A more practical approach to corn

silage processing is in field processing mechanisms that reduce time spent harvesting and

processing the silage.

Recutter Screens

Early attempts at mechanical processing of corn silage in the field began with the use

of recutter screens. Recutter screens are cylindrical plates fitted behind the cutterhead

such that the knives of the cutterhead pass the recutter screen with very little clearance.

Small openings in the recutter screen act as shear bars to produce fine particles.

Recutter screens are used to ensure that kernels are completely damaged when

harvesting ear corn or whole plant corn silage and to further reduce particle size of plant

material. There is an extra power input of at least 15% with the use of recutter screens

(Johnson et al., 1999). There is also reduced harvester capacity with the use of recutter

20

screens. Capacity and power input are determined by knife sharpness, knife- to- screen

clearance, number and size of openings in the screens, and characteristics such as

moisture content of the plant (Johnson et al, 1999).

Buck et al. (1969) examined the use of recutter screens to reduce kernel size at two

stages of corn maturity and with two different sizes of recutter screens. Silage was cut in

the field at a TLC of 6.4 cm and then recut at the silo through a 5.0 cm screen or a 2.5 cm

screen. The DMI and milk yield were not affected by processing with the recutter

screens. Recutting the corn plant before ensiling reduced the amount of kernel dry

matter in the feces from 2.2% in the control (no recutting) to 1.1 and 0.4% in the 5.0 cm

and 2.5 cm recut silages respectively, but had no effect on the total digestible nutrients

(TDN) of the silages. The minimal response to processing in this study is most likely due

to the relatively short TLC. Particle size was too small to stimulate adequate mastication

and rumen function, and rate of passage of nutrients was most likely too fast to elicit a

response. Examination of the horsepower requirements for field chopping showed that

the addition of a 2.5 cm recutter screen to the harvester required 3 times more horsepower

hours per ton than required for regular cutting (8.20 hphr/metric ton vs. 2.76 hphr/metric

ton).

Miller et al (1969) did not observe any differences in DMI or milk yield of cows fed

control or recut silages. Rechopping did reduce milk fat yield. However, as noted in the

previous study, chop length was relatively short at 1.3 cm and most likely did not provide

adequate mastication and rumen function required to maintain milk fat production.

21

Pull-type and Self-propelled Forage Harvesters

Until recently, onboard kernel processors were only available on expensive self-

propelled type forage harvesters. However, in the late 1990’s, manufacturers began

producing more reasonably priced pull-type forage harvesters with processors. The

addition of an onboard kernel processor to a self-propelled harvester ranges from $12,000

to $15,000 and in pull-type harvesters, $7,000 to $9,000 (Shinners, 1999). Mechanical

processors consist of two counter rotating serrated rollers operating at different speeds

and positioned between the cutterhead and the blower. The axis of rotation of the rollers

is generally parallel to the cutterhead. Roll clearance is manually or electrically

adjustable, ranging from 1mm (fully active) to 15mm (inactive).

Studies comparing the differences between self-propelled and pull-type forage

harvesters indicate little or no difference between the machines when considering the

physical properties of the silage produced. Shinners et al. (2000) compared machine

variables using a Gehl model 1275 pull-type harvester and a John Deere model 6750 self-

propelled forage harvester, both equipped with onboard processing rollers. Silage was

harvested at ¾ milkline and 60% moisture. Each machine was operated at a roll

clearance of 1, 3, and 5 mm and an average TLC of 1.97 cm. At 1mm roll clearance,

both machines produced a similar whole plant and kernel fraction particle size and

estimated surface area of the kernel fraction. As roll clearance increased to 5mm, the

changes in kernel fraction physical properties were greater for the self-propelled

harvester compared to the pull-type harvester most likely because of an increased feed

rate with the self-propelled harvester. Because the thickness of the mat of the material

being fed into the self-propelled harvester was greater than the pull-type harvester, some

22

material may have been cushioned from the effects of the roll forces and not fully

processed. With the pull-type harvester, the percent of broken kernel mass decreased

from 94.1 to 73.1% as roller clearance increased to 5mm. With the self-propelled

harvester, the percent of broken kernel mass decreased from 100.0 to 74.7% as roller

clearance increased to 5mm.

The use of mechanical processors can increase the power requirement for harvesting

corn silage and may reduce the throughput capacity of the harvester. Roberge et al.

(1998) reported that the average power requirement for harvesting corn with a pull-type

forage harvester was 48.5 kW with the processor engaged and 44.7kW with the processor

inactivated. The processor required a small (7%) but statistically significant increase in

specific energy (4.07 vs. 3.80 kWh/metric ton DM).

Roll clearance has the most significant effect on harvester specific energy

requirements. In the study by Shinners et al. (2000), as roll clearance increased from 1 to

3 to 5mm, specific energy requirements were reduced by 20 and 9% respectively.

However, increasing TLC can offset the effects of roll clearance on specific energy

requirements. The processed treatments at 0.3, 0.5 and 1.9 cm TLC (2.27 and 2.22

kWh/Mg) required similar specific energy requirements as unprocessed corn silage at a

TLC of 0.95 cm (2.21 kWh/Mg).

Conclusions

While current research recommends increasing TLC of whole plant corn silage to 1.90

cm with mechanical processing, limited research has been published on the effects of

increasing chop length to TLC greater than 1.90 cm with kernel processing on digestion,

23

silage fermentation characteristics, and performance characteristics in lactating dairy

cattle. Research is also limited when examining the degree of kernel processing required

to stimulate maximum response to increased starch digestibility in processed corn silage.

Previous research focused on either increasing TLC or manipulating degree of kernel

processing, not both. Further research is necessary to examine the effects of increasing

chop length above 1.90 cm with varying degrees of kernel processing on digestion and

performance characteristics in lactating dairy cattle.

24

CHAPTER 3

EFFECT OF LENGTH OF CUT AND KERNEL PROCESSING ON NUTRIENT

UTILIZATION OF CORN SILAGE BY LACTATING DAIRY COWS1

__________________________

1Cooke, K.M. and J.K. Bernard. 2003. To be submitted to Journal of Dairy Science.

