MISCELLANEOUS REPORTS 4

SOME ASPECTS OF THE PHYSIOLOGY OF

DIGESTION IN THE RUMINANT WITH PARTICULAR

REFERENCE TO ACIDOSIS

s. EcoDomides

AGRICULTURAL RESEARCH INSTITUTE

MINISTRY OF AGRICULTURE AND NATURAL RESOURCES

NICOSIA -­ CYPRUS

MARCH 1982

SOME ASPECTS OF THE PHYSIOLOGY OF

DIGESTION IN THE RUMINANT WITH PARTICULAR

REFERENCE TO ACIDOSIS

S. Economides

SUMMARY

In ruminants, 70% of the digestion is completed in the rumen through the enzymatic activity of micro­organisms. Many changes and pathological conditions may result from feeding ruminants excessive amounts of concentrates. The clinical designation "rumen acidosis" is a collective term for several digestive disturbances resulting in depression of the pH of tbe rumen contents. Mortality or loss of production from acidosis can be minimized with gradual adaptation of tbe animals to concentrate diets, with a correct balance between the roughage and concentrate components of the diet (roughage should constitute 40 % of the dry matter of the total diet), separate feeding of animals having different energy requirements. and with the supplementation of the diet with buffer substances.

INTRODUCTION

Ruminant animals (cows, sheep and goats) are of low productivity in their natural environ­ment, and have, presumably, evolved on diet~ made up largdy of forages. When forages are the predominant feed a very healthy type of metabolism is maintained. Ever since ruminants have been domesticated their feeding and management have changed. It is expected of ruminants to have a rapid rate of growth, early maturity, large litter size and high milk yield. In order to supply sufficient energy for maximum production, forage diets are supplemented with high levels of cereal grain. Under these condi­tions the normal metabolism of the animals is derailed and health problems arise.

Microbial digestion in the rumen

The main feature of ruminants is their stomach, which is composed of four compart­ments, i.e. the rumen, reticulum, oma.mm and abomasum. In simple-stomach animals, like pigs, the digestion of feed is of a chemical nature, and ingested carbohydrates are hydrolyzed to glucose. In ruminants digestion is microbial in the rumen, where feed ingredients are hydro­lyzed by enzymes secreted by microorganisms, and chemical in the abomasum and small intestine. Nearly 70% o-f the digestion in ruminants is c2-mpl~!ed in the _.rumen where carbohydrates are fermented to acids, mainly ac~tic, propionic and butyric, which are absorbed through the rumen wall into the blood system. Despite

the continuous production of acids in the rumen the pH of the rumen fluid remains fairly constant, between 6.6 and 6.8, mainly because the increased acidity is buffered by the saliva secreted from the parotid glands.

According to Hobson (1972) all natural microbiological systems are mixtures of organisms balanced in type and number in response to environmental conditions. A change in the enviro-nment will lead to a change in the balance of organisms. A large change in the environment can result in the breakdown of the system from death of the microbes or the proliferation of undesirable organisms. The rumen microorga­nisms have evolved to deal with low substrate concentrations and slow changes in input. Mod:::rn feeding systems are producing high substrate concentrations and rapid changes in inputs, which make microbial systems unstable.

Factors affecting the microbial population and acidity in the rumen

The amount and quality of feed eaten by ruminants, and the type of feeding are the main factors governing the composition and activities of the rumen microbial population.

When roughages are fed, fermentation in the rumen is regulated by the activities of the cellulolytic organisms. Even when the diet supports the growth of higWy active cellulolytic flora in the rumen the enzymatic hydrolysis is slow. The rate of production of end products does not exceed the animals' capacity to dispose of them and as a result a favourable rumen envi­

ronment and a healthy metabolism is maintained (Schwartz and Gilchrist, 1975).

When high barley diets are fed ad libitum rumen pH fluctuate.> from day to day, which shows thaTIfie con 1 ions lotl:fe rumen on such-­diets are variable (Eadie and Mann, 1970). When an excess of barley grain is consum(;d there may be a sudden increa&e in bactelial ac~ivity, which lead.> to the accumtt.lation of acids, suppres­sion of certain of the strictly anaerobic bacteria, a marked increase of streptococci followed by lactobacilli, and elimination of ciliate protozoa. The rumen conditions can be appreciably improved by feeding the same high barky diet in amounts below appetite given in fmall portions throughout the day, though these conditions differ from those of roughage feeding. However, largely becaus~ of valiatjoDs between animals in susceptibility to rumen changes, the conditions produced are variable.

