J. O. Sanders and S. B. Smith S. G. May, N. S. Burney, J. J. Wilson, J. W. Savell, A. D. Herring, D. K. Lunt, J. F. Baker, produced by different generations of angus sires Lipogenic activity of intramuscular and subcutaneous adipose tissues from steers 1995, 73:1310-1317. J ANIM SCI http://jas.fass.org/content/73/5/1310 on the World Wide Web at: The online version of this article, along with updated information and services, is located www.asas.org by guest on July 14, 2011 jas.fass.org Downloaded from
Transcript
J. O. Sanders and S. B. SmithS. G. May, N. S. Burney, J. J. Wilson, J. W. Savell, A. D. Herring, D. K. Lunt, J. F. Baker,
produced by different generations of angus siresLipogenic activity of intramuscular and subcutaneous adipose tissues from steers
1995, 73:1310-1317.J ANIM SCI
http://jas.fass.org/content/73/5/1310on the World Wide Web at:
The online version of this article, along with updated information and services, is located
www.asas.org
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Lipogenic Activity of Intramuscular and Subcutaneous Adipose Tissues from Steers Produced by
Different Generations of Angus Sires'
S. G. May2, N. S. Burney, J. J. Wilson3, J. W. Savell, A. D. Herring, D. K. Lunt4, J. F. Bake+, J. 0. Sanders, and S. B. Smith6
Department of Animal Science, Texas Agricultural Experiment Station, Texas A&M University, College Station 77843-2471
ABSTRACT Simmental and Hereford cows ( n = 74) were inseminated with semen from purebred Angus bulls from the 1960s or with semen from purebred Angus bulls from the 1980s. The F1 calves provided the foundation for two investigations, one addressing growth and carcass characteristics, and another measuring the impact of sire generation on lipid metabolism and adiposity. Calves sired by the 1980s-type bulls had greater ( P < .05) birth, weaning, and final live weights and carcass weights. They also had larger ( P < .05) hip heights and hip widths at weaning and larger ( P < .05) hip heights and lower ( P < .05) body condition scores at slaughter. There were no differences ( P > .05) in any measure of fatness between groups (adjusted fat thickness, kid- ney, pelvic, and heart fat, or marbling scores), but yield grade was higher numerically ( P < . l ) for the 1980s steers. The second aspect of this research addressed the influence of different generations of Angus sires on specific carcass traits and adipose tissue metabolism. A subset of six steers for each generation type (from Simmental cows) were selected
and samples were collected at slaughter for measure- ments in vitro. For both generation types, intramuscu- lar (i . m. adipocytes had lesser ( P < .05) cell volumes than subcutaneous (s.c. adipose tissue. Correspond- ingly, i.m. adipose tissue exhibited lower ( P < .05) rates of 14C-labeled acetate incorporation into lipids as measured immediately after slaughter. Intramuscular and S.C. adipocytes from 1980s-type steers were smaller ( P < .05) than those from the 1960s-types steers, with correspondingly more cells per gram of tissue. Thus, the newer generation-type steers required more adipocytes to achieve the same amounts of i.m. and S.C. fat. There was no difference between generation types in 14C-labeled acetate incor- poration into neutral lipids and, of the lipogenic enzyme activities measured, only 6-phosphogluconate dehydrogenase was significantly lower in adipose tissues of the 1980s-type steers. Thus, in spite of differences in sire generation and cellularity, carcass traits associated with fatness and in vitro measures of lipogenesis were not different between 1960s- and 1980s-type Angus crossbred steers.
Key Words: Angus, Steers, Lipogenesis, Carcass
'Tech. art. no. 31795 Texas Agric. Exp. St. Funded by the Texas
'Present address: Westreco, Inc., 3916 Pettis Rd., St. Joseph, MO
'Present address: Dept. of Food Sci. and Human Nutr., Colorado
4Dept. of M m . Sci., Texas A&M University, McGregor Research
'Present address: University of Georgia Coastal Plain Station,
6To whom correspondence should be addressed. Received June 3, 1994. Accepted December 6, 1994.
Agric. Exp. Sta.
64503.
St. Univ., Ft. Collins 80523.
Center, Rt. 1, Box 148, McGregor 76657.