25

ABSTRACT

Forty Holstein cows were used in a randomized block design study to determine the

effects of theoretical length of cut (TLC) and kernel processing of whole plant corn silage

on nutrient intake and digestibility, milk yield, and milk composition. The 8-wk trial

consisted of a 2-wk standardization period followed by a 6-wk collection period. Corn

silage was harvested at three-quarter milk line stage of maturity at two theoretical lengths

of cut of 1.90 cm and 2.54 cm. At each TLC, corn silage was processed at two degrees of

kernel processing of 2 and 8 mm roll clearance using a John Deere 3975 pull-type forage

harvester equipped with an onboard kernel processor. Control silage was harvested at

1.90 cm without kernel processing. Corn silage provided 38% of the dietary DM in the

experimental diets. There were no differences in intake of DM and nutrients among

treatments. Crude protein and starch digestibility was greater for cows fed corn silage

processed at 2 mm versus 8 mm. Milk yield tended to be higher for cows fed diets

containing kernel processed corn silage compared with unprocessed corn silage. There

were no differences in milk fat or protein concentrations among treatments. There was an

interaction between TLC and kernel processing due to greater yield of milk, fat, and

protein for cows fed corn silage processed at 2-mm versus 8-mm at 2.54 cm chop length.

Kernel processing increased plasma glucose concentrations and decreased plasma urea

nitrogen concentrations compared with unprocessed corn silage. These results indicate

that kernel processing becomes more important as the TLC of corn silage increases to

maintain nutrient digestibility and performance in lactating dairy cows.

(Key words: corn silage, mechanical processing, chop length)

26

Abbreviation key: TLC = theoretical length of cut, 1.95 NP = corn silage chopped at

1.95 cm with no processing, 1.95 M = corn silage chopped at 1.95 cm and 2-mm

processing, 1.95 P corn silage chopped at 1.95 cm and 8-mm processing, 2.54 M = corn

silage chopped at 2.54 cm and 2-mm processing, 2.54 P = corn silage chopped at 2.54 cm

and 8-mm processing.

27

INTRODUCTION

Whole plant corn silage serves as the primary forage source for lactating dairy cattle

in the southeastern United States. Over the past decade, there has been increased interest

in kernel processing of whole plant corn silage using forage harvesters equipped with

onboard processing rolls. Kernel processing of corn silage increases starch

concentrations and digestibility and decreases NDF concentrations (Rojas- Bourrillon et

al., 1987; Johnson, 1996; Bal et al., 2000, Johnson et al., 2003a). Kernel processing has

also been reported to decrease particle size of corn silage by 15 to 30 % and increase

machinery power requirements by 7 to 15 % (Schurig and Rodel, 1993; Roberge et al.,

1998). Because of the reduction in particle size, it is recommended that theoretical length

of cut (TLC) be increased when corn silage is mechanically processed to offset any

potential problems associated with lack of physically effective fiber (Johnson et al.,

2003a). Also, extending the TLC of whole plant corn silage reduces power requirements

associated with kernel processing (Johnson et al., 2003a) and supports greater rate of

harvest.

Recent research has focused on the impact of TLC or the interaction of TLC and

kernel processing of whole plant corn silage on intake and production of dairy cattle.

However, the degree at which kernels should be processed and the interaction of TLC

and degree of kernel processing have not been thoroughly examined. The objective of

this study is to examine the effects of TLC and degree of kernel processing of whole

plant corn silage on nutrient intake, digestibility and performance of lactating dairy cows.

28

MATERIALS AND METHODS

Forty Holstein dairy cows (18 primiparous and 22 multiparous) in mid lactation were

used in a randomized block design trial to examine the effect of TLC and kernel

processing of corn silage on milk yield and composition and nutrient intake and

digestibility. Experimental treatments were arranged in an incomplete 3 x 2 factorial

design. The five treatment groups consisted of 1.95 cm corn silage with no kernel

processing (1.95NP, control), 1.95 cm corn silage with 2mm processor roll clearance

(1.95P, aggressively processed), 1.95 cm corn silage with 8 mm processor roll clearance

(1.95M moderately processed), 2.54 cm corn silage with 2mm processor roll clearance

(2.54P), and 2.54 cm corn silage with 8mm processor roll clearance (2.54M). The trial

was conducted from January 16, 2003 to March 7, 2003. Cows were housed in a freestall

barn at the University of Georgia Dairy Research Center in Tifton. Cows averaged 144 ±

45 DIM and 34 ± 6 kg/d of milk at the beginning of the trial. Experimental diets

contained approximately 38% forage from unprocessed or kernel processed corn silage,

6% alfalfa hay, and 56% concentrate and mineral mix (DM basis, Table 3.1).

Corn (Pioneer 33J56, Pioneer Hi-Bred International, Inc., Des Moines, IA) was

planted on March 31, 2002 in a Tift loam soil at a seeding rate of approximately 69,135

plants per hectare. Corn was irrigated with dairy waste effluent to provide nutrients with

additional fresh water to meet water requirements. Corn was harvested from July 10,

2002 to July 12, 2002 at ¾ milk line stage of maturity and 37%DM. Processed corn

silage was harvested using a John Deere 3975 pull-type forage harvester (Deere and

Company, Moline, IL) equipped with an onboard kernel processor. Processing rolls with

a diameter of 20.3 cm were located between the cutterhead with 24 knives. The TLC was

29

adjusted by changing the speed of feeder rolls to provide TLC of 1.95 or 2.54 cm. Corn

silage was harvested and processed at 1.95 cm with 2mm and 8mm processor roll

clearance, and 2.54 cm with 2mm and 8mm processor roll clearance. The control corn

silage of 1.95 cm unprocessed corn silage was cut using a Claas 880 self-propelled forage

harvester (Claas of America, Columbus, IN) set for a TLC of 1.95 cm with the kernel

processor removed. Chopped corn was stored in 2.4-m plastic silage bags (Kelly Ryan,

Blaine, NE) and allowed to ferment.

