ENHANCING SUSTAINABILITY OF BEEF PRODUCTION BY ELUCIDATING SUBSPECIES? DIFFERENCES IN UREA RECYCLING IN RESPONSE TO SUPPLEMENTATION

Our primary research goal is to promote sustainability of global beef production by improving the prediction of nitrogen utilization, thus allowing precision diet formulation to be implemented in formulating cattle diets. Reducing the over provision of protein by 10% potentially reduces nitrogen excretion from cattle by approximately 22 pounds per head per year or 704 million pounds for the U.S. beef industry per year. Potential savings, on a soybean meal equivalent basis, by reducing the amount of protein (nitrogen)fedto and excreted bycattle by 10% is $1.4 billion per year. Ultimately, precision diet formulation will reduce both overfeeding and underfeeding of N and increase the environmental, economic, and social sustainability of beef production.We have chosen to address the problem of over and underfeeding of protein in cattle diets by elucidating differences in N utilization and recycling in Bos taurus taurus and Bos taurus indicus subspecies to improve our capacity to describe urea recycling and microbial capture of recycled N, both essential to precision diet formulation. Focusing on the the two subspecies of cattle improves the likelihood of our findings affecting beef production across all the major beef producing regions.Our work focuses on three main objectives:Objective 1. Quantify urea recycling and forage utilization responses in Bos taurus taurus and Bos taurus indicus consuming LQF supplemented with graded levels of ruminal degradable and undegradable protein.Objective 2. Quantify urea recycling and forage utilization in Bos taurus taurus and Bos taurus indicus consuming LQF when supplemented with graded levels of starch and protein.Objective 3. Elucidate differences in urea recycling, N metabolism, and LQF utilization between Bos taurus indicus and Bos taurus taurus to address deficiencies in nutritional models.Current standard feeding models, NASEM (2016) and BR-CORTE (Valadares Filho et al., 2016), recognize the importance of urea recycling in addressing microbial N requirements in the text, but neither model incorporates urea recycling into prediction equations. Both models default to predicting that all N required for microbial synthesis must come from RDP, which generally leads to over fortification of diets with RDP. NASEM (2016) states, "The RDP requirement is computed assuming an efficiency of use of RDP by ruminal microbes of 100%. The RDP supply does not include ruminally recycled N." (Equations 19-121 through 126). In recognition of these limitations, NASEM (2016) recommended research to support development of "equations to accurately predict recycled nitrogen across a wide variety of diets" and to determine the "efficiency with which RDP is converted to MCP". Therefore, our studies are designed to address these deficiencies in relation tourea recycling, microbial N capture,and supplementation strategies in Bos taurus indicus and Bos taurus taurus consuming LQF.Completion of proposed projects will provide data allowing for precise delivery of supplemental N for cattle grazing LQF across a wide-array of production systems. Capturing data in both subspecies enhances the global utility of these projects for meeting the increasing demand for animal proteins.