P.O. Box 748, Tifton 31793-0748.
J. h i m . Sci. 1995. 73:1310-1317
Introduction
Selective breeding during the past several decades has produced Angus cattle that are larger framed, taller, and longer in conformation than their 1960s counterparts. Because of these differences, contem- porary Angus cattle are thought by some to be later maturing with a lesser propensity to accumulate carcass fat and, notably, marbling. However, the recent National Beef Quality Audit (Lorenzen et al., 1993) indicated that few carcass traits have changed between 1974 and 1991, with the exception of carcass weight. The results of Lorenzen et al. (1993) indi- cated that contemporary cattle have changed in conformation, but do not produce carcasses that are
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leaner or that produce lower USDA quality grades than cattle of a previous era.
The National Beef Quality Audit compared car- casses across a wide variety of breed types, but could not address specific changes over the past two decades within a breed type. This research specifichlly ad- dressed differences between Angus conformation types in aspects of carcass fatness. To accomplish this, we took advantage of the availability of semen from 1960s bulls to produce 1960s-type crossbred steers, and compared these offspring to those produced with semen or natural service from 1980s-type bulls. We measured lipogenesis and lipogenic enzymes in i.m. and S.C. adipose tissues obtained at slaughter in order to provide a cellular mechanism for putative differ- ences in marbling scores between 1960s- and 1980s-type cattle.
Materials and Methods
Growth and Carcass Characteristics. Two groups of Angus x Simmental crossbred calves, bred t o differ in phenotypic characteristics, were used in the study. Crossbred calves were produced by inseminating Simmental cows with semen from bulls from two different beef industry era, the 1960s and 1980s. The 1960s sires were smaller framed, earlier maturing than 1980s sires, which were larger framed, later maturing cattle. Semen from 18 1960s-type Angus bulls was obtained from an earlier crossbreeding study at the McGregor Research Center, Texas. Semen from 12 1980s-type Angus bulls was obtained from contem- porary commercial sources. In addition, one Simmen- tal cow was bred to a 1980s-type bull by natural service. Sixty-seven Simmental cows and seven Hereford cows, varying in age from 2 to 10 yr , were used to produce the calves.
Semen from the two generation types was assigned randomly over 2 yr for insemination of the cows. In 1989, 69 F1 calves were born from January through April, and 28 F1 calves were born froln February to May of 1990. At birth, calves were weighed and measured for heart girth circumference and cannon bone length. Date of birth, sex of calf, calving ease, calf vigor, and nursing codes also were recorded.
The calves were weaned at approximately 7 mo of age, immunized for respiratory and clostridial dis- eases, and implanted with RalgroTM. Weight, hip height, and body condition scores were recorded at weaning and repeated every 30 d, whereas hip width was recorded at weaning and repeated every 90 d. RalgroTM was reimplanted every 90 d. After weaning, a starter diet was fed for 3 wk, followed by an intermediate diet for 2 wk, before switching to a finishing diet (Schiavetta et al., 1990), which was fed until slaughter. Steers and heifers born in 1989 were fed for 217 and 202 d, respectively, whereas steers and
heifers born in 1990 were fed for 222 and 229 d, respectively. The animals were weighed before ship- ping, which was used as the final live weight.
Metabolic Studies. A subset of the cattle from the growth study was used for measurements of adipose tissue metabolism and cellularity. Six Simmental cows were selected for each generation group, representing offspring of six purebred Angus bulls from the 1960s and five purebred Angus bulls from the 1980s. Steers from both sire generations were born between January and April 1990, and were slaughtered the end of May 1991. There was no difference in age at slaughter. All steers used in this subset were bred and managed at the Texas A&M University, McGregor Research Cen- ter.
Animal Slaughter and Carcass Characteristics. Calves born in 1989 were slaughtered at three different facilities and evaluated by two different evaluators, whereas 1990-born calves were slaugh- tered at the Rosenthal Meat Science and Technology Center at Texas A&M University. The animals were slaughtered humanely in compliance with the Hu- mane Methods of Slaughter Act of 1978. Immediately after exsanguination, the 2nd t o 6th lumbar region of the loin was removed and transported to the labora- tory in Krebs-Henseleit bicarbonate buffer with 5 mM glucose (pH 7.4) at 37°C. Subcutaneous and i.m. adipose tissue samples were obtained for cellularity, lipogenic enzyme assays, and 2 h in vitro incubations for lipogenic activity for acute ( 0 h ) and long term (40 h ) cultures. Carcasses were chilled for 24 h and USDA quality and yield grade data were collected (USDA, 1989) by two trained Texas A&M University person- nel.