The trial consisted of a 2-week standardization period followed by a 6-week

experimental period. Cows were trained to operate electronic gate feeders (American

Calan, Northwood, NH) before the standardization period began. Cows were fed a TMR

once daily (0800) in amounts to provide approximately 10% orts for ad libitum

consumption. Amounts of TMR offered and refused were recorded daily. Cows were

milked twice daily at approximately 0400 and 1500 and daily milk yield was recorded

electronically (Alpro, DeLaval, Kansas City, MO). All protocols were reviewed and

approved by the University of Georgia Institute of Animal Care and Use Committee.

Sample Collection and Analysis. Milk samples were collected from two consecutive

p.m. and a.m. milkings during each week of the trial period. Milk samples were sent to

Southeast Milk Inc. in Belleview, FL for analyses of fat, protein, and somatic cell

concentrations.

Corn silage treatments, feed ingredients, experimental diets, and orts were collected

three times each week. Samples were composited each week of the trial and stored at

0°C until analysis. Samples were analyzed for DM (forages-Goering and Van Soest,

1970, grains, mixed feeds, concentrates, and byproducts-AOAC, 1990), CP (Leco FP-528

30

Nitrogen Analyzer, St. Joseph, MI), ADF (AOAC, 1990), NDF and lignin (Goering and

Van Soest, 1970), starch (Holm et al., 1986), and pH (Mettler DL12 Titrator, Columbus,

OH). Acetic, propionic, butyric, and isobutyric acids were measured by gas

chromatography (Shimadzu GC-14A, Columbia, MD).

Particle size of corn silage and TMR samples was determined using the Nasco Penn

State Forage Particle Size Separator (Heinrichs, 1996). Representative subsamples of

each composited sample were weighed to ensure that at least 250 g of sample were wet

sieved through three plastic separator boxes. The diameter of screens used was 1.95 cm

for the upper sieve and 0.775 cm for the middle sieve. Mean particle length of samples

was determined by calculating the proportion of sample remaining on each sieve.

During wk 5 of the trial, fecal grab samples were taken from a subgroup of 20 cows,

four from each treatment group for four consecutive days at 12 h intervals with a 2 h

declining schedule each day. Collection times were at 0500 h, 1700 h, 0700 h, 1900 h,

0900 h, 2100 h, 1100 h, and 2300 h. Samples were composited by cow and dried in a

forced-air oven at 60° C for 72 h along with composited weekly TMR and orts samples.

All samples were ground to pass through a 1-mm screen using a Wiley mill (Arthur

Thomas, Philadelphia, PA) and analyzed for DM, CP, ADF, NDF, lignin, and starch as

described previously. Samples were also analyzed for indigestible ADF as an internal

marker using the techniques described by Cochran et al. (1986) using an Ankom Daisy II

Incubator (Ankom Technology, Macedon, NY).

Body weights were recorded on two consecutive days during the standardization

period and week 6 of the experimental period and once during weeks 2 and 4. To

31

Table 3.1. Ingredient composition of diets containing unprocessed or processed corn silage cut atdifferent theoretical lengths of cut (DM basis).

Ingredient % of DM

Corn silage 1 38.31Steam flaked corn 18.05Brewers grains, wet 12.81Whole cottonseed w/lint 6.43Alfalfa hay 6.05Urea 0.40Base concentrate2 10.67Energy-protein supplement3 6.62

1 Corn silage treatments - 1) 1.95 cm TLC, no processing, 2) 1.95 cm TLC, 2 mmprocessing, 3) 1.95 cm TLC, 8 mm processing, 4) 2.54 cm TLC, 2 mm processing, and 5)2.54 cm TLC, 8 mm processing.

2Base concentrate contained (% of DM) 4.07% soybean meal, 1.85% cottonseed hulls,1.83% citrus pulp, 0.71% limestone, 0.61% sodium bicarbonate, 0.37% vitamin- tracemineral premix, 0.29% salt, 0.24% Diamond V® XP yeast (Diamond V Mills, CedarRapids, IA), 0.22% dicalcium phosphate, 0.20% K-minus® (Church and Dwight Co.,Inc., Princeton, NJ), 0.16% magnesium oxide, and0.12% pot-mag-sulfate.

3Energy- protein supplement contained (% of DM) 3.56% soybean meal (48% CP),1.53% ground corn, 1.02% calcium salts of long chain fatty acids, 0.31% , and 0.20% K-Minus® (Church and Dwight Co., Inc., Princeton, NJ).

32

minimize variation, all BW were recorded after the evening milking before animals had

access to feed or water. Body condition scores were recorded during weeks 1, 4, and 6 of

the experimental period by two independent evaluators according to Edmondson et al.

(1989).

Blood was collected from the tail (coccygeal artery) during the standardization period,

wk 4, and at the end of the trial. Tubes were allowed to clot, and serum was harvested by

centrifugation. Samples were analyzed at the University of Georgia Veterinary

Diagnostic Laboratory in Tifton for blood urea nitrogen and glucose using a Bodhringer

Mannheim/Hitachi 912 automated chemistry analyzer (Roche Laboratory Systems,

Indianapolis, IN).

Statistical analysis. Dry matter intake, milk yield and composition, and BW

data were analyzed as a randomized block using PROC MIXED procedures of SAS

(1989). The model included effects of covariate, block, treatment, week, and their

interactions.

The corresponding value from the preliminary period was included as a covariate in the

model. The model included cow within treatment as a random variable and week as a

repeated measure. The following orthogonal contrasts were in the model: 1) 1.91 versus

2.54 cm chop length; 2) no kernel processing versus processed; 3) 2 versus 8 mm kernel

processing; and 4) the interaction of chop length and kernel processing. Chemical

content of silage, particle size of silage, diets, and orts, nutrient intake and apparent

digestibility data were analyzed using PROC GLM procedures of SAS (1989). The

model included block and treatment. Orthogonal contrasts as outline above were also

included in the model.

