Using real-time ultrasound and carcass measurements to estimate total internal fat in beef cattle over different breed types and managements

The objective of this study was to re-evaluate our previously published technique of estimating total physically separable internal fat (IFAT) in beef cattle using real-time ultrasound (RTU) and carcass measurements from live animals by including more breed types and genders under different management scenarios. We expanded the original database and performed additional analyses. The database was gathered from 4 studies and contained 110 animals (16 bulls, 16 heifers, and 78 steers), being Angus (n = 56), Angus× 5/8 Angus × 3/8 Nellore (n = 18), and Angus crossbreds (n = 36). Ultrasound measurements were obtained 7 d before slaughter, including the 12th to 13th rib fat thickness (uBF) and ultrasound kidney fat depth (uKFd). The uKFd was measured in a cross-sectional image collected between the first lumbar and 13th rib as previously published. Carcass data were collected 48 h post-mortem and consisted of backfat thickness (cBF), kidney fat depth (cKFd) and KPH weight, live BW, and HCW. Whole gastrointestinal tracts were removed and dissected to obtain IFAT weights. Weight of IFAT was highly correlated with KPH weight (0.88) and cKFd (0.81) and moderately correlated with uKFd (0.71). Prediction equations were developed for estimating IFAT, KPH weight, and cKFd with the PROC REG of SAS using the stepwise statement. The best predictors of IFAT were KPH weight or cKFd and cBF (r(2) = 0.84 and 0.83 and root mean square errors (RMSE) of 4.23 and 4.33 kg, respectively). Ultrasound measurements of uKFd and uBF had an r(2) of 0.65 and RMSE of 6.07 kg when both were used to predict IFAT. The results of cross-validation analyses indicated that equations developed either with KPH weight or cKFd weight and cBF had greater precision than the equation developed with uKFd and uBF. Most of the errors associated with the mean square error of prediction were due to random, uncontrolled variation. These results were consistent with previously published evaluation of this technique. These findings confirm that this RTU technique allows the measurement of IFAT in a non-invasive way that may improve our ability to estimate IFAT in beef cattle, be used to more accurately formulate rations, and be applied in sorting cattle at feedyard.     

Genetic parameters for ultrasound and carcass measures of yield and quality among replacement and slaughter beef cattle.

Real time ultrasound (RTU) measures of longissimus muscle area and fat depth were taken at 12 and 14 mo of age on composite bulls (n = 404) and heifers (n = 514). Carcass longissimus muscle area and fat depth, hot carcass weight, estimated percentage lean yield, marbling score, Warner-Bratzler shear force, and 7-rib dissectable seam fat and lean percentages were measured on steers (n = 235). Additive genetic variances for longissimus muscle area were 76 and 77% larger in bulls at 12 and 14 mo than the corresponding estimates for heifers. Heritability estimates for longissimus muscle area were 0.61 and 0.52 in bulls and 0.49 and 0.47 in heifers at 12 and 14 mo, respectively. The genetic correlations of longissimus muscle area of bulls vs heifers were 0.61 and 0.84 at 12 and 14 mo, respectively. Genetic correlations of longissimus muscle area measured in steer carcasses were 0.71 and 0.67 with the longissimus muscle areas in bulls and heifers at 12 mo and 0.73 and 0.79 at 14 mo. Heritability estimates for fat depth were 0.50 and 0.35 in bulls and 0.44 and 0.49 in heifers at 12 and 14 mo, respectively. The genetic correlation of fat depth in bulls vs heifers at 12 mo was 0.65 and was 0.49 at 14 mo. Genetic correlations of fat depth measured in bulls at 12 and 14 mo with fat depth measured in steers at slaughter were 0.23 and 0.21, and the corresponding correlations of between heifers and steers were 0.66 and 0.86, respectively. Live weights at 12 and 14 mo were genetically equivalent (r(g) = 0.98). Genetic correlations between live weights of bulls and heifers with hot carcass weight of the steers were also high (r(g) > 0.80). Longissimus muscle area measured using RTU was positively correlated with carcass measures of longissimus muscle area, estimated percentage lean yield, and percentage lean in a 7-rib section from steers. Measures of backfat obtained using RTU were positively correlated with fat depth and dissectable seam fat from the 7-rib section of steer carcasses. Genetic correlations between measures of backfat obtained using RTU and marbling were negative but low. These results indicate that longissimus muscle area and backfat may be under sufficiently different genetic control in bulls vs heifers to warrant being treated as separate traits in genetic evaluation models. Further, traits measured using RTU in potential replacement bulls and heifers at 12 and 14 mo of age may be considered different from the corresponding carcass traits of steers.     