Adipose Tissue Cellularity. Procedures outlined by Etherton et al. (1977) as modified by Prior (1983) were used to determine adipocyte cellularity. Sub- cutaneous and i.m. adipose tissue samples were frozen at -25°C and sliced in l-mm thick sections to facilitate tissue fuation. Fixed cells were filtered through 240-, 64-, and 20-pm nylon mesh screens using .01% Triton in .l54 M NaC1. Cell fractions were collected from the 64- and 20-pm mesh screens for cell size and number determination using a Coulter Counter, Model ZM equipped with a channelizer, Model 256 (Hialeah, FL).
Lipogenic Enzyme Activities. Fresh adipose tissue samples (1 g s.c.; 100 mg i.m.) were homogenized on ice in 3 volumes (wthol) of .l54 M KC1: .l M phosphate buffer (pH 7.4, 25°C). The homogenate was centrifuged at 3,000 x g for 15 min at 4°C and decanted. The supernate was centrifuged at 15,000 x g for 30 min at 4°C. The resulting supernate was assayed immediately for selected enzymes associated with fatty acid synthesis. Fatty acid synthetase activity was assayed according to procedures of Martin et al. (1961). The activity of NADP-malate de- hydrogenase was assayed as described by Ochoa
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( 19 5 5). Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were determined as described by Bernt and Bergmeyer ( 1974). All enzyme assays were determined in triplicate using spectrophotometric absorbance of solutions in cuvettes at 340 nm. Slopes of the linear rates of NADPH oxidation (fatty acid synthetase) or reduction (all other enzymes) were used to calculate enzyme activi- ties. All enzyme activities were linear to at least 10 times the amount of homogenate added, and were assayed under substrate conditions that yielded max- imal velocities.
Lipogenesis In Vitro. Two-hour in vitro incubations were performed with S.C. and i.m. adipose tissue immediately after slaughter (0 h ) and after a 40-h explant culture period. Adipose tissue explants (50 to 100 mg) were incubated in 3 mL of incubation media containing Krebs-Henseleit bicarbonate buffer (pH 7.41, 5 mM sodium acetate, 5 mM glucose, 10 mM Hepes buffer and 1 pCi of [U-l4C1acetate. Vials were gassed for 1 min with 95% 0 2 5 % COz, capped and incubated for 2 h in a shaking water bath at 37°C. At the end of the incubation period, reactions were terminated by addition of 3 mL of 5% trichloroacetic acid. Explants were rinsed sequentially with .l54 M NaCl and Krebs-Henseleit bicarbonate buffer to re- move free lipid and unincorporated substrate. The neutral lipids were extracted using the Folch et al. (1957) procedure as modified by Mersmann (1987). Samples were evaporated to dryness, resuspended in 10 mL of scintillation cocktail, and radioactivity was counted with a scintillation counter. Incorporation of 14C-labeled acetate into neutral lipids was calculated as acetate incorporated per 2-h incubation period per 105 cells.
Adipose Tissue Explant Culture. Long-term adipose tissue explant cultures were conducted according to procedures described by Etherton and Evock ( 19861, as modified by Miller et al. ( 199 1). Tissue explants for subcutaneous and intramuscular adipose tissue were incubated for 40 h at 37°C at pH 7.4 in a NuAire Model NU1500 (Plymouth, MN) and maintained under a gas phase of 95% 0 2 5 % C02. Adipose tissue was sliced to obtain explants of 30 to 50 mg. Approximately 100 mg of adipose tissue were trans- ferred to each well in 12-well, 17.6-mm x 22.1-mm well diameter culture plates containing 5 mL of culture media. All explants were in culture within 30 min postmortem. Explant medium was Medium 199 (with Hank's salts and L-glutamine) sup- plemented with .35 g/L of NaHC03, 10% bovine calf serum, 20 mM Hepes buffer (pH 7.41, 5 mM glucose, 5 mM sodium acetate, and antibiotics (100 pg/mL of penicillin, 10 pg/mL of streptomycin and, 10 pg/mL of amphotericin B). To maintain proper pH and nutrient supply, media were changed every 6 h.