33

RESULTS AND DISCUSSION

Chemical Characteristics of Corn Silage and TMR. Chemical composition of whole

plant corn silage (WPCS) treatments is presented in Table 3.2. The total mixed ration

(TMR) for animals fed silage cut at 2.54 cm and processed at 8 mm was numerically

lower in DM content compared with all other TMR (39.9 %; data not statistically

analyzed). All other treatments averaged 43.2% DM. The reason for the lower DM

content is unclear as the DM content of the 2.54 P corn silage was similar to all other

corn silage treatments. Degree of kernel processing had an effect on starch content of the

TMR. Diets with corn silage processed at 8-mm roll clearance had lower starch content

than those more aggressively processed at 2-mm, regardless of TLC. Nonfibrous

carbohydrate (NFC) concentrations were also lower in diets containing corn silage

processed at 8 mm roll clearance compared with silage processed at 2-mm roll clearance.

Diets containing kernel processed corn silage had lower NFC concentrations than those

containing unprocessed corn silage (40.6 vs. 43.3 % of DM). Weiss and Wyatt (2000)

noted that NFC concentrations decreased with kernel processing of corn silage compared

to unprocessed corn silage. The differences in NFC concentrations of these diets was

unlikely due to organic acid contents of the silage as there was little difference between

diets in lactic, acetic, or propionic acid concentrations and was most likely due to the

increase of starch contents of the 2-mm processed diets. All other nutrient

concentrations, NEL, and RUP were similar among diets.

Kernel processing affected some of the chemical characteristics of corn silage (Table

3.3). Dry matter content was lower in kernel processed corn silage (average 35.3%)

compared with unprocessed corn silage (37.5%). The majority of the difference between

34

the processed and unprocessed silage can be explained by the 1.95 M corn silage, which

contained only 32.2 % DM. Johnson et al. (2002b, 2003a) also noted a decrease in DM

content of processed corn silage compared with unprocessed silage. Others have found

no differences in DM content of kernel processed and unprocessed corn silage (Rojas-

Bourillon et al., 1987; Andrae et al., 2001; Zobell et al., 2002). It should be noted that

DM content may have been slightly affected by harvesting procedures. Corn was

chopped for silage over a 3 day period in July, 2002 with the kernel processed corn cut on

d1 and d2 and unprocessed corn silage cut on d3. The extra days spent in the field may

have slightly increased the DM content of the unprocessed corn silage.

The DM content of corn silage also tended to decrease with more aggressively kernel

processed corn silage (P=0.10). Corn silage processed at 2 mm roll clearance average

34.6 % DM while silage processed at 8 mm roll clearance averaged 36.1 % DM. The

variability can be attributed to the lower DM content of the 1.95 M corn silage, which

had the lowest DM content of all corn silage at 32.2%. Despite the lower DM content,

the 1.95 M corn silage had similar silage fermentation characteristics and nutrient

contents compared to all other kernel processed corn silage. While DM content was

lower for the 1.95 corn silage, DM contents were within an acceptable range for all corn

silage treatments.

Kernel processed corn silage had lower CP concentrations. Crude protein content of

unprocessed corn silage was 11.1% of DM while processed corn silage averaged 9.1 % of

DM. The majority of the variation in CP was due to the 2.54M diet (7.9% of DM).

Previous research (Rojas-Bourillon et al., 1987; Johnson et al., 2002b; Zobell et al., 2002;

Johnson et al., 2003b, Johnson et al., 2003c) has reported no differences in CP content of

35

Table 3.2. Chemical composition of diets containing unprocessed or processed corn silagecut at different theoretical lengths of cut (DM basis).

Diets1 Nutrient composition 1.95NP 1.95M 1.95P 2.54M 2.54P

----------------------------- % -----------------------------------

DM 43.6 43.6 43.0 42.7 39.9

------------------------------% DM--------------------------------CP, % 17.7 17.6 18.3 17.6 18.1NDF, % 30.4 31.0 32.0 31.8 32.2ADF, % 20.5 21.1 21.8 21.7 21.9Starch, % 31.5 30.1 27.1 29.2 25.7NFC, % 43.3 42.2 39.8 41.5 38.9Ash, % 7.1 6.8 7.0 6.9 7.3NEL, Mcal/kg 1.67 1.66 1.65 1.66 1.66RUP, % of CP 35.6 35.4 35.3 35.3 35.4

1 Diets containing corn silage cut at 2 different TLC and 3 different degrees of kernelprocessing - 1) 1.95 cm TLC, no processing, 2) 1.95 cm TLC, 2 mm processing, 3) 1.95cm TLC, 8 mm processing, 4) 2.54 cm TLC, 2 mm processing, and 5) 2.54 cm TLC, 8mm processing.

36

kernel processed corn silage compared with unprocessed silage. There is no explanation

for the decrease in CP content of processed corn silage in this trial.

There was no difference in NDF or ADF concentrations of corn silage associated with

kernel processing. This result differs from what was expected. Theoretically, kernel

processed corn silage should have decreased fiber concentrations due to increased starch

concentrations associated with kernel processed corn silage. A decrease in NDF

concentrations has previously been observed with kernel processing (Rojas-Bourillon et

al., 1987; Doggett et al., 1998; Johnson et al., 2002b). Rojas-Bourillon et al. (1987)

explained that decreased NDF concentrations in kernel processed corn silage were

attributed to increased concentrations of total alpha-glucosides in the rolled silage. Weiss

and Wyatt (2000) observed an increase in NDF concentrations of kernel processed corn

silage compared with unprocessed corn silage. These researchers attributed the increased

NDF concentrations to greater fermentation losses of readily fermentable substrates in the

processed corn silage.

Kernel processing had no effect on starch concentrations in whole plant corn silage.

This was opposite of reports in recent literature that noted an increase in starch

concentrations with kernel processing (Rojas-Bourillon et al., 1987; Doggett, 1998;

Johnson et al., 2003a). Bal et al. (2000) noted that increased starch concentrations of

kernel processed corn silage may be due to more uniform sampling of more aggressively

processed corn silage. Particle size is more uniform for aggressively processed corn

silage. Therefore, when sampled, kernel processed silage will more than likely represent

both stover and grain fraction more evenly compared to unprocessed corn silage.