Genetic parameters by son-sire covariances for growth and carcass traits of Hereford bulls in a nonselected herd

Growth rates and weights at weaning, 365 d, and at slaughter were obtained on 616 bulls in a nonselected Hereford herd over a 10-yr period beginning in 1978. Carcass data were obtained for 401 of these bulls at 16 mo of age and on 101 that were sires or alternates and slaughtered at 30 mo of age. Fifty-five bulls slaughtered at 30 mo of age sired 301 male offspring on which growth data were obtained and 30 sired 169 male offspring on which carcass data were obtained. Bulls gained an average of .75 kg/d preweaning and 1.16 kg/d postweaning on a 168-d feed test. Rate of daily gain from the end of feed test to slaughter ranged from .7 to 1.2 kg/d. Time from the end of the feed test to slaughter ranged from 48 to 140 d. Slaughter weight, marbling score (Small = 12, Traces = 6), longissimus muscle area, fat covering over the 12th rib, percentage of kidney, pelvic and heart fat (KPH), and dressing percentage for bulls slaughtered as yearlings were 470 kg, 7.6 score, 82.5 cm, 8.2 mm, 1.0%, and 58.8%, respectively. The 30-mo-old bulls were slaughtered directly from range pastures. Marbling was devoid or practically devoid and fat covering over the 12th rib and KPH fat were insufficient to measure or estimate accurately. Sufficient variation was not available for statistical analyses of these traits. Slaughter weight, longissimus muscle area, and dressing percentage of 30-mo-old bulls were 583 kg, 91.8 cm, and 54.0%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Leptin as a predictor of carcass composition in beef cattle

Our objective was to determine if serum concentrations of leptin could be used to predict carcass composition and merit in feedlot finished cattle. Two different groups of crossbred Bos taurus steers and heifers were managed under feedlot conditions near Miles City, MT. The first group consisted of 88 1/2 Red Angus, 1/4 Charolais, and 1/4 Tarentaise composite gene combination steers (CGC) harvested at the ConAgra processing facility in Greeley, CO. The second group (Lean Beef Project; LB) consisted of 91 F2 steers and heifers born to Limousin, Hereford, or Piedmontese by CGC F1 cows crossed to F1 bulls of similar breed composition and harvested at a local processing facility in Miles City, MT. Blood samples were collected approximately 24 h before harvest (CGC) or approximately 3 d before and at harvest (LB). No differences in serum concentrations of leptin were detected (P > 0.10) between Hereford, Limousin, or Piedmontese F2 calves nor between LB steers and heifers. Positive correlations (P < 0.01) existed between serum leptin and marbling score (r = 0.35 and 0.50), fat depth measured between the 12th and 13th rib (r = 0.34 and 0.46), kidney, pelvic, and heart fat (KPH) (r = 0.42 and 0.46), and quality grade (r = 0.36 and 0.49) in CGC and LB cattle, respectively. Serum leptin was also positively correlated with calculated yield grade for CGC steers (r = 0. 19; P = 0. 10) and LB cattle (r = 0.52; P < 0.01). Longissimus area was not correlated with serum leptin in CGC steers (r = 0.12; P > 0.10). However, a negative correlation existed between longissimus area and serum leptin in the LB cattle (r = -0.45; P < 0.01). Serum concentrations of leptin were significantly associated with carcass composition (marbling, back fat depth, and KPH fat) and quality grade in both groups of cattle studied and may provide an additional indicator of fat content in feedlot cattle.     

Predicting carcass composition of beef cattle using ultrasound technology.

We evaluated 20 slaughtered cattle with ultrasound before hide removal to predict fat thickness and ribeye area at the 12th rib for possible use in carcass composition prediction. Carcasses were fabricated into boneless subprimals that were trimmed progressively from 2.54 to 1.27 to .64 cm maximum fat trim levels. Stepwise regression was used to indicate the relative importance of variables in a model designed to estimate the percentage of boneless subprimals from the carcass at different external fat trim levels. Variables included those obtained on the slaughter floor (ultrasound fat thickness and ribeye area; estimated percentage of kidney, pelvic, and heart [KPH] fat; and warm carcass weight) and those obtained from carcasses following 24 h in the chill cooler (actual fat thickness, actual ribeye area, estimated percentage of KPH fat, warm carcass weight, and marbling score). At all different subprimal trim levels, percentage KPH was the first variable to enter the model. In the models using measures taken on the slaughter floor, ultrasound fat thickness was the only other variable to enter the model. Ultrasound fat thickness increased R2 and decreased residual standard deviation (RSD) in models predicting subprimals at 2.54-cm maximum fat trim; however, at 1.27- and .64-cm trim levels, R2 and RSD increased. Models using the same two variables (except actual fat instead of ultrasound) in the cooler were similar to those using data from the slaughter floor. However, as more cooler measurement variables entered the models, R2 increased and RSD decreased, explaining a greater amount of the variation in the equation. Ultrasonic evaluation on the slaughter floor may be of limited application compared with the greater accuracy found in chilled carcass assessment.     