Statistical Analyses. Data analyses for the growth and carcass data (all animals) were conducted using
the GLM procedures of SAS ( 1985 1. For birth, weaning, and carcass traits, the fixed effects in the model included year of birth, sex of calf, generation of sire, age of dam, and breed of dam. Age of dam was separated into categories of 2-yr, 3-yr, 4-yr, or 5- to 10-yr olds. A random sire within generation effect was included to serve as the proper error term to test the significance of generation of sire.
For birth trait analyses, date of conception within year was used as a covariate to account for the tendency in central Texas for calves born later in the Spring calving season to be heavier due to the increased nutritional plane of the cows. Age at weaning was used as a covariate for weaning trait analyses, whereas age at slaughter was used as a covariate for analysis of carcass characteristics.
Statistical analyses of the data from steers used in the metabolic studies were made using the GLM procedures of SAS ( 1985 1. For carcass grade traits, the statistical model included generation type as the main effect. For lipogenic enzyme activities and cellular characteristics, the statistical model included generation type and depot site as main effects and generation type x depot site as interaction. The model for 14C-labeled acetate incorporation into lipid in- cluded generation type, depot site and culture period as the main effects and the generation type X depot site, generation type x culture period, depot site x culture period, and generation type x depot site x culture period interactions. The residual mean square was used as the error term. If an interaction was not significant at P < .05, then it was removed from the model and the new residual mean square was used as the error term. For significant F tests ( P < .05), mean separations were performed using Tukey's test (Steel and Torrie, 1980).
Results and Discussion
Birth Traits. Age of dam significantly influenced gestation length and birth weight, whereas breed of dam affected gestation length, birth weight cannon bone length, and heart girth circumference (data not shown in tabular form). As expected, 1980s-type bulls produced heavier calves with longer cannon bones and greater heart girth circumferences ( P < .05; Table 1). Males calves were heavier ( P < .05) than female calves.
The difference in size of offspring from different generation sires represented only one-half of the genetic difference between generations of Angus bulls, because all progeny had similar dams. Cundiff et al. (1993) evaluated calves from bulls born from 1968 to 1970, and calves from bulls born from 1982 to 1984 (all mated to A n g u s and Hereford cows). Similar to the results of this investigation, the newer generation bulls produced calves that were 2.7 kg heavier a t birth. Earlier, the American Angus Association ( 1990) reported an increase of 1.04 kg in birth weight
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Table 1. Least squares means and standard errors for birth, weaning, and slaughter traits for all animals In = 74)
Group Significant
Timekrait 1960s 1980s Male Female effectsa
Birth Weight, kg 32.3 f 1.2 37.0 f 1.2 36.3 f 1.3 33.2 f 1.1 G, S Cannon bone, cm 27.2 f .36 28.7 Ir .36 28.2 f .38 27.9 f .33 G Heart girth, cm 71.6 f 1.0 74.9 i 1.1 73.9 f 1.2 72.4 f 1.0 G
Weight, kg 198.9 f 8.2 221.7 k 8.2 222.5 f 8.9 198.1 f 8.0 G, S Hip height, cm 104.4 f 1.2 110.7 f 1.3 108.2 f 1.4 106.7 f 1.2 G Hip width, cm 12.7 k .2 13.2 f .2 13.2 f .2 12.7 f .2 G , S Body condition score 5.3 k .l 5.1 f . l 5.1 f . l 5.2 Ir . l NS
Weight, kg 425.0 f 37.3 498.2 f 37.1 512.7 f 37.6 410.7 f 46.9 G , S Hip height, cm 117.5 f 2.2 127.9 f 2.2 127.3 f 2.2 118.0 f 2.8 G , S Hip width, cm 18.7 f .4 19.1 f .4 19.0 f .7 18.8 f .5 NS Body condition score 6.6 f .6 6.0 f .6 6.3 f .6 6.4 f .7 G
Weaning
Slaughter
aSignificant generation ( G ) or sex (S) effects ( P < ,051. NS = no significant effects.
expected progeny differences ( E P D ) for Angus cattle born from 1972 to 1989.
