Differences in growth, chemical body composition and visceral organ development were evaluated in three genetic stocks: Beltsville Highfat (HF) and Lowfat (LF) Duroc-Yorkshire composites and a Hampshire X Large White cross (CX). Ten sets of littermate barrows were used from each stock. One pig from each set was slaughtered at 10, 17 and 24 wk of age. After slaughter, each pig was dissected into three fractions: carcass, head and feet, viscera and blood. Backfat was measured at three locations and visceral organs were weighed separately. Each fraction was frozen, ground, sampled and analyzed in duplicate for protein, fat, water and ash. The CX pigs were heaviest at all ages and contained the most fat-free mass (FFM). The HF pigs were smallest and contained the most fat, while LF pigs tended to be intermediate. The LF pigs deposited a greater proportion of weight in head and feet and a greater proportion of total FFM in the carcass than HF and CX pigs. Estimated allometric growth coefficients for non-fat chemical components relative to empty body weight (EBWT) were lower for HF than LF and CX, which were similar. Coefficients for fat were similar among stocks yet intercepts differed widely. Relative to total FFM, water increased at a faster rate and ash a slower rate in CX pigs compared to HF and LF. Growth coefficients were calculated for internal organs relative to EBWT. Coefficients for organs of the digestive tract were not different among stocks. However, significant differences among stocks were found for heart, lung, spleen and liver that were not explained by differences in body composition.

Deterministic computer models were used to simulate the cow-calf segment of an integrated production system. Angus, Charolais, Hereford, Limousin, and Simmental breeds were included in three mating systems: pure-breeding (PB) or two- (2R) or three-breed (3R) rotational crossbreeding. Breed data were taken from the literature. Herds were evaluated over the production year. Sires represented breed averages and were available from sources outside their herds, and 100 replacement heifers were saved annually. Females in 3R had the highest average energy requirements (8,144 Mcal of ME.cow-1.yr-1) and production costs ($322.31.cow-1.yr-1), and PB females had the lowest average requirements (7,748 Mcal of ME.cow-1.yr-1) and costs ($313.2.cow-1.yr-1). Purebred systems were the least biologically and economically efficient (64.9 Mcal of ME/kg of steer equivalent, $2.35/kg of steer equivalent), respectively, and 3R systems were the most efficient (56.6 Mcal of ME/kg of steer equivalent, $1.95/kg of steer equivalent). On average, 3R systems were more efficient biologically and economically than 2R systems. However, some 2R systems were as efficient as some 3R systems. Crossbred combinations containing Angus and(or) Hereford ranked more biologically and economically efficient than other breed combinations. Conversely, British purebreds ranked more biologically efficient, whereas Continental purebreds ranked more economically efficient.

Two studies, using 128 crossbred, multiparous beef cows grazing native winter and spring rangeland, were conducted to determine effects of protein supplementation on nutrient status and subsequent calf and reproductive performance. Postparturient cows (body condition score [BC] = 3.9) were fed 1.82 kg of one of two supplements (54% CP at 490 g of CP/d) per cow on alternate days. Supplements were formulated to contain 50% (245/490 g of CP) ruminally undegradable (UD) CP (RU50) or 25% (119/490 g of CP) UD CP (RU25) and were fed until breeding (BC = 4.2). Late-calving cows (Study 1) were individually fed supplement while grazing native range. Early-calving cows (Study 2) were group-fed supplement on alternate days while grazing native range and were daily fed approximately 5.4 kg of medium-quality grass hay (10.5% CP). Late-calving cows had similar (P > .22) BW losses postpartum, whereas early-calving cows fed RU50 lost 39 kg less (P < .01) BW than did cows fed RU25. Cows that received RU25 returned to estrus sooner (P < .02), in the late-calving study, than cows fed RU50 (47 vs 56 d, respectively). The percentage of cows serviced in the first 21 d of breeding (96 and 100% for late- and early-calving studies, respectively) and pregnancy rates (88 and 86% for late- and early-calving studies, respectively) did not differ regardless of postpartum protein supplement. When adequate CP was provided in the diet for optimum function of the rumen, addition of ruminally undegraded CP decreased weight loss of mature, postpartum beef cows grazing native range.(ABSTRACT TRUNCATED AT 250 WORDS)

Results from a 10-yr study of five dam breed groups in a range beef production system were used as inputs to a simulation model to evaluate life cycle biological and economic performance. Dam breed groups were straightbred Herefords (HH), Angus-Hereford and Simmental-Hereford F1 crosses (AH and 1S1H), 75% Hereford-25% Simmental (1S3H), and 25% Hereford-75%. Simmental (3S1H). Biological measures were number of lifetime matings, calves weaned and kilograms of calf weight sold per cow exposed, and metabolizable energy (ME) consumed per kilogram of calf weight sold and per kilogram of total weight sold. Simulated economic measures were break-even steer prices, total production costs, net profit per cow exposed, and net profit for a ranch of fixed size. The HH and 3S1H dams consumed the least ME per kilogram of total weight sold yet had the highest steer break-even prices and lowest profits. The F1 dams yielded consistently higher profits than other dam breed groups. Breed group rankings were not sensitive to prices paid for hay or calf prices relative to cow prices. Regression techniques were used to estimate maternal heterosis and maternal breed substitution effects from the simulated data. Maternal heterosis effects were large and highly significant for all measures of system performance and were desirable for all traits except ME/kilogram of total weight sold. Maternal heterosis for net profit was nearly $70.cow-1.yr-1 and $20,400/yr for a ranch of fixed size (2,700 AUM range forage). Maternal breed substitution effects were generally much smaller than heterosis effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Twelve Hereford bulls were used to sire calves in each of three locations in North Carolina over 6 yr. Three bulls were bred artificially to a random one-third of the cows at each location each year. Locations represented the Mountain, Piedmont and Coastal Plain regions of the Southeast. As yearlings, steer progeny were fed a concentrate diet in a feedlot or grazed on pasture and then slaughtered in the fall. Performance records from 816 calves were used to evaluate sire X location interaction effects for birth weight, preweaning average daily gain and weaning weight. Performance records from 355 steers were used to investigate sire X location and sire X diet interactions for average daily gain, carcass weight and percentage fat in the rib section. For all traits, sire X location and sire X diet interactions were not significant. Estimates of genetic correlations of sire progeny performance across environments ranged from .50 to 1.25. However, when estimates of sire variances within each environment were used to adjust the genetic correlations for bias due to scale effects, all genetic correlations were greater than .90. These results suggest that sires rank similarly for breeding values across the range of locations and diets used in this study.