Serum hormone concentrations relative to carcass composition of a random allotment of commercial-fed beef cattle

Cattle (n = 995 steers and 757 heifers) were randomly selected from a commercial abattoir (Emporia, KS) to determine the relationships between USDA quality and yield grade characteristics and serum concentrations of leptin, IGF-I, and GH. Animals were randomly selected postexsanguination on the slaughter line on 4 occasions (March, May, August, and January). Blood was collected at exsanguination and transported to the University of Missouri for analysis. Sex and hide color were recorded. Carcass data included HCW, 12th-rib fat thickness, KPH, LM area, and marbling score, which were collected from each carcass approximately 24 h postmortem. Average serum leptin concentrations were greater (P = 0.008) for heifers (11.9 ng/mL) than steers (10.9 ng/mL). Heifers had lighter carcasses (331.9 vs. 352.2 kg, P < 0.001), greater 12th-rib fat measurements (1.3 vs. 1.1 cm, P < 0.001), greater KPH (2.5 vs. 2.4%, P < 0.001), and more marbling (Small(40) vs. Small(10), P < 0.001) than steers. Positive correlations (P < 0.01) existed between leptin concentration and marbling score (r = 0.28), 12th-rib fat depth (r = 0.37), KPH (r = 0.23), and USDA yield grade (r = 0.32). Negative correlations were found between leptin and IGF-I (r = -0.11; P < 0.001) and leptin and GH (r = -0.32; P < 0.001). Negative correlations (P < 0.01) were observed for IGF-I and KPH (r = -0.23) and marbling score (r = -0.20), whereas GH was most highly negatively correlated with KPH (r = -0.23; P < 0.001). Leptin concentration accounted for variation (P < 0.001) in a model separating least squares means across USDA quality grade, separating USDA standard (8.5 ng/mL), select (10.3 ng/mL), low choice (12.2 ng/mL), and upper 2/3 choice/prime (>12.9 ng/mL) carcasses. There was no difference (P = 0.31) observed in leptin concentrations between the upper 2/3 choice and prime carcasses (12.9 and 14.2 ng/mL, respectively). Relationships within endocrine profiles and between endocrine concentrations and carcass quality characteristics may prove to be a useful tool for the prediction of beef carcass composition.     

Factors influencing intermuscular fat and other measures of beef chuck composition.

Carcasses from 59 steers produced from the mating of Braford, Simbrah, Senepol, and Simmental bulls to Brahman- and Romana Red-sired cows and Brahman bulls mated to Angus cows were used in this study. Effects of sire breed and feeding calves vs yearlings on fat depots in the chuck, when steers were fed to 1.0 cm external fat, were determined. Breed of sire and feeding calves vs yearlings had no effect (P greater than .05) on percentage of intermuscular fat. However, carcasses from Braford-sired steers had a higher (P less than .05) percentage of dissectable subcutaneous fat on the chuck than did those from other breed groups. Carcasses from Simmental-sired steers were superior (P less than .05) to those from Braford-sired steers in USDA yield grade and had a higher average marbling score (P less than .05) than the Simbrah-sired group. Estimated kidney, pelvic, and heart (KPH) fat was higher (P less than .05) in carcasses from Brahman-, Simbrah-, and Senepol-sired steers than in Braford-sired steers. Steers fed as calves had higher percentages (P less than .05) of KPH fat and major chuck muscles than did those fed as yearlings. The best single predictor of percentage of intermuscular fat within the chuck was adjusted fat over the ribeye (R2 = .46).     

Evaluation of ultrasound for prediction of carcass fat thickness and longissimus muscle area in feedlot steers.