Growth Traits. Age of dam significantly affected weaning weight, hip height, hip width, body condition score, and hip height at weaning. Also, breed of dam significantly affected weight, hip height, and hip width at slaughter. Simmental cows produced wean- ling calves that were 27.1 kg heavier, 5.5 cm taller, and 2.3 cm wider at the hip, than calves produced from Hereford cows. At slaughter, offspring of Sim- mental cows were 49.5 kg heavier, 10.4 cm taller, and had greater body condition scores than those from Hereford cows (data not shown in tabular form).
Steers and heifers sired by 1980s-type bulls were heavier, and had greater hip heights and hip widths at weaning; at slaughter, the 1980s-sired animals were heavier, with greater hip heights and body condition scores ( P < .05; Table 1). As anticipated, steers were heavier a t weaning and slaughter, had greater hip width at weaning, and had taller hip heights at
slaughter ( P < .05; Table 1 1. The differences between sire generations in weaning and slaughter weights (22.8 and 73.1 kg, respectively) are somewhat more than those reported by Cundiff et al. ( 1993) ( 13 and 40 kg, respectively). The American Angus Association ( 1990) reported an even smaller, 8.8 kg increase in weaning weight EPD for Angus cattle born from 1972 to 1988. The sires used in the current investigation represented a larger range in years of birth, which would explain the greater differences in growth traits observed in this study than in the earlier reports.
Carcass Characteristics. Simmental dams produced calves with 33.4 kg heavier carcasses, and with 8.9 cm2 more longissimus muscle area than calves from Hereford dams ( P < .05; data not shown). Marbling scores for calves from Simmental and Hereford dams were Slight74 and Smallz2, respectively ( P < .05). As anticipated, male offspring had significantly heavier carcasses and lower marbling scores than females (Table 2).
Table 2. Least squares means and standard errors for carcass traits for all animals (n = 74)
Group Significant
Timehai t 1960s 1980s Male Female effectsa
Carcass wt, kg Overall maturityb Fat thickness, cm Adjusted fat thickness, cm Longissimus muscle area, cm2 Kidney, pelvic, and heart fat, % USDA yield grade Marbling scored
~~~
269.3 f 9.4 140.9 f 3.0
1.17 k . l0 1.24 f . l0
78.7 k 2.4 2.46 k .20 2.6 k .2
410 f 22
314.7 Ir 9.3 140.3 f 3.2
1.24 f . l0 1.32 Ir . l0
2.61 k .20 3.0 f .2
81.3 f 2.4
386 Ir 21
312.1 f 10.5 138.1 f 4.7
1.14 Ir . l 3 1.19 f . l 0
82.6 f 2.7 2.57 f .23 2.8 f .2
365 f 24
271.9 k 9.1 143.1 k 2.8
1.27 f . l 0 1.37 k . l0
77.4 f 2.3 2.50 f .20 2.8 f .2
432 f 21
G, S NS NS NS S NS NSC S
aSignificant generation ( G ) or sex ( S ) effects (P < .05). NS = no significant effects bMaturity: 100-199 = A maturity. CThe generation effect was < . l0 for USDA yield grade. dMarbling score: 300-399 = Slight, 400-499 = Small.
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Only carcass weight was different ( P < .05) between generation types (Table 2). Although longis- simus muscle area was numerically greater for the 1980s-type offspring, longissimus muscle area per unit of carcass weight was greater for the older generation- type cattle; for this reason, USDA yield grade tended ( P < . l o ) to be greater for the 1980s-type cattle. Because cattle from both generation types were slaughtered at the same age, carcass maturity was the same ( P > . l o ) between generation types.
Cundiff et al. (1993) reported that carcass weight was 25.4 kg heavier, fat thickness at the 12th rib was 1 mm greater, and longissimus muscle area was .3 cm2 larger for calves sired by current generation Angus and Hereford bulls than for calves from older generation bulls. Additionally, marbling score was significantly lower in the newer generation calves
vs Small3I; Cundiff et al., 1993). The results of current investigation are similar to those of the American Angus Association ( 1991 j , which reported that carcass weight EPD increased, longissimus mus- cle area EPD increased slightly, and marbling score EPD remained relatively constant for Angus sires born from 1970 to 1988.
