159. Effects of Dietary Crude Protein Concentration and Urea Source on Nitrogen Utilization in Finishing Nellore Bulls.

Abstract The objective of this study was to evaluate the effects of dietary crude protein (CP) concentration and urea source on nitrogen (N) utilization in finishing Nellore bulls fed corn-based diets. A total of 84 Nellore bulls (initial body weight = 448 ± 40.5 kg) were assigned to a 2 × 2 factorial arrangement, with treatments allocated across 28 pens (7 pens per treatment; 3 bulls per pen). Treatments consisted of two CP concentrations (11 or 14% of dry matter [DM]) and two urea sources: feed-grade urea (U) or coated urea (CU). Diets were offered as total mixed rations ad libitum twice daily. The CU source contained 82% urea on a DM basis, and in an in vitro assay, 27% of this urea was released in the rumen. The experimental period lasted 90 d. The N intake, fecal and urinary N excretion, and N retention from growth performance were determined to evaluate N utilization efficiency. No CP concentration × urea source interactions were observed (P ≥ 0.31) for N balance variables. Bulls fed diets containing 14% CP had greater N intake compared with those fed diets containing 11% CP (290.6 vs. 222.5 g/d; P < 0.001). Consequently, urinary N excretion was also greater in bulls fed diets containing 14% CP (156.2 vs. 94,8 g/d; P < 0.001). Nitrogen absorption was greater in bulls fed diets containing 14% CP (206.2 vs. 140.2 g/d; P < 0.001). However, reducing dietary CP concentration improved nitrogen utilization efficiency. Bulls fed diets containing 11% CP had greater N retention relative to N intake (20.2 vs. 17.1%; P < 0.001) and greater N retention relative to apparent N absorption (32.2 vs. 24.2%; P < 0.001) compared with bulls fed diets containing 14% CP. Urea source also affected nitrogen losses. Bulls fed diets containing CU had lower fecal N excretion compared with those fed diets containing U (79.2 vs. 87.2 g/d; P = 0.04). In addition, CU increased N retention efficiency relative to N intake compared with U (19.5 vs. 17.9%; P = 0.03). These results indicate that modifying dietary protein concentration and the form of non-protein nitrogen supplementation can influence N partitioning and its overall efficiency in feedlot cattle. In conclusion, reducing dietary CP concentration increased N utilization efficiency by enhancing N retention relative to N intake and absorbed N. In addition, inclusion of coated urea reduced fecal N excretion and increased nitrogen retention efficiency compared to feed-grade urea, demonstrating that these strategies improve N utilization efficiency in finishing cattle. For image description, please refer to the figure legend and surrounding text.