Four hundred fifty-two yearling steers from two experiments were measured for subcutaneous fat thickness and longissimus muscle area between the 12th and 13th ribs using real-time linear array ultrasound equipment. Ultrasonic predictions were compared to corresponding carcass measurements to determine accuracy of ultrasound measurements. In Exp. 1, 74% of the ultrasonic estimates of fat thickness were within 2.54 mm of carcass values (r = .81) and muscle area was predicted within 6.45 cm2 for 47% of all carcasses (r = .43). Although similar correlation coefficients between ultrasonic and carcass fat thickness were obtained in Exp. 2 (r = .82), estimates were more biased; only 62% of ultrasound estimates were within 2.54 mm of carcass measurements. Improvement in longissimus muscle area estimates was noted in Exp. 2, in which 54% of ultrasonic estimates were within 6.45 cm2 of carcass values (r = .63). The extremes for each trait proved most difficult to predict; fat thickness was underestimated on fatter cattle and muscle area was underpredicted on more heavily muscled steers. Ultrasonic measurements of fat thickness are precise and accurate in determining carcass fat thickness, but muscle area estimates are inconsistent and warrant further investigation.     

Residual feed intake of purebred Angus steers: effects on meat quality and palatability

Relationships between residual feed intake (RFI) and other performance variables were determined using 54 purebred Angus steers. Individual feed intake and BW gain were recorded during a 70-d post-weaning period to calculate RFI. After the 70-d post-weaning test, steers were fed a finishing ration to a similar fat thickness (FT), transported to a commercial facility, and slaughtered. A subsample of carcasses (n = 32) was selected to examine the relationships among RFI, meat quality, and palatability. Steers were categorized into high (> 0.5 SD above the mean; n = 16), medium (mid; +/- 0.5 SD from the mean; n = 21), and low (< 0.5 SD below the mean; n = 17) RFI groups. No differences were detected in ADG, initial BW, and d 71 BW among the high, mid, and low RFI steers. Steers from the high RFI group had a greater DMI (P = 0.004) and feed conversion ratio (FCR; DMI:ADG; P = 0.002) compared with the low RFI steers. Residual feed intake was positively correlated with DMI (r = 0.54; P = 0.003) and FCR (r = 0.42; P = 0.002), but not with initial BW, d 71 BW, d 71 ultrasound FT, initial ultrasound LM area, d 71 ultrasound LM area, or ADG. The FCR was positively correlated with initial BW (r = 0.46; P = 0.0005), d 71 BW (r = 0.34; P = 0.01), and DMI (r = 0.40; P = 0.003) and was negatively correlated with ADG (r = -0.65; P = 0.001). There were no differences among RFI groups for HCW, LM area, FT, KPH, USDA yield grade, marbling score, or quality grade. Reflectance color b* scores of steaks from high RFI steers were greater (P = 0.02) than those from low RFI steers. There was no difference between high and low RFI groups for LM calpastatin activity. Warner-Bratzler shear force and sensory panel tenderness and flavor scores of steaks were similar across RFI groups. Steaks from high RFI steers had lower (P = 0.04) off-flavor scores than those from low RFI steers. Cook loss percentages were greater (P = 0.005) for steaks from low RFI steers than for those from mid RFI steers. These data support current views that RFI is independent of ADG, but is correlated with DMI and FCR. Importantly, the data also support the hypothesis that there is no relationship between RFI and beef quality in purebred Angus steers.     

Weaning,yearling, and preharvest ultrasound measures of fat and muscle area in steers,bulls, and heifers

Longissimus muscle area and fat thickness were measured following weaning, at yearling, and prior to harvest using real-time ultrasound, and corresponding carcass measurements were recorded 3 to 7 d following the preharvest scan in composite steers (n = 116, 447 +/- 19 d), bulls (n = 224, 521 +/- 11 d), and heifers (n = 257,532 +/- 12 d). Although fat deposition was limited in bulls and heifers from weaning to yearling, coefficients of variation ranged from 8.46 to 13.46% for muscle area, and from 27.55 to 38.95% for fat thickness, indicating that significant phenotypic variance exists across genders. Residual correlations, adjusted for the effects of year of birth, gender, and age at measurement, were high and ranged from 0.79 to 0.87 among ultrasound and carcass measures of muscle area. Residual correlations among ultrasound and carcass measures of fat thickness were also high, ranging from 0.64 to 0.86. Weaning and/or yearling ultrasound muscle area yielded similarly accurate predictions of carcass muscle area. Yearling ultrasound fat thickness accounted for 13% more of the observed variance in carcass fat thickness than the weaning ultrasound measure in single-trait prediction models. When both weaning and yearling ultrasound measures were used to predict carcass fat thickness, partial R2 values were 0.15 and 0.61 for weaning and yearling ultrasound fat thickness, respectively. The difference between predicted and carcass measures with respect to muscle area (fat thickness) was less than 6.45 cm2 (2.5 mm) for 80.2 to 88.9% (90.3 to 95%) of animals. Preharvest ultrasound measures yielded standard errors of prediction of less than 4.95 cm2 for muscle area and 1.51 mm or less for fat thickness. These results indicate that ultrasound measures taken between weaning and yearling provide accurate predictors of corresponding carcass traits in steers, bulls, and heifers.     