When low-roughage diets are kd in con­junction with concentrates the activities of the amylolytic and saccharolytic bacteria are no longer limited by the cellulolytic flora. They are able to hydrolyze starch and break hexos.;s down faster than the metabolic hydrogen so produced can be disposed of by the normal path­ways, the most important of which is its conversion by methanogenic bact~ria to methane (Sqhwartz and Gilchrist, 1975). When an animal is abruptly 'fed a readily fermentable feed, hydrogen accu­mulates in the rumen and is c.onverted mainly to lactate. If the production of lactic acid is above the capacity of the bacteria using it, lactic acid accumulates in the rumen. As a res ult the pH of the rumen falls and the microbial popu,lation is changed drastically.

Prevention of accum ulation of lactic acid is' not, however, in itself sufficient to enSUre a favourable ruminal pH for the maintenance of a balanced population of microbes. Rapid production of volatile fatty acids can bring the pH down below 5.5, if insufficienl saliva is secreted to n~utralize them (Emmanuel et al., 1970; Lawlor et al., 1966). The amount of saliva s~reted depends on the coarseness of the diet and th~ amount of time spent on chewing and ruminating. The quantity of saliva secreted is reduced when concentrate diets are fed, and when the quantity of roughage is low Or is ground and pelleted.

Adaptation of microorganisms to starchy diets

If the animal is adapted gradually to high concentrate rations, instead of being switched over abruptly, it can tolerate a high level of

readily fermentable carbohydrate in the rumen without breakdown of rumen function. The key organisms in successful adaptation are the ciliate protozoa, the methanogenic bacteria and the lactic-acid-utilizing bacteria. Protoz<,>a take up starch grains and soluble sugars and convert them to storage polysaccharides. In this way the quantities of substrate available to the faster growing bacter;a ar~ greatly reduced. Growth of protozoa is very sensitive to changes in rumen pH. If the pH of the rumen goes below 5.0 the protozoa die lapidly. The failure of the methanogenic bacteria to increase their activity to match the hydrogen production resulting from feeding large amounts of concentrate rations is due to th~ir sensitivity to acid. Time;s needed to enable lactic-acid-ulilizing bacteria to incr~ase their numbers to match the production of lactic acid in the rumen.' This helps also to prevent rumen pH from falling to a level that endangers the protozoa and methanogenic bacteria.

Pathological effects

Numerous authors have reported the follow­ing changes and pathological conditions due to high concentrate intakE.:: Decreased salivation, rwnination, and rwnen pH, changes in rumen microorganisms, reduction or loss of appelile, production of lactic acid, craving, wool eating, ruminal atony, reduCed motility of the dig~stive

tr act, rumenitis, gastric impaction, displac~d

abomasum, metastatic infections of tissues. laminitis, and acidosis (Dough~rty el al., 1965; Lawlor etal., 1966; Nelson el al., 1968; Mi1kr and O'Dell, 1969; Dougherty, 1970; Dirk';Gn, 1970; Eadie and Mann, 1970; Sharpe, 1972; Schwartz and Gilchrist, 1975).

Other workers have suggested tha, the ingestion of large amounts of carbohydrates and the resulting depre~sion of rumen pH and the pronounced changes in rumen microorganisms cause the production of toxic substances, lik~ histamine and tyrarnine(Dirksen, 1970; Dougherty, 1970; Mullenax el al., 1966; Dougherty et al., 1965). From the rumen contents of overfed sheep different sp,~cies of lactobacilli Were isolated which were capable of forming histamine from histidine by decarboxylation (Rodwell, 1953, cited by Dirksen, 1970). Administration of antihistamine counteracted ruminal stasis' and successfully reinstituted el uctation in sheep (Dougherty et al., 1965).