Carcass data also are presented for the subset of animals from which adipose tissues were obtained (Table 3). Subcutaneous fat thickness did not differ ( P > .05) between generation type groups (Table 3). However, there also was no difference in age at slaughter, indicating that the 1960s- and 1980s-type steers had achieved the same S.C. fat thickness at the same rate. As seen for the full group of steers and heifers, the larger framed 1980s-type steers produced heavier ( P < .05) car- casses. For this subset of steers, the 1980s-type cattle produced carcasses with numerically lower longissi- mus muscle cross sectional areas, which resulted in higher ( P < .05 j USDA yield grades than the carcasses from the 1960s-type steers. Other carcass
grade traits did not differ ( P > .05), most notably marbling score. Additionally, all carcasses were “ A maturity (data not shown). These results indicate that our preconception that 1960s-type steers would produce greater marbling scores than 1980s-type steers was incorrect. These data are in agreement with the data for the complete set of cattle, from which these steers are a subset. Also, these results are consistent with the National Beef Quality Audit (Lorenzen et al., 1993) which showed that, with the exception of carcass weight, few carcass traits have changed between 1974 and 1991.
Adipose Tissue Cellularity. Whereas the number of adipocytes per gram of tissue was not significantly different between depot sites, S.C. adipocytes exhibited greater ( P < .05 j mean diameters and mean volumes than i.m. adipocytes (Table 4). Similar results have been reported previously (Cianzio et al., 1985; Schiavetta et al., 1990; Miller et al., 1991). It has been demonstrated consistently that i.m. adipose tissue possesses more cells per gram and(orj a smaller mean cell volume than S.C. adipose tissue. Based on similar adipocyte size and lesser rates of lipogenesis, we and others (Hood and Allen, 1973) have concluded previously that i.m. adipose tissue is a less mature depot than S.C. adipose tissue at the time of slaughter in conventional management regimens.
There were more ( P < .05) cells per gram of tissue for adipose tissues from the 1980s-type steers than for the 1960s-type steers (Table 4). This corresponded t o smaller ( P < .05) adipocytes in both i.m. and S.C.
adipose tissues of the 1980s-type steers. Because both generation types exhibited the same marbling scores and fat thicknesses, this would suggest a greater number of S.C. and i.m. adipocytes in the adipose tissues of 1980s-type steers. Thus, 1980s-type cattle achieved the same gross measures of fatness (sub- cutaneous fat thickness and marbling scores) with smaller adipocytes.
Table 3. Carcass traits of crossbred steers produced by different generations of Angus sires for the subsample used for the measurements
of adiposity and lipogenesis (n = 12)
Sire peneration
Trait 1960s 1980s SEM
n Age a t slaughter, d Fat thickness, cm Adjusted fat thickness, cm Longissimus muscle area, 02 Kidney, pelvic, and heart fat, LT0 Hot carcass wt, kg USDA yield grade Marbling scorea Live wt, kg
6 447b
1.22b 1.25b
2.83b
2.7b
9O.Bb
342b
362b 532b
6 45Bb
1.21’ 1.27b
2.41’
3.1‘
87.2’
381‘
33Bb 593‘
-
5 .09 .09
.23
.2
3.9
15
22 21
aMarbling score: 300-399 = Slight. b,cMeans within the same row lacking a common superscript differ ( P < .05). SEM is the pooled
standard error of the mean.
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Table 4. Cellularity of subcutaneous and intramuscular adipose tissue from crossbred steers produced by different generations of Angus sires
Item
~
Fat depovsire generation
Subcutaneous Intramuscular Significant
1960s 1980s 1960s 1980s SEM effectsa - Cells per gram, x ~ o - ~ 15.8 20.9 16.3 23.4 2.04 G Mean diameter, pm 60.5 58.1 54.2 45.6 3.41 G, D Mean volume, pL 422 407 274 185 37.2 G, D
aG = Sire generation effect ( P < ,051; D = fat depot effect ( P < ,051. SEM is the pooled standard error of the mean.
