Growth in animals is defined as accretion of protein, fat and bone. Although growth typically is measured as the change in live weight, nutrient retention is estimated more precisely by measuring empty body weight and composition, whereas production economics are measured ideally through carcass weights and quality. As a percentage of live weight gain, carcass weight gain usually is a much higher percentage during the feedlot phase than during the growing phase of production because dressing percentage (ratio of carcass:live weight) increases with maturation and is greater with concentrate than with roughage diets. At a given fraction of mature body size (maximum body protein mass), body fat percentage seems to be a constant. Mature size may be altered genetically and nutritionally. Protein accretion declines to zero when cattle reach their mature body size (approximately 36% fat in empty body weight in modern cattle) even though mature animals can continue to accrete fat. Although fat accretion can be reduced by limiting the supply of net energy, rate of fat accretion by finishing steers given ad libitum access to high-concentrate diets seems to reach a plateau at approximately 550 g daily. Protein mass, in contrast, increases in proportion to empty body weight. The protein:fat ratio of the carcass can be increased through increasing mature size, by administering hormones or hormonal modifiers, by limiting energy intake during the growing period or finishing period, or by slaughtering cattle at an earlier stage of maturity. Energetically, efficiency of accretion of fat is approximately 1.7 times that of protein. But because more water is stored with deposited protein than with deposited fat, lean tissue gain is four times as efficient as accretion of fat tissue. Conversion of protein to fat is very inefficient, suggesting that excess protein is utilized inefficiently.

Objective of this study was to assess the association of SNP in the diacylglycerol O-acyltransferase 1 (DGAT1), thyroglobulin (TG), and micromolar calcium-activated neutral protease (CAPN1) genes with carcass composition and meat quality traits in Bos indicus cattle. A population of Brahman calves (n = 479) was developed in central Florida from 1996 to 2000. Traits analyzed were ADG, hip height, slaughter weight, fat thickness, HCW, marbling score, LM area, estimated KPH fat, yield grade, retail yield, sensory panel tenderness score, carcass hump height, and cooked meat tenderness measured as Warner-Bratzler shear force at 7, 14, and 21 d postmortem. Single nucleotide polymorphisms previously reported in the TG and DGAT1 genes were used as markers on chromosome 14. Two previously reported and two new SNP in the CAPN1 gene were used as markers on chromosome 29. One SNP in CAPN1 was uninformative, and another one was associated with tenderness score (P < 0.05), suggesting the presence of variation affecting meat tenderness. All three informative SNP at the CAPN1 gene were associated with hump height (P < 0.02). The TG marker was associated with fat thickness and LMA (P < 0.05), but not with marbling score. No significant associations of the SNP in the DGAT1 gene were observed for any trait. Allele frequencies of the SNP in TG and CAPN1 were different in this Brahman population than in reported allele frequencies in Bos taurus populations. The results suggest that the use of molecular marker information developed in Bos taurus populations to Bos indicus populations may require development of appropriate additional markers.

Our objective was to characterize further the acute-phase response following endotoxin (i.e. lipopolysaccharide; LPS) exposure in the bovine. Nine pure-bred Angus castrated males (i.e. steers; average body weight ¼ 299 AE 5 kg) were used in a randomized complete block design in environmentally controlled chambers, set at thermoneutral level, to characterize the acute physiological, endocrine, immune, and acute-phase protein responses following an i.v. bolus administration of 2.5 mg of LPS/kg body weight. One day before administration of LPS, all steers were fitted with an indwelling jugular vein catheter for serial blood collection. Blood samples were collected at 30-min intervals from À2 h to 8 h relative to the LPS challenge (time 0), and serum was harvested and stored at À80 C until analyzed for concentrations of cortisol, pro-inflammatory cytokines, and acute-phase proteins. Indicators of thermal status (i.e. rectal temperature, ruminal temperature, respiration rate, sweat rate, and skin temperatures) were measured at 30-min intervals from À1 h to 6 h relative to the challenge. Endotoxin exposure increased (P50.05) serum concentrations of cortisol, tumor necrosis factor-a (TNF-a), interleukin 1-b (IL-1b), IL-6, interferon-g (IFN-g), and serum amyloid A. Respiration rate, rectal temperature, and rump skin temperature also were increased (P50.05) following LPS administration. Endotoxin exposure dramatically decreased ear skin temperature (P ¼ 0.002), but tended to increase (P50.10) ruminal temperature, shoulder skin temperature, and shoulder sweat rate. Serum concentrations of acid soluble protein, a-acid glycoprotein, IL-4 and IL-2, and rump sweat rate were not altered (P40.24) by the challenge. To our knowledge, this report is the most complete characterization of the bovine acute-phase response to a bolus-dose endotoxin challenge conducted under thermoneutral conditions and should provide foundation data for future research.