Forms of acidosis, clinical signs, course and complications

According to Dirksen (1970) the clinical designation "rumen acidosis" has to be considered

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as a collective term for several digestive disturb­ances resulting in depression of rumeD pH. He divides rumen acidosis into acute Or peracute form, (1 umen acid?si~, l~ctjc acidosis, grain engorgement, toxic mdig~tlOn, rume.n. overload, rumen impaction), chrOUlc or subchmcal form, and "off fe~d" form. Acidosis may develop under the following conditions: 1) when rumI­nants unaccUstomed to conc~ntrate food (baIley, wheat, 6Orghum, maize, sugar beets, potato~s,

apples, baker's waste) suddenly c.onsurne a large quantity of it, 2) when potentially dangerous food has already been eaten for seve:al weeks and the ration is then modelately mcrcas:~d, 3) when the roughage component of the ratIOn is low in relation to the concentrates, 4) when the proportion of roughage is lowered with a.n un­changed concentrate intake. 5) when rummants are suddenly changed from straw or hay to green pasture (high in soluble cal bohydrates and low in crude fiber).

The clinical signs differ with the degree. of ruminal acidosis the resulting local and absorptive changes, and the stage of the diseas~. ~ild cases show a transient loss of appetite ( off feed") and diminished rumen contractions. M~re severely affected animals sho",,: co.mplde anoreXia, apathy, muscular tremors, gnndmg of ~he te-eth, unrest increased heart rate and usually Illcreascd respir~tion rate and lameness. PeraCULe acidosis leads to recumb~Qcy and coma in a "milk-fever­like" attitude within 12-24 h.

The course and complications of acidosis depend on its form, the d:i{:t and the de~ee of damage to the absorptive sites. In subchmcal or chronic and mild acidosis the course depends on whether the rumen flora adapts to concentrate food or whether the composition of the ration is corrected. Otherwise, subclinical acidosis with the rumen flora in a labile state leads to acute indigestion even after small changes in the ~atio1?' In cases of acute indigestion, if the ratIOn IS corrected in time, appetite usually returns to normal within a few days. In more severely affected animals the course Of the disorder is determined by the degree of rumenitis, and by che blood chang(;s and degenerative altera~ions in body tissues. The animal may. die Within 4-8 days, it may have a slow .or .ral2Id r~covery, or it may suffer from chromc mdJgestIOn due to ulcerative rumenitis. Rumenitis of varying form and degree according to the s~verity of rumina I acidosis is a characteristic sign. As soon as the mucosa of the rumen wall loses its protective epithelia, bacteria or fungi __may invade and pJ:oduce deep ulcers and metastatic infections in other organs (liver, kidneys).

WORK AT THE AGRICULTURAL RESEARCH INSTITUTE

The increasing intensification of animal production, the use· of improved .breeds with high milk yields and lalge litter .sIZe, and ~he scarcity of roughage, have necessJta:ed feeding increased quan.ities of concentrates. Health pro­blems to ruminants from acidosis have already b~n indicated (Economides, 1973; LaWIor and Economides, 1974). In a trial with ewes, an attempt was mad~ to simulate cases of acidosis. Thirty lactating ewes were divided into three groups to receive the following diets: .

A. Concentrates ad libitum

B. Concentrates ad libitum plus chopped st'raw ad libitum

C. Concentrates ad libitum plus lucerne straw ad libitum

Only ewes on treatment C were h~althy. Ewes on treatment A ·went "off feed" and therr appetite was only restored when th<?y reCeived lucerne hay, and their concentrate mtak~ was reduced. Two eweJ on treatment B were ~nously affected, one within 36 hours and the other within 3 days from the change of the diets. Both sho~ed apathy, muscular tremors, frothing f~om the mouth, increased respiration rate and Jllcreas~d heart rate. The calcium levels of blood samples taken before any therapy were normal, i.e. 10.9 and 11.0 mgj I00 ml serum in both ewes. Follow­ing injection of calcium borogluconate all sym­ptoms disappean:d. Appetite was restored when they were offered lucern~ hay and only a small quantity of concentrates.

In another experiment with pregnant ewes, feed composed of lucerne meal and concentrates was given in pelleted form. Ewes soon went "off feed" and appetite was only restored when they WCl e 'offel ed Iucel ne hay. During the tri~l ewes had a desire for craving the walls of theIr individual pens. In other caSES they were chewing their wooden feeding troughs. This desire for chewing wood or craving the walls are mainly attlibutcd to lack of fib::r in their diets.