37

Theoretical length of cut had an effect on the CP content of corn silage (P=0.03). As

TLC increased from 1.95 to 2.54 cm, CP content decreased in the corn silage. Results

from the literature are variable, although most show no difference in CP content of corn

silage cut at different TLC. Sudweeks et al.(1979) noted that as TLC increased from

0.63 to 1.27 to 1.91 cm, CP content (% DM) increased (8.4, 9.4, and 10 % DM

respectively). Others (Clark and Armentano, 1999; Dhiman et al., 2000) have reported no

change in CP content with increased TLC. Bal et al. (2000a) compared corn silage cut at

0.95 cm, 1.45 cm and 1.90 cm and found no difference in CP content of silages.

There was no significant effect of TLC on NDF or ADF concentrations of corn silage.

Results from the literature on the effect of TLC on NDF concentrations are variable.

Schwab et al. (2002) reported that as TLC increased from 1.3 to 1.9cm in unprocessed

corn silage and 1.9 to 3.2 cm in processed corn silage, NDF concentrations increased

(25.5% vs. 26.6% for unprocessed, and 24.6% vs. 25.4% of DM for processed silage).

Others (Bal et al., 2000a; Dhiman et al., 2000) have reported similar results. Again, this

variation in NDF concentrations can be attributed to more uniform sampling of finer

chopped corn silage with more uniform particle size.

There was a significant interaction between kernel processing and TLC among

treatments for DM content (P= 0.002). As TLC increased in kernel processed corn silage

from 1.95 to 2.54 cm, DM content increased on average by 1.1% (34.8% vs. 35.9%

respectively). This variation was due to the relatively low DM content of the 1.95 M

corn silage (32.2%). There was also an interaction between TLC and kernel processing

noted for CP content of corn silage for this trial (P= 0.02). As chop length increased

from 1.95 to 2.54 cm in kernel processed corn silage, CP content (% of DM) decreased.

38

This decrease was due to the 2.54 M diet, which contained only 7.9% CP, slightly lower

than recommended NRC (2001) CP values for corn silage.

There was an interaction between TLC and kernel processing for NDF concentrations

in treatment silages in this research trial. As chop length increased in kernel processed

corn silage, NDF concentrations decreased by 1% (35.5 vs. 34.5% of DM). Others (Bal

et al., 2000a) have reported increased NDF concentrations with increasing TLC. That

trial compared unprocessed corn silage and kernel processed corn silage cut at 0.63, 1.27,

and 1.91 cm and reported that as TLC increased in kernel processed corn silage, NDF

concentrations increased. The variability in fiber concentrations of kernel processed corn

silage among trials can again be attributed to more uniform sampling of finely chopped

corn silage.

There was also a noted interaction between TLC and kernel processing for ADF

concentrations in treatment corn silages. This is in agreement with Bal et al. (2000a) and

Schwab et al.(2002). Schwab et al. (2002) observed that as TLC increased from 1.9 to

3.2 cm in kernel processed corn silage, ADF concentrations increased from 15.4 ± 0.2 to

16.5 ± 0.9 % of DM. Again, this variation of fiber concentrations is most likely due to

sampling techniques (Bal et al., 2000a).

An interaction between TLC and kernel processing was observed for starch

concentration of processed corn silages. Starch concentrations were greater for kernel

processed corn silage chopped at 2.54 cm TLC compared to 1.95 cm TLC. These results

differ from results observed in recent studies. Bal et al. (2000a) compared unprocessed

corn silage to kernel processed silage cut at 0.95, 1.45, and 1.90 cm and found that the

more finely chopped kernel processed corn silage had the highest starch concentrations

39

(27.3 % DM versus 25.5% for medium TLC kernel processed silage, 25.0 % DM for long

TLC kernel processed silage, and 24.0 % DM for unprocessed control silage). The

variation in starch concentrations in that trial was attributed again to more uniform

sampling of finely chopped and kernel processed corn silage. In the current trial, silage

fermentation characteristics are similar among kernel processed treatments and indicate

adequate fermentation occurred. There is no clear explanation for the increased starch

concentrations in 2.54 cm processed corn silage compared to 1.95 cm processed corn

silage in this trial.

Fermentation characteristics. Theoretical length of cut and kernel processing affected

fermentation characteristics of the whole plant corn silage. There was an increase in pH

of corn silage with kernel processing (3.7 unprocessed versus average of 4.0 for

processed). This is in agreement with Dhiman et al. (2000) and Weiss and Wyatt (2000).

Lactic acid concentrations were also affected by kernel processing. Unprocessed corn

silage had greater (P= 0.0005) lactic acid concentrations (4.9 %) compared with

processed silage (average lactic acid concentration = 2.5%). These results differ from

recent published research examining fermentation characteristics of kernel processed

corn silage. Others (Rojas Bourillon et al., 1987; Johnson et al., 2002b; Johnson et al.,

2003b- experiment 2) have reported increased lactic acid contents with kernel processed

corn silage. The greater lactic acid content of kernel processed corn silage for these trials

was attributed to greater packing density of processed silage. More tightly packed silage

excludes more oxygen in the silo and an anaerobic environment is achieved more

quickly. Fermentation was more efficient allowing for increased carbohydrate

availability (Bal et al., 2000a). The higher lactate concentrations in unprocessed corn

40

silage in the current trial would explain the decrease in pH for unprocessed silage

compared with kernel processed silage. The higher lactate concentrations of unprocessed

corn silage may also indicate more efficient and better fermentation of the 1.95 NP corn

silage.

Particle size. Particle size distribution of post ensiled corn silage, diets, and orts is

presented in Table 3.4. As expected, percentage of as-fed post ensiled corn silage

retained on the top screen was greatest for 2.54 P and lowest for 1.95 M. Contrast

statements indicate that TLC and degree of kernel processing have an effect on

percentage of corn silage remaining on the top screen. As TLC increased from 1.95 to

2.54 cm, percent of sample remaining on the top screen increased (average 9.25 vs. 18.7

%). With less aggressive kernel processing (8 mm roll clearance), an average of 19.2 %

of silage material remained on the top screen compared to just 12.7% of the corn silage

processed at 2 mm roll clearance. Percentage of as-fed post ensiled corn silage retained

on the middle screen was greatest for 1.95 M and lowest for 2.54 M. Increased TLC

significantly reduced the amount of material left on the middle screen (P= 0.01). At a

TLC of 2.54 cm, average percentage of material 8-19 mm in size was 63% compared to

71.9% for the 1.95 cm TLC. The 2.54 M corn silage contained the greatest percentage of

particles < 8 mm in size at 25.2 %, followed by the 1.95 M corn silage at 17.4 %. This

supports the hypothesis that aggressive kernel processing significantly fractures the

kernel portion of the silage to allow for greater rumen microbial attack and starch

digestion by the animal. Aggressive kernel processing of corn silage not only reduces

particle size of the kernel fraction but further reduces the particle size of the stover

fraction of corn silage for increased microbial attack of the fiber portion of the plant.