Heritabilities and genetic and phenotypic correlations were estimated from feedlot and carcass data collected from Brahman calves (n = 504) in central Florida from 1996 to 2000. Data were analyzed using animal models in MTDFREML. Models included contemporary group (n = 44; groups of calves of the same sex, fed in the same pen, slaughtered on the same day) as a fixed effect and calf age in days at slaughter as a continuous variable. Estimated feedlot trait heritabilities were 0.64, 0.67, 0.47, and 0.26 for ADG, hip height at slaughter, slaughter weight, and shrink. The USDA yield grade estimated heritability was 0.71; heritabilities for component traits of yield grade, including hot carcass weight, adjusted 12th rib backfat thickness, loin muscle area, and percentage kidney, pelvic, and heart fat were 0.55, 0.63, 0.44, and 0.46, respectively. Heritability estimates for dressing percentage, marbling score, USDA quality grade, cutability, retail yield, and carcass hump height were 0.77, 0.44, 0.47, 0.71, 0.5, and 0.54, respectively. Estimated genetic correlations of adjusted 12th rib backfat thickness with ADG, slaughter weight, marbling score, percentage kidney, pelvic, and heart fat, and yield grade (0.49, 0.46, 0.56, 0.63, and 0.93, respectively) were generally larger than most literature estimates. Estimated genetic correlations of marbling score with ADG, percentage shrink, loin muscle area, percentage kidney, pelvic, and heart fat, USDA yield grade, cutability, retail yield, and carcass hump height were 0.28, 0.49, 0.44, 0.27, 0.45, -0.43, 0.27, and 0.43, respectively. Results indicate that sufficient genetic variation exists within the Brahman breed for design and implementation of effective selection programs for important carcass quality and yield traits.

A study was performed to evaluate differences in thermoregulatory ability of 2 Bos taurus breeds with known differences in heat tolerance. Nine Angus (AG; 304 +/- 7 kg of BW) and 9 Romosinuano (RO; 285 +/- 7.5 kg of BW) steers were transported to the Brody Environmental Center at the University of Missouri. Steers were housed for 18 d at thermoneutrality (TN; 21 degrees C) before initiation of heat stress (HS), which consisted of daily cyclic air temperature (26 degrees C, night; 36 degrees C, day) for 14 d. Rectal temperature and respiration rate were measured 6 times daily throughout the study. Sweat rates at shaved skin sites were recorded on specific days. Blood samples were taken once per week. Angus steers maintained rectal temperature 0.5 degrees C greater than RO at TN (P < 0.001). Likewise, respiration and sweat rates were greater (P < 0.001) in AG than RO at TN (P < 0.05). Rectal temperature increased during HS for both breeds with AG maintaining greater temperatures (P < 0.001). Both breeds increased respiration rate during HS, with AG steers exhibiting the greater rate (P < 0.001). Sweat rate increased more than 4-fold during HS (P < 0.001), followed by reduction after 7 d. Even after HS acclimation, AG exhibited the greater sweat rate (P < 0.001). Breed differences for serum leptin, creatinine, and cholesterol were found throughout the study with AG being greater than RO. Although there were no breed differences (P = 0.21) at TN, only AG steers exhibited a HS-induced increase (P < 0.05) in prolactin, creatinine, and cholesterol concentrations to suggest that an increase in rectal temperature is required for this effect. Use of rectal temperature along with endocrine markers, such as prolactin, may aid in the identification of B. taurus sensitivity to heat.