Mechanisms underlying feed intake regulation and nutrient utilization in beef steers

The beef industry is under constant pressure to improve sustainability, including economic and environmental. Maximizing feed efficiency is of the utmost importance. Optimizing nutrient utilization efficiency is a key aspect of improving feed efficiency because as nutrients are more efficiently utilized, fewer waste products are excreted by the animal. Further, when discussing feed efficiency, feed intake and weight gain are vital components as these are used in common measures of feed efficiency. Investigating mechanisms contributing to these phenotypes is important. The hepatic oxidation theory suggests ruminant feed intake is regulated by hepatic oxidation of fuels such as propionate. Oxidation of propionate results in increased circulating glucose and likely an increase in insulin to stimulate glucose uptake into tissues such as muscle. Understanding mechanisms related to hepatic oxidation and glucose uptake is important. The primary objectives of this work were to: 1. Investigate mechanisms involved in hepatic feed intake regulation and skeletal muscle glucose uptake, and 2. Elucidate the effect of propionate on insulin secretion. To address objective 1, 54 Angus steers were utilized, with steers of high and low dry matter intake (n = 8 each) selected to investigate mechanisms involved in hepatic feed intake regulation, and an intravenous glucose tolerance test was conducted to select steers with high and low rates of glucose clearance (n = 8 each). To address objective 2, 4 Angus steers were utilized in a 2 × 2 crossover design and abomasally infused with propionate or saline. Our results suggest that insulin signaling is important for hepatic feed intake regulation as well as adipocytokine signaling, and factors related to myocyte function and insulin signaling were altered in divergent glucose clearance cattle. Further, abomasal propionate infusion stimulated insulin secretion, confirming that propionate induces insulin secretion upon oxidation to glucose. Overall, this work suggests that the ruminant liver plays a key role in feed intake regulation. Further, alterations in glucose clearance may be driven by myocyte structure and factors related to insulin signaling. Given that propionate stimulates insulin secretion, there is potential that increased hepatic oxidation of propionate to glucose also stimulates glucose uptake into skeletal tissue.