41

Table 3.3. Chemical composition of corn silage cut at 2 different TLC and 3 different degrees of kernel processing.

Contrasts1 TLC (cm) 1.95 1.95 1.95 2.54 2.54KP NP M P M P SE 1 2 3 4

Variable------------------------------------------ % ----------------------------------

DM 37.5 32.2 37.3 36.8 34.9 0.8 NS 0.04 0.10 0.002

------------------------------------------ % DM ---------------------------------- CP 11.1 9.9 8.7 7.9 10.0 0.4 0.03 0.0008 NS 0.02

SIP 7.0 5.9 5.0 4.5 6.4 0.5 NS 0.01 NS 0.01

ADF 20.4 22.3 19.6 19.9 21.2 0.8 NS NS NS 0.04

NDF 33.9 37.3 33.6 32.8 36.3 0.9 NS NS NS 0.003

Starch 35.2 33.3 38.9 38.9 35.5 1.0 NS NS NS 0.0009

NSC 48.7 46.1 51.2 52.6 46.7 1.4 NS NS NS 0.04

Ash 3.3 3.7 3.5 3.6 4.0 0.5 NS NS NS NS

pH 3.7 4.1 4.1 4.0 4.0 0.1 NS 0.06 NS NS

Ammonia 0.8 0.9 0.7 0.4 0.9 0.1 NS NS NS 0.006

Lactic acid 4.9 2.8 2.0 2.3 2.9 0.4 NS 0.0005 NS NS

Acetic acid 1.2 1.7 1.4 1.7 1.9 0.2 NS NS NS NS

Propionic acid 0.1 0.2 0.2 0.1 0.3 0.1 NS NS NS NS1Contrasts- 1) chop length effects, 2) processing effects, 3) 2 vs. 8 mm processing effects, 4) interaction between chop length and processing effects.

42

Table 3.4. Particle size distribution of corn silage, diets, and orts for diets containing corn silage cut at 2 different TLC and 3different degrees of kernel processing.

Contrasts1 TLC (cm) 1.95 1.95 1.95 2.54 2.54KP NP M P M P SEM 1 2 3 4

% of sample remaining on screen (as fed basis)

Corn silage > 19 mm 18.6 6.4 13.1 12.1 25.3 2.0 0.005 0.08 0.0001 NS 8-19 mm 65.6 76.2 74.1 62.7 63.3 3.3 0.01 NS NS NS < 8 mm 15.8 17.4 12.9 25.2 11.4 3.9 NS NS 0.03 NS

Diet > 19 mm 6.9 7.9 7.3 9.7 11.6 2.5 0.01 NS NS NS 8-19 mm 45.9 46.8 47.9 43.9 41.7 2.4 0.05 NS NS NS < 8 mm 47.1 45.3 44.7 46.5 46.7 2.6 NS NS NS NS

Orts > 19 mm 25.6 16.9 18.2 22.6 38.4 4.2 NS NS NS NS 8-19 mm 45.0 54.3 53.6 48.3 39.6 4.0 NS NS NS NS < 8 mm 29.4 28.7 28.1 29.0 22.3 4.4 NS NS NS NS

1Contrasts- 1) chop length effects, 2) processing effects, 3) 2 vs. 8 mm processing effects, 4) interaction between chop length and processing effects.

43

Theoretical length of cut of the diet had an effect on particle distribution estimated

with the Penn State Forage Particle Separator. As TLC increased from 1.95 to 2.54 cm,

the amount of particles remaining on the top sieve increased (P = 0.01). The amount of

particles remaining on the middle sieve (between 8 and 19 mm in size) was lowest for the

diets containing corn silage of the longer TLC. As TLC increased to 2.54 cm, the

percentage of particles remaining on the middle sieve averaged 42.8% compared with

46.9% for the 1.95-cm diets. As TLC increased in diets containing processed silage, the

amount of particles left on the bottom pan (< 8-mm) was numerically greater for the

longer TLC. This data indicates that increasing TLC tends to increase the percentage of

particles left on the top and bottom sieves of the Penn State Forage Particle Separator

while reducing the percentage of particles left on the middle screen. Johnson et al.

(2003a) observed similar results with corn silage chopped at 1.11 and 2.78 cm.

Intake and digestibility. Nutrient intake data is presented in Table 3.5. Dry matter

intake did not differ among corn silage treatments. Crude protein intake was reduced as

TLC increased. There is no explanation for these results as CP content of all diets was

similar while DMI did not differ among treatments. Similar results have been reported

by Johnson et al. (2003a). Others have reported no effect of TLC on CP intake (Whitlow

and Hopkins, 1998; Dhiman et al., 2002; Johnson et al., 2002). Theoretical chop length

had no effect on NDF, ADF, or starch intake in this trial.

Increasing TLC tended to decrease NDF digestibility of diets (P = 0.08). However,

this was most likely due to the greater digestibility of the 1.95 NP diet, which was on

average 15% greater in digestibility than all other diets. When comparing the NDF

digestibility of kernel processed corn silage, silage chopped at the 2.54-cm TLC appears

44

to have greater digestibility than silage chopped at 1.95 cm TLC. This is in agreement

with current research. Previous studies (Rojas-Bourillon et al., 1987; Bal et al., 2000a;

Johnson et al., 2003a) have noted that NDF digestibility increases with increased chop

length. With increased particle size of corn silage harvested at longer TLC, stimulation

of rumination increases, passage rates are slower, thereby increasing fiber digestibility.

Theoretical chop length had no effect on CP, ADF, or starch digestibility in this trial.