Hood and Allen ( 1975) examined the influence of breed (beef versus dairy) on cellularity of Hereford x Angus and Holstein steers, and concluded that fewer and smaller adipocytes were present in adipose tissue from Holstein steers than in Hereford x Angus steers. This difference resulted in less subcutaneous fat accumulation for the Holstein steers. We have found similar results. Miller et al. (19911, in a comparison of the cellularity of adipose tissue from Angus and Santa Gertrudis steers, demonstrated that i.m. adi- pose tissue from Santa Gertrudis steers possessed small cells, less mean cell volume and more cells per gram than intramuscular adipose tissue from Angus steers. These differences in cellularity corresponded to a lower mean marbling score for carcasses from Santa Gertrudis steers than those from the Angus steers.
In studies with pig breeds, Hood and Allen ( 1977) found that during the growth phase, differences in animal fatness between lean and fat breeds of pigs were due to cellular hypertrophy. This conclusion was reached because the leaner breed of pigs possessed more cells per gram than the fatter breed. By slaughtering the steers in the present investigation at a constant age, with consequently the same fat thickness, any differences in cellularity due to body condition or physiological maturity of the animals should have been eliminated. Correspondingly, there were no differences in marbling scores or S.C. fat thickness between the 1960s- and 1980s-type cattle. However, the 1980s-type cattle had more cells per
gram in S.C. and i.m. adipose tissue, hence more adipocytes in these depots. The finding that 1980s-type steers had smaller adipocytes suggests that their i.m. and S.C. adipose tissues were less mature than those of the 1960s-type steers, in spite of the similarity in physiological maturity between generation types.
Lipogenic Enzymes. Of the lipogenic enzymes meas- ured, only 6-phosphogluconate dehydrogenase activity was significantly different between generation types. No significant differences in enzyme activities were observed between S.C. and i.m. adipose tissues (Table 5), which was unexpected. Chakrabarty and Romans ( 19 7 2 ) documented higher activities of ATP-citrate lyase, fatty acid synthetase, and acetyl-coenzyme A carboxylase in S.C. adipose tissue than in i.m. adipose tissue. Similarly, in an investigation of lipogenesis in adipose tissues of purebred Angus steers, Smith and Crouse (1984) observed greater activities of ATP- citrate lyase, NADP-malate dehydrogenase, NADP- isocitrate dehydrogenase, glucose-6-phosphate de- hydrogenase, and 6-phosphogluconate dehydrogenase in S.C. adipose tissue than in i.m. adipose tissue. Although Miller et al. ( 199 1) found no differences in fatty acid synthetase, ATP-citrate lyase, or NADP- malate dehydrogenase between S.C. and i.m. adipose tissues, they observed the typical differences in pentose cycle reductase activities between these adi- pose tissue types. The lack of difference between i.m. and S.C. adipose tissues for lipogenic enzyme activities
Table 5. Lipogenic enzyme activities in subcutaneous and intramuscular adipose tissue from crossbred steers produced by different generations on Angus sires
we observe in some studies seems to be related to the overall lipogenic activities of the tissues. In those studies in which i.m. adipose tissue is relatively more active (i.e., displays a greater rate of acetate incorpo- ration into fatty acids in vitro), there are fewer differences between S.C. and i.m. adipose tissue in lipogenic enzyme activities.
Lipogenesis In Vitro. Acute incubations (i.e., with- out prior culture) of i.m. and S.C. adipose tissue samples produced results similar t o those we have reported previously (Smith and Crouse, 1984; Miller et al., 1991). Acetate incorporation into neutral lipids in S.C. adipose tissue was 8- to 10-fold greater in S.C. adipose tissue than in i.m. adipose tissue (Figure 1).
Corresponding to the lack of differences in enzyme activities between generation types, there were no significant differences for acetate incorporation into neutral lipids in S.C. adipose tissue for acute (0 h ) incubations or after 40 explant culture (Figure 1) . Hood and Allen ( 1975 1 compared the lipogenic activity of adipose tissue from Hereford x Angus and Holstein steers and concluded that S.C. and perirenal adipose tissue from Hereford x Angus steers had higher lipogenic enzyme activity and acetate incorpo- ration rates than adipose tissues from Holstein steers. However, the animals in that study differed considera- bly in external fat thickness.
