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Effect of Dietary Protein Level on Nitrogen Metabolism in the Growing Bovine: II. Diffusion into and Utilization of Endogenous Urea Nitrogen in the Rumen¹

University of Kentucky, Lexington 40546-0215

Abstract

Six Angus heifer calves (234 kg) were assigned to either a high (HP; 126.1 g N/d) or low (LP; 66.5 g N/d) protein intake to evaluate ruminal criteria associated with movement of blood urea-N (BUN)-derived NH₃-N from the rumen wall into interior ruminal digesta. Calves received 4.8 kg DM/d of diets containing 30% cottonseed hulls and 70% corn-soybean meal in equal portions at 4-h intervals. Following single i.v. injections of ¹⁵N-urea, ruminal fluid was collected serially for 4 h postinjection from digesta located adjacent to the rumen wall (wall-proximate digesta; WPD) and from the center of the rumen digesta mass after manual agitation (center mixed digesta; CMD). Mean ruminal NH₃-N (RAN) concentrations were higher (P < .05) for HP than for LP, but were not affected (P > .05) by digesta sampling site. Ruminal urease activity was higher (P < .05) for LP than for HP and tended (P = .14) to be higher for WPD than for CMD. Area under the 15N enrichment curve (AUC) ratios between sampling sites (WPD/CMD x 100) for RAN were greater (P < .05) for LP than for HP. However, AUC ratios for bacterial N were not affected (P > .05) by protein level. Whereas BUN-derived ¹⁵NH₃ appeared to thoroughly equilibrate with RAN in interior ruminal digesta with HP, there appeared to be a declining enrichment gradient for RAN from the rumen wall to the interior ruminal digesta with LP. Data are interpreted to suggest that bacteria at or near the rumen wall may preferentially utilize some BUN-derived NH₃-N entering through the rumen wall in calves fed LP diets.

¹ This paper (No. 87-5-3-313) is published with the approval of the Director of the Kentucky Agric. Exp. Sta.

² Dept. of Anim. Sci.

³ USDA-ARS and Dept. of Agron.