Influence of dietary energy source on in vitro substrate utilization and insulin sensitivity of muscle and adipose tissue of beef cattle

Beef carcass value is influenced by the quantity and distribution of adipose tissue. Elucidation of metabolic controls of caloric partitioning between adipose depots could lead to development of production solutions that enhance beef carcass value. Historical trends in Choice and Select beef supply and short-term demand structures for Choice and Select boxed beef were explored. Recent stabilization in slaughter mix may suggest an optimum is being approached. Evaluation of short-run demand supports this premise, and suggests that Choice and Select products may not be strong substitutes. Growth-based prediction equations relating carcass traits to growth traits using ultrasound measurements as the basis of projections under different growing systems were explored. Accuracy of carcass fat predictions from growth-based equations is influenced by weight gain between ultrasound and endpoint, breed, and gender; scans out to 120 d pre-harvest may be accurate. Angus steers were used to test effects of dietary energy source on muscle and adipose tissue metabolism and insulin sensitivity. Results suggest that feeding hay limited both glucose supply and tissue capacity to increase glucose utilization in response to insulin without altering acetate conversion to fatty acids. Because subcutaneous (s.c.) adipose tissue consistently utilized more acetate and oxidized more glucose than intramuscular (i.m.), these results suggest that hay-based diets may alter i.m. adipose tissue metabolism with less impact on s.c. adipose tissue. Additionally, s.c. adipose tissue may become resistant to insulin in steers fed to an excessive s.c. fat thickness. A final experiment was designed to test the effects of dietary energy source during backgrounding and compositional endpoint on adipose tissue metabolism and insulin sensitivity. Feeding hay during backgrounding may have differential effects on tissue lipogenesis. Feeding hay increased both glucose oxidation and incorporation of acetate into fatty acids; in i.m. insulin failed to stimulate glucose conversion to lipid. As physiological maturity increases, glucose conversion to CO2 and lactate increased, but the ability of insulin to stimulate lipid synthesis from glucose may be reduced. These data provide foundation for a hypothesis regarding diet-mediated regulation of differential adipose tissue metabolism. Validation of these hypotheses could generate nutritional strategies that alter the rate and site of adipose deposition.