In vivo estimation of goat carcass composition and body fat partition by real-time ultrasonography

The accuracy of ultrasound measurements to assess goat carcass composition and the partition of body fat depots was evaluated. An ultrasound machine with a 5-MHz probe and image analysis was used to assess in vivo fat thickness and muscle depth in 56 Spanish Celtiberica adult goats, in lumbar and breast body regions. The goats were slaughtered and the weight of body fat depots recorded. Measurements corresponding to the in vivo ultrasound fat thickness and muscle depth were taken on carcasses. The left sides of carcasses were completely dissected into their components. The best relationships (r = 0.94, P < 0.01) between in vivo and carcass measurements of fat thickness were obtained when measurements were taken at the sternum, and the best anatomical point was located between the third and fourth sternebrae. The best correlation coefficients (r = 0.84) for muscle depth were found for measurements taken between the third and the fourth lumbar vertebrae at 2 cm from the middle of the vertebral column. Body weight and ultrasound measurements were used to fit the best multiple regression equations to predict carcass composition and the partition of body fat depots. All equations, with the exception of those for muscle quantity, omental, and total body fat depot amounts, were computed after performing a logarithmic transformation. Body weight in association with the ultrasound measurement taken at largest LM muscle depth, between the first and second lumbar vertebrae accounted for 90% of the muscle weight. Body weight was the first variable admitted into the prediction models of muscle, mesenteric fat, and total body fat and accounted for 82, 67, and 79% of the variation in tissue weights, respectively. The ultrasound measurement of fat thickness taken at the third sternebra was the first variable admitted into the prediction models for intermuscular fat, kidney and pelvic fat, and total carcass fat and accounted for by 73, 75, 71, and 79% of the variation in the weight of these fat depots, respectively. The ultrasound measurements taken in the breast region, particularly at the third and fourth sternebrae, were the most suitable for assessing fat thickness. The results of this experiment suggest that BW associated with some in vivo ultrasonic fat measurements allow the accurate prediction of goat carcass composition and body fat depots.     

Growth, carcass quality, and protein and energy metabolism in beef cattle with different growth potentials and residual feed intakes

Twenty-four beef steers (predominantly Angus x Hereford, 14 to 18 mo of age, 403 +/- 3 kg of BW), were housed and fed in individual pens for about 122 d. Twelve steers came from a herd that had been selected for growth (high growth; HG) and the other 12 from a herd with no selection program (low growth; LG). Another 6 steers (3 from each group) were slaughtered at the beginning to obtain the initial composition. All steers were fed the same corn-based diet (3.06 Mcal of ME/kg of DM, 13.6% CP) on an ad libitum basis. Two weeks before slaughter, total urine was collected for 5 d for estimation of 3-methylhistidine excretion and myofibrillar protein breakdown rates. Compared with LG steers, HG steers had less initial BW but greater final BW, DMI (7.52 vs. 6.37 kg/d), ADG (1.33 vs. 0.853 kg/d), G:F (0.176 vs. 0.133 kg/kg), ME intake (0.233 vs. 0.201 Mcal x kg of BW(0.75) x d(-1)), and retained energy (RE; 0.0711 vs. 0.0558 Mcal x kg of BW(0.75) x d(-1)); gained more fat (676 vs. 475 g/d); and tended to gain more whole body protein (100 vs. 72 g/d), with no difference in residual feed intake (RFI). Estimated net energetic efficiency of gain (k(g)) and ME for maintenance (ME(m)) did not differ between the 2 groups, averaging 0.62 and 0.114, respectively. The HG steers had greater HCW (350 vs. 329 kg), backfat (16.1 vs. 11.6 mm), and yield grades (3.53 vs. 2.80), with a similar dressing percent, KPH fat, LM area, and marbling score. Skeletal muscle protein gain (70.2 vs. 57.6 g/d) and fractional protein accretion rate (0.242 vs. 0.197%/d) tended to be greater in HG than in LG steers. Steers were classified into low (-0.367 kg/d) and high (0.380 kg/d) RFI classes. Compared with the high RFI steers, low RFI steers consumed less DM (6.61 vs. 7.52 kg/d) and ME (0.206 vs. 0.234 Mcal x kg of BW(0.75) x d(-1)) and tended to gain less fat (494 vs. 719 g/d), but were similar for initial and final BW, ADG, G:F, protein gain, HCW, dressing percent, backfat, KPH fat, LM area, marbling score, and yield grade, as well as for all observations related to myofibrillar protein metabolism. Residual feed intake may be positively [corrected] correlated with ME for maintenance. The maintenance energy requirement increased by 0.0166 Mcal x kg(-0.75) x d(-1) for each percentage increase in fractional protein degradation rate, confirming the importance of this process in the energy economy of the animal.     