PREVENTIVE MEASURES

Mortality 01 loss of production from acidosis may be high. In order to m.inimiz~ such losses the foJIov.ing preventive measures are suggested:

IL_GraduaLadaptation_tfr-c_onc.cntralLdiets ov~r a period of at least two weeks. This. change­OVer period will be applied whenever Ulcreases

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in energy intake are required because of Lawlor, M.J., and S. Economides. 1974. Report 00 tbe increasing energy requiremenls (late pre- incidence of mortality and illness among the ewes

at Margo and Athalassa farms duriog 1973-1974. gnancy-early lactation) or dUi ing changes Mimeographed. Agricultural Research Institute of the gl ains in the diet. Nicosia. '

2) There-rollSLaLwaY-s--be-a.-b.alancLbctw.eeJ1----L-awlor,--MoJ~D:-Giesecke;-and--K-:-Wa"lser=Karst. 196-6. the roughage and concentrate componenls Comparative studies on the digestive physiology of the diet. General recommendations are of sheep fed. on se~!-purified or roughage-that the crude fibre content of the diet should ~~~~~~~t~ dIets. British Journal of NutTitioll

not be less than 18% or 14% of Total Dige- .. stible Nutrients for milking or fattening Miller, W.J., and G.D. O'Dell. 1969. Nutritional problems animals, respectively. This is II or 8% of of using maximum forage or maximum con-the total ra'ion. Alternatively the roughag~ centrates in dairy cows. Journal of Dairy Science.

52:1144-1154. component of the diet should constitute 40% of the dry maUer of the total ration. Mullenax, CH., R.F. Keeler, and M.l. Allison. 1966. Physio­

logic responses of ruminants to toxic factors 3) Feeding more than once daily. extracted from rumen bacteria and rumen fluid.

American Journal of Veterinary Research 27:When group feeding is practised, adequate4) 857-868. space for all animals in the feeding troughs is necessary. More aggressive animals, or Nelson, B.D., H.D. Ellzey,. E.B. Morgan, and M. Allen. 1968.

Effects of feeding lactating dairy cows varyinganimals with higher energy requirements forage to concentrate rations. Journal of Dairy

should be fed separately. Science 51 :1796-1800.

5) Inclmion of buffN substances (NaHCO)) Schwartz, H. M., and F.M.C Gilchrist. 1975. Microbial in the feed when diet> are changed or concw­ interactions with the diet and the host animal. lrate intake is incr~ased, though they are In Digestion and metabolism in tberuminant.

The University of New England Publishingnot always effective. Unit, Armidale.

Sharpe, M.E. 1972. The effect of manipulation of rumen fermentation by change in diet from high roughage to high cereals on various aspects of the rumen

REFERENCES microorganisms. Paper presented at the '·Meeting on manipulation of rumen fermentation".

Dirksen, G. 1970. Acidosis. In PbysiO:ogy of digestion and Hannah Research Institute, Ayr.metabolism io the ruminant. Oriel Press Ltd., Newcastle upon Tyne.

Dougherty, R.W., CH. Mullenax, and M.J. Allison. 1965. Physiological phenomena associated with eructa­tion of ruminants. In Physiology of digestion in the ruminant. Butterworth Inc., Washington DC

Dougherty, R. W. 1970. Physiopathology of the ruminant digestive tract. In Duke's Physiology of domestic animals. Cornell University Press, Ithaca and London.

Eadie J. Margaret, and S.O. Mann. 1970. Development of the rumen microbial population: High starch diets and instability. In Pbysiology of digestion and metabolism in the ruminant. Oriel Press Ltd., Newcastle upon Tyne.

Economides, S. 1973. Report on the incidence of bypocalcaemia among ewes at Margo farm. Mimeographed. Agricultural Research Institute, Nicosia.

Emmanuel, B., M.J. Lawlor, and D. McAlleese. 1970. The effect of phosphate and carbonate-bicarbonate supplements on the rumen buffering systems of sheep. British Journal of Nutrition 24 :653-660.

Hobson, P.N. 1972. Microbiological studies related to the function of the rumen microbial population. Paper presented at the "Meeting on Manipulation of Rumen Fermentation". Hannah Research Institute, Ayr.

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