Kernel processing had no effect on intake of DM, NDF, or starch in this trial. There

was an effect of degree of kernel processing on CP intake (P= 0.006). Crude protein

intake was greater for corn silage processed at 8 mm compared to 2 mm (average 4.3

versus 3.5 kg/d, respectively). Because there was no difference in DMI, this effect was

attributed to the slightly higher CP percent of diets containing 2 vs. 8-mm processed

silage (average 17.6 vs. 18.2 % respectively).

There was a tendency (P=0.08) for kernel processing to affect ADF intake. Cows fed

kernel processed corn silage tended to have greater ADF intakes than cows fed

unprocessed corn silage (4.8 versus 4.2 kg/d, respectively). The average ADF

concentration of diets containing kernel processed corn silage was slightly higher than

unprocessed corn silage (21.6 compared to 20.5%) which may have contributed to the

increased intake. Also, because kernel processing also reduces particle size of fibrous

fractions of the whole plant such as the cob, less sorting of processed silage may occur

and therefore increase ADF intake of cows consuming these diets.

Crude protein digestibility was decreased with kernel processing. Crude protein

digestibility of unprocessed corn silage was 73.1 % while kernel processed corn silage

45

averaged 57.9 %. Furthermore, as degree of kernel processing increased from 8 to 2-

mm, CP digestibility further decreased. The 1.95 M diet had the lowest CP digestibility

of 50.8 %, which accounts for the majority of the variation between the 2 and 8-mm

kernel processed corn silages. There was an interaction between TLC and kernel

processing for this study for CP digestibility. As TLC increased in kernel processed corn

silage, CP digestibility also increased. Again, the majority of this variation was attributed

to the lower digestibility of the 1.95 M diet.

Kernel processing decreased NDF digestibility from 44.7 (unprocessed) to an average

of 29.7 % (P=0.0009). Others (Doggett et al., 1998; Dhiman et al., 2000; Andrae et al.,

2001; Schwab et al., 2002) have reported similar results. Reduction of NDF digestibility

may be attributed to several reasons. The increased starch digestibility associated with

kernel processed corn silage may create a competitive inhibition of cellulolytic rumen

bacteria thereby decreasing NDF digestibility. However, in this trial, there was an

observed decrease in starch digestibility with kernel processing, most notably in the less

aggressively processed corn silage. When comparing the more aggressively processed

corn silage to unprocessed silage, this explanation for decreased NDF digestibility is

possible. Also, because particle size and cob fraction of unprocessed corn silage is

greater compared to kernel processed corn silage, cows fed unprocessed corn silage have

greater cob refusal and therefore consume more digestible NDF fractions of the silage.

Animals fed kernel processed corn silage eat more of the less digestible cob fraction

leading to decreased NDF digestibility.

Kernel processing also decreased ADF digestibility from 41.2 % (unprocessed corn

silage) to 29.5% (average for kernel processed corn silage). This is in agreement with

46

Table 3.5. Nutrient intake and digestibility of diets containing corn silage cut at 2 different TLC and 3 different degrees ofprocessing.

Contrasts1 TLC (cm) 1.95 1.95 1.95 2.54 2.54KP NP M P M P SEM 1 2 3 4

Intake, kg/d

DM 21.9 21.8 22.2 22.1 21.9 0.4 NS NS NS NS

CP 4.3 3.8 4.3 3.1 4.2 0.2 0.05 NS 0.006 NS

ADF 4.2 5.1 4.9 4.5 4.8 0.3 NS 0.08 NS NS

NDF 6.8 7.9 7.4 6.4 7.1 0.4 NS NS NS NS

Starch 6.9 6.6 6.7 5.6 6.7 0.4 NS NS NS NS

Nutrient digestibility, %

DM 68.9 48.6 54.3 59.2 52.8 1.5 NS <0.0001 NS 0.001

CP 73.1 50.8 62.6 57.9 60.1 2.1 NS <0.0001 0.004 0.04

ADF 41.2 26.8 27.7 39.5 23.9 2.9 NS 0.002 0.03 0.02

NDF 44.7 29.7 30.6 35.4 23.2 3.3 0.08 0.0009 NS 0.08

Starch 85.2 83.1 75.8 87.7 75.3 1.7 NS 0.03 <0.0001 NS

1Contrasts- 1) chop length effects, 2) processing effects, 3) 2 vs. 8 mm processing effects, 4) interaction between chop length and processing effect

47

Doggett et al. (1998) and Andrae et al. (2001). The degree of kernel processing also

affected ADF digestibility. However, most of the variation was associated with the

2.54M diet, which was an average of 13.4 % more digestible than all other kernel

processed silage. There was also a noted interaction between TLC and kernel processing,

again mainly attributed to the 2.54 M diet. As TLC increased in kernel processed corn

silage, ADF digestibility increased.

Kernel processing did decrease starch digestibility by 4.8 %. This response is different

from response noted in the literature. Others (Andrae et al., 2001; Weiss and Wyatt,

2000; Johnson et al., 2002c) have reported increased starch digestion with kernel

processing of corn silage. However, these results indicate that the decrease in starch

digestibility with kernel processing can be attributed to the degree of kernel processing of

the corn silage. Corn silage processed at 8-mm roll clearance was significantly lower

than that processed at 2 mm roll clearance. As kernel processing became more

aggressive, starch digestibility increased greatly. Corn silage processed at 8-mm roll

clearance had an average starch digestibility of 75.6 % while corn silage processed at 2-

mm roll clearance averaged 85.4 % digestibility.

Production response. Production responses to TLC and degree of kernel processing

are presented in Table 3.6. Theoretical chop length had no effect on yield of milk, fat or

protein. There was a tendency for TLC to affect efficiency, defined as energy corrected

milk/ kg of DMI (P=0.10). However, the majority of this effect was due to the 2.54 P

diet, which had a significantly lower efficiency (1.56 ECM/kg DMI) compared to all

other diets, which ranged from 1.70 to 1.74 ECM/kg DMI. Theoretical chop length also

tended to decrease blood glucose concentrations (P= 0.08). This result differs from what

48

was initially hypothesized. Starch and NDF digestibilities were not significantly affected

by increasing TLC and therefore the response observed in the trial for increased blood

glucose with increasing TLC is unexplained.