Accuracy of predicting weight and percentage of beef carcass retail product using ultrasound and live animal measures

Five hundred thirty-four steers were evaluated over a 2-yr period to develop and validate prediction equations for estimating carcass composition from live animal ultrasound measurements and to compare these equations with those developed from carcass measurements. Within 5 d before slaughter, steers were ultrasonically measured for 12th-rib fat thickness (UFAT), longissimus area (ULMA), rump fat thickness (URPFAT), and body wall thickness (UBDWALL). Carcasses were fabricated to determine weight (KGRPRD) and percentage (PRPRD) of boneless, totally trimmed retail product. Data from steers born in Year 1 (n = 282) were used to develop prediction equations using stepwise regression. Final models using live animal variables included live weight (FWT), UFAT, ULMA, and URPFAT for KGRPRD (R2 = 0.83) and UFAT, URPFAT, ULMA, FWT, and UBDWALL for PRPRD (R2 = 0.67). Equations developed from USDA yield grade variables resulted in R2 values of 0.87 and 0.68 for KGRPRD and PRPRD, respectively. When these equations were applied to steers born in Year 2 (n = 252), correlations between values predicted from live animal models and actual carcass values were 0.92 for KGRPRD, and ranged from 0.73 to 0.76 for PRPRD. Similar correlations were found for equations developed from carcass measures (r = 0.94 for KGRPRD and 0.81 for PRPRD). Both live animal and carcass equations overestimated (P < 0.01) actual KGRPRD and PRPRD. Regression of actual values on predicted values revealed a similar fit for equations developed from live animal and carcass measures. Results indicate that composition prediction equations developed from live animal and ultrasound measurements can be useful to estimate carcass composition.     

Performance and carcass quality of steers supplemented with zinc oxide or zinc methionine

Forty-five Angus steers (avg initial wt 330 kg) were individually fed for 112 d to assess the value of supplemental Zn and source on performance and carcass quality. Steers had ad libitum access to a control diet (81 ppm Zn) of 33% whole corn, 33% ground milo, 15% cottonseed hulls and 13% cottonseed meal, or this control diet with 360 mg Zn/d added from either zinc methionine or zinc oxide. Steers were slaughtered on d 114, and carcass composition was determined by specific gravity. Average daily gain and feed efficiency were not affected by dietary treatments. Steers fed zinc methionine had a higher (P less than .05) USDA quality grade than those fed the control and zinc oxide diets. Marbling score was higher (P less than .05) for steers fed zinc methionine than for those fed control and zinc oxide treatments (4.4 vs 4.0 and 4.0, respectively, where 3 = slight, 4 = small, 5 = modest). Steers fed zinc methionine tended to have more (P less than .10) external fat (13 mm) than steers fed the control diet (10 mm); steers supplemented with zinc oxide had intermediate amounts of external fat (11 mm). Steers fed zinc methionine had 10.5 and 12.8% more (P less than .05) kidney, pelvic and heart (KPH) fat than steers fed control or zinc oxide diets, respectively. The effects of zinc methionine on carcass quality grade and marbling score may be due to Zn and (or) methionine. Regardless of the mechanism, the difference represents a potential economic benefit to producers.     

Genetic evaluation of retail product percentage in Simmental cattle.

The objective of this study was to estimate genetic parameters required for genetic evaluation of retail product percentage (RPP) in Simmental cattle. Carcass weight (HCW), subcutaneous fat thickness (FAT), longissimus muscle area (REA) and kidney, pelvic, and heart fat (KPH) records were available to compute RPP on steers (n = 5171) and heifers (n = 1400) from the American Simmental Association database; animals were sired by 561 Simmental bulls and out of 5886 crossbred dams. Genetic parameters were estimated using residual maximal likelihood and a four trait animal model for the components of RPP including fixed harvest contemporary group effects, random animal genetic effects, and a linear covariate for age at harvest. Heritability estimates were 0.51 +/- 0.05, 0.36 +/- 0.05, 0.46 +/- 0.05, and 0.18 +/- 0.05 for HCW, FAT, REA and KPH respectively. Non-zero genetic correlations were estimated between HCW and REA (rg = 0.51 +/- 0.06) and between REA and FAT (rg = -0.43 +/- 0.08), but other genetic correlation estimates among the component traits were low. As a linear function of its components, heritability and genetic correlations involving RPP were estimated using index methods. The heritability estimate for RPP was 0.41, and genetic correlations were -0.17, -0.83, 0.67, and 0.01 with HCW, FAT, REA and KPH respectively. Therefore, RPP was strongly associated with muscle and fat deposition, but essentially independent of carcass weight and internal body cavity fat. Genetic evaluation of RPP would be straightforward using multiple trait index methods and genetic regression, although the inclusion of KPH would be of marginal value.     