In a comparison of large and small breed types, Scott and Prior ( 1980) noted few differences in enzyme activities between conformation types. The authors contributed the lack of difference to similarity in physiological maturity between animals used in their study. This could also explain the lack of differences seen in the present study, in which cattle not only were the same chronological age at slaughter but were of the same physiological maturity and had the same S.C. fat thickness at the 12th rib.
As seen previously (Etherton and Evock, 1986; Miller et al., 1991), acetate incorporation into lipids declined markedly after culture of S.C. adipose tissue. In the study of Miller et al. ( 199 1) we demonstrated that i.m. adipose tissue from Angus steers lost no lipogenic activity during explant culture. In fact, Miller et al. ( 199 1) demonstrated that acetate incor- poration into lipids in i.m. adipose tissue from Santa Gertrudis steers actually increased during explant culture. The i.m. adipocytes from the Santa Gertrudis steers were smaller than their counterparts from the Angus steers, indicating that smaller adipocytes exhibit better survival during extended explant cul- ture. The current study provides similar results.
Summary. Lorenzen et al. (1993) reported trends for carcass traits that occurred over a 17-yr period. Over this period, cattle were selected for larger weaning weights and heights. Whereas this has resulted in larger carcasses, it has not produced the desired numerical reductions in yield grade. The current investigation provided similar results for
ET AL.
Angus-sired calves and steers. The overall conclusion is that, although larger, contemporary cattle do not produce leaner carcasses. This interpretation may be too simplistic; although adjusted fat thickness was not different between the 1960s- and 1980s-type steers, it was a lesser proportion of total carcass weight in the newer generation steers. From that perspective, the contemporary cattle may be considered leaner. How- ever, by USDA grading standards, which balance longissimus muscle area against carcass weight and kidney, pelvic, and heart fat, the subset of 1980s-type steers used for our metabolic studies were “fatter” (i.e., had greater USDA yield grades).
The lipogenesis and cellularity data suggest an interesting phenomenon. In the process of selecting for larger, taller offspring, the cattle industry also may have selected for S.C. adipocyte hyperplasia. In our subset, the 1980s-type steers achieved the same marbling scores and S.C. fat thicknesses as 1960s-type steers with smaller adipocytes. We re- cently provided evidence for s . ~ . and i.m. preadipocyte proliferation in Wagyu crossbred and Angus purebred steers, and indicated that Wagyu adipose tissues
* 1980s S.C.
+ 1960s S.C.
-A-, 1980s i.m. -f- 1960s i.m.
-1 0 0 I O 20 30 40 50 Time in culture, h
Figure 1. Acetate incorporation into neutral lipids in S.C. and i.m. adipose tissues before (0 h) and after 40-h explant culture. Each data point is the mean of six animals per tissue and sire generation type. The pooled standard error of the mean has been affixed to the lines for S.C. adipose tissue from the 1980s-type cattle. Significant effects were observed for adipose tissue depot (P = .0001), incubation time (P = .0003), and the depot x time interaction (P = .0013). Significant effects were not observed for sire generation (P = .38), sire generation X depot (P = 381, or the sire generation X
incubation time interaction (P = .87).
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displayed greater rates of 3H-labeled thymidine incor- poration into DNA (May et al., 1994). Greater rates of preadipocyte proliferation in contemporary cattle than in older generation cattle has not yet been demon- strated.
Implications
In spite of substantial differences in the conforma- tion of the 1960s- and 1980s-type Angus x Simmental crossbred steers evaluated in this investigation, there were no differences in carcass fatness. We demon- strated differences in cellularity between s . ~ . and i.m. adipose tissue, and this resulted in greater survival of i.m. adipose tissue during an extended period of explant culture. Adipocytes from the 1980s-type steers were smaller, suggesting that they were less mature than their counterparts in the 1960s-type steers in spite of the lack of differences in physiological maturity and carcass fatness of the animals. However, the differences we noted in adiposity between the two generation types apparently were of little practical significance in dictating carcass characteristics. The results of this study corroborate those of others in that they indicate that, for this crossbreed type, genera- tional changes in carcass conformation have not resulted in a reduction of carcass fat or substantially reduced marbling ability.
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