Blood urea nitrogen tended to decrease as chop length increased from 1.95 to 2.54 cm

(P=0.09). However, this difference was mainly due to the 2.54 M diet (16.8 mg/dl) and

the 1.95 NP diet (20.2 mg/dl). All other diets ranged from 18.1 to 18.7.

Degree of kernel processing tended to increase milk yield in cows fed silage processed

at 2-mm roll clearance compared to cows fed silage processed at 8-mm roll clearance

(P=0.07). Cows fed the 2.54 M diet exhibited the greatest milk yield at 37.9 kg/d. All

others averaged 35.4 kg/d milk yield. Bal et al. (2000) noted an increase in milk yield of

1.2 kg/d with corn silage processed at 1-mm roll clearance compared to unprocessed

silage, regardless of TLC. The increase in milk yield with aggressively processed corn

silage is most likely related to increased starch digestibility of the 2-mm processed silage.

There was no significant effect of kernel processing on percent of milk fat or protein.

There was an interaction between TLC and kernel processing for protein yield (kg/d). As

TLC increased from 1.95 to 2.54 cm, protein yield was reduced in kernel processed corn

silage from an average of 2.41 to 2.38 kg/d. An interaction also occurred between TLC

and kernel processing for energy corrected milk (ECM). As TLC increased in kernel

processed corn silage, ECM decreased. The variation in ECM was attributed to the 2.54

P diet, which had an ECM of 75.9 kg/d compared to other kernel processed diets with

ranged from 81.2 to 84.0 kg/d. Efficiency, defined as kg milk/ kg DMI, was greatest for

the 2.54 M diet at 1.73 %. Because DMI did not differ among diets, the greater starch

49

and fiber digestibility of this diet compared to all other diets explains the increased

efficiency.

Kernel processing had an effect on blood glucose. Cows fed kernel processed corn

silage had greater blood glucose concentrations compared to cows fed unprocessed

silage. This increase in blood glucose in cows fed kernel processed silage can be

attributed to greater starch digestibility of the processed diets.

50

Table 3.6. Least square means of performance parameters for lactating dairy cows fed corn silage cut at 2 different TLC and 3different degrees of kernel processing.

Contrasts1 TLC (cm) 1.95 1.95 1.95 2.54 2.54KP NP M P M P SE 1 2 3 4

Variable

DMI, kg/d 21.9 21.8 22.2 22.1 21.9 0.4 NS NS NS NS

Milk, kg/d 35.4 36.1 36.0 37.9 34.1 1.0 NS NS 0.07 0.09

Fat, % 3.86 3.92 3.88 3.95 3.81 0.12 NS NS NS NS

Fat, kg/d 3.00 3.00 3.09 3.11 2.80 0.11 NS NS NS 0.09

Protein, % 3.13 3.09 3.11 3.15 3.04 0.04 NS NS NS NS

Protein, kg/d 2.43 2.35 2.47 2.49 2.32 0.07 NS NS NS 0.01

ECM, kg/d 81.2 81.2 83.2 84.0 75.9 2.2 NS NS NS 0.03

Efficiency,milk/kg DMI 1.62 1.67 1.64 1.73 1.56 0.04 NS NS 0.03 NS

Efficiency,ECM/kg DMI 1.70 1.72 1.71 1.74 1.56 0.04 0.10 NS 0.03 0.04

BW, kg 628.3 627.5 624.7 634.5 623.7 5.3 NS NS NS NS

Plasma metabolites, mg/dlGlucose 64.2 68.2 65.3 67.5 68.5 1.3 0.08 0.03 NS NS

Urea nitrogen 20.2 18.1 18.2 16.8 18.7 0.7 0.09 0.006 NS NS1Contrasts- 1)chop length effects, 2) processing effects, 3) 2 vs. 8 mm processing effects, 4) interaction between chop length and processing effects.

51

CONCLUSIONS

Theoretical length of cut, kernel processing, and degree of kernel processing all had an

effect on starch and fiber digestibility of corn silage and milk yield in cows. As TLC

increased from 1.95 to 2.54 cm in whole plant corn silage, NDF digestibility tended to

decrease. Furthermore, kernel processing also decreased the NDF digestibility of the

diets. Degree of kernel processing influenced starch digestibility. The more aggressively

processed corn silage (2-mm) had greater starch digestibility compared to the less

aggressively processed corn silage (8-mm). The greater starch digestibility of the 2-mm

processed diets contributed to an increase in milk yield (P= 0.07) for cows fed these

diets. Degree of kernel processing also increased the efficiency (milk/ kg DMI) of the

cows fed diets processed at 2-mm roll clearance. The results from this study suggest that

with an increase in TLC from 1.95 to 2.54 cm, aggressive kernel processing is needed to

increase starch digestibility and offset any decrease in NDF digestibility of the diet.

Further research is needed to determine the optimal degree of kernel processing needed at

longer TLC to elicit the greatest production response in dairy cattle.

52

CHAPTER 4

CONCLUSIONS

The practice of kernel processing is a useful tool in the production of quality whole

plant corn silage. Kernel processing fractures the waxy outer coating of the kernel

fraction of the corn plant, exposing the inner starchy granule and allowing for greater

microbial attack and starch digestion. Processing further reduces particle size of the

plant, increasing surface area and further increasing rumen microbial attachment for

greater nutrient digestion.

Kernel processing in corn silage chopped at longer theoretical lengths of cut

improved starch digestibility. More aggressively processed corn silage also shows a

greater starch digestibility compared with less aggressively processed silage. This greater

starch digestibility led to an increase in milk production in cows fed aggressively

processed silage. Animals fed aggressively processed silage had a greater efficiency,

defined as kg milk/ kg DMI, compared with those fed less aggressively processed silage.

As theoretical chop length increased form 1.95 to 2.54 cm, NDF digestibility tended to

decrease. Furthermore, kernel processing also decreased the NDF digestibility of the

diets. The results from this study indicate that kernel processing as chop length increases

is a beneficial tool to increase starch digestibility and milk yield in dairy cattle.

53

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