Effects of winter stocker growth rate and finishing system on: I. Animal performance and carcass characteristics

Angus-crossbred steers (n = 216) were used in a 3-yr study to assess the effects of winter stocker growth rate and finishing system on finishing performance and carcass characteristics. During winter months (December to April) steers were randomly allotted to 3 stocker growth rates: low (0.23 kg x d(-1)), medium (0.45 kg x d(-1)), or high (0.68 kg x d(-1)). Upon completion of the winter phase, steers were randomly allotted within each stocker treatment to a corn silage-concentrate or pasture finishing system. All steers regardless of finishing treatment were finished to an equal-time endpoint to eliminate confounding of treatments with animal age or seasonal factors. Upon completion of the finishing period, steers were slaughtered in 2 groups (one-half of pasture and one-half of feedlot cattle each time) and carcass data were collected. Winter data were analyzed as a completely randomized design, with winter treatment, pen replicate, year, and the winter x year interaction in the model. Finishing performance and carcass data were analyzed in a split-plot design with finishing system in the whole plot, and winter growth rate and winter x finish in the split-plot. Winter treatment mean within finishing replication was the experimental unit, and year was considered a random effect. Winter stocker phase treatments resulted in differences (P < 0.001) in final BW, ADG, and ultrasound LM area between all treatments for that phase. Pasture-finished cattle had lower (P < 0.001) final BW, ADG, HCW, LM area, fat thickness, KPH, dressing percent, USDA yield grade, and USDA quality grade. Winter stocker treatment influenced (P < 0.05) final BW and HCW, with low and medium being less than high. Steers with low stocker gain had greater (P < 0.05) finishing ADG. Dressing percent was greater (P < 0.001) for high than low, and USDA quality grade was greater (P < 0.05) for high than low and medium. Carcass LM area, fat thickness, KPH, and USDA yield grade were not influenced (P > 0.05) by winter rate of gain. Cattle on low during winter exhibited compensatory gain during finishing but were unable to catch the high group regarding BW or HCW. The USDA quality grade was greater for high than low or medium. Animal performance during the winter stocker period clearly impacts finishing performance, carcass quality and beef production in both pasture- and feedlot-finishing systems, when cattle were finished to an equal-time endpoint.     

Estimates of parameters between direct and maternal genetic effects for weaning weight and direct genetic effects for carcass traits in crossbred cattle

Estimates of heritabilities and genetic correlations were obtained for weaning weight records of 23,681 crossbred steers and heifers and carcass records from 4,094 crossbred steers using animal models. Carcass traits included hot carcass weight; retail product percentage; fat percentage; bone percentage; ribeye area; adjusted fat thickness; marbling score, Warner-Bratzler shear force and kidney, pelvic and heart fat percentage. Weaning weight was modeled with fixed effects of age of dam, sex, breed combination, and birth year, with calendar birth day as a covariate and random direct and maternal genetic and maternal permanent environmental effects. The models for carcass traits included fixed effects of age of dam, line, and birth year, with covariates for weaning and slaughter ages and random direct and maternal effects. Direct and maternal heritabilities for weaning weight were 0.4 +/- 0.02 and 0.19 +/- 0.02, respectively. The estimate of direct-maternal genetic correlation for weaning weight was negative (-0.18 +/- 0.08). Heritabilities for carcass traits of steers were moderate to high (0.34 to 0.60). Estimates of genetic correlations between direct genetic effects for weaning weight and carcass traits were small except with hot carcass weight (0.70), ribeye area (0.29), and adjusted fat thickness (0.26). The largest estimates of genetic correlations between maternal genetic effects for weaning weight and direct genetic effects for carcass traits were found for hot carcass weight (0.61), retail product percentage (-0.33), fat percentage (0.33), ribeye area (0.29), marbling score (0.28) and adjusted fat thickness (0.25), indicating that maternal effects for weaning weight may be correlated with genotype for propensity to fatten in steers.     

Adipose tissue growth and cellularity: changes in bovine adipocyte size and number

Forty crossbred steers of similar birth date and fed the same growing-finishing diet were used to study adipocyte changes in six fat depots during growth from 11 to 19 mo of age. Steers were slaughtered at 2-mo intervals. Adipose tissue samples were obtained from kidney, mesenteric and brisket fat and subcutaneous, intermuscular and intramuscular fat from the 10th to 12th rib section. The osmium tetroxide fixation technique was used for determination of cell size and number. Except for three brisket fat samples, distributions of adipocyte diameters from six different fat depots were monophasic during the age range considered in this study. At 17 mo of age, the mean adipocyte diameter, in decreasing order, was: kidney fat greater than mesenteric greater than subcutaneous greater than intermuscular greater than intramuscular greater than brisket fat. Fat deposition during growth to 19 mo of age occurred mainly by hypertrophy of adipocytes. An apparent cell hyperplasia occurred in the intramuscular fat depot from 11 to 15 mo and in the brisket fat depot after 15 mo of age. Based on cellularity characteristics, evidence exists to classify intramuscular and brisket fat depots as late-developing ones. Cell number/gram of intramuscular adipose tissue was a better predictor of marbling score than was fat cell diameter.     

Estimates of genetic parameters for live animal ultrasound, actual carcass data, and growth traits in beef cattle

Growth and carcass measurements were made on 2,411 Hereford steers slaughtered at a constant weight from a designed reference sire program involving 137 sires. A second data set consisted of ultrasound measures of backfat (USFAT) and longissimus muscle area (USREA) from 3,482 yearling Hereford cattle representing 441 sires. Restricted maximum likelihood procedures were used to estimate genetic parameters among carcass traits and live animal weight traits from these two separate data sets. Heritability estimates for the slaughter weight constant steer carcass backfat (FAT) and longissimus muscle area (REA) were .49 and .46, respectively. In addition, FAT had a negative genetic correlation with REA (-.37), weaning weight (-.28), and yearling weight (-.13) but positive with marbling (.19) and carcass weight (.36). Marbling was moderately heritable (.35) and highly correlated with total postweaning average daily gain (.54) and feedlot relative growth rate (.62). Heritability estimates for weight constant USFAT and USREA were .26 and .25, respectively. The genetic correlation between weight constant USFAT and USREA was positive (.39), indicating that in these young animals USFAT does not seem to be an indication of maturity. Mean USFAT measures and variability were small (.48 +/- .17 cm, n = 3,482). Results indicate that carcass fat on slaughter steers and ultrasound measures of backfat on young breeding animals may have different relationships with growth and muscling. These relationships need to be explored before wide scale selection based on ultrasound is implemented.     

Effects of growth type on carcass traits of pasture- or feedlot-developed steers.

Carcasses of 342 steers of known genetic backgrounds from four fundamentally different growth types were developed either on pasture or feedlot regimens to study differences in carcass traits. Growth types were large framed-late maturing (LL), intermediate framed-intermediate maturing (II), intermediate framed-early maturing (IE), and small framed-early maturing (SE). Five calves from each growth type were assigned to each regimen in each year of a 9-yr study. Eighteen steers were removed from the study because of accident or illness. Data collected were preslaughter shrunk BW (SBW); hot carcass weight (HCW); chilled carcass weight (CCW); dressing percentage (DRESS); fat thickness at the 12th and 13th-rib interface (FAT); percentage kidney, pelvic, and heart fat (KPH); longissimus muscle area (LMA); marbling score (MARB); quality grade (QG); and yield grade (YG). Differences in carcass traits reflected genetic differences among growth types. The LL steers had heavier BW, HCW, and CCW and larger LMA (P < .05) than steers of other growth types, regardless of development regimen. Among pasture-developed steer carcasses, IE and SE steers had higher (P < .05) MARB and QG than either LL or II steers. Carcasses of large framed-late maturing steers had the lowest (P < .05) MARB and QG of the growth types. Carcasses of the II, IE, and SE steers had a higher (P < .05) numerical value for YG than carcasses of the LL steers. Among the carcasses of the feedlot-developed steers, IE and SE steers had the highest (P < .05) MARB and QG. Carcasses from the IE and SE steers were fatter (P < .05) than those from LL or II steers. Carcasses of the LL steers had the lowest percentage of KPH of growth types developed in the feedlot. No difference was observed in KPH for carcasses of II, IE, and SE steers. The LL steer carcasses had the lowest numerical value for YG of all growth types. These data indicate that variation existed among carcass traits for the four growth types and that carcass traits influenced by fatness were greater and more attainable in the feedlot-developed steers using current methods of evaluation.