Effects of ruminally degraded nitrogen source and level in a high concentrate diet on site of digestion in yearling Boer × Spanish wether goats

Eight yearling Boer × Spanish wether goats (36.6 ± 2.3 kg average initial BW) with ruminal and duodenal cannulae were used in an experiment with two simultaneous 4×4 Latin squares to study effects of dietary level of CP, the ratio of ruminally degraded intake N or protein (DIP) to TDN and source of supplemental DIP on site of digestion with a high concentrate diet. Diets were formulated to be (DM basis) 9.2% CP, without inclusion of urea (U0) or soybean meal (SBM; S0); 11.3%, CP achieved with 0.73% urea (U1) or 4.48% SBM (S1); 13.3% CP, via use of 1.46% urea (U2) or 8.90% SBM (S2); or 15.2% CP, derived through use of 2.16% urea (U3) or 13.2% SBM (S3). The ratio of DIP:TDN was 0.073, 0.104, 0.136, 0.167, 0.073, 0.093, 0.113 and 0.132 for U0, U1, U2, U3, S0, S1, S2 and S3, respectively. Diets contained 30% cottonseed hulls and were corn-based and fed at 2% BW (DM basis). Apparent ruminal N digestibility increased quadratically as level of supplemental CP rose (−71.6, −39.9, −20.5, 8.5, −60.3, −12.5, −8.4 and −3.5% for U0, U1, U2, U3, S0, S1, S2 and S3, respectively; S.E. = 6.6). Microbial OM and N flows to duodenum decreased linearly as CP level increased. Ruminal and total tract NDF digestibilities (e.g. total tract: 51.3, 57.6, 57.7, 57.4, 49.7, 52.3, 53.2 and 53.2% for U0, U1, U2, U3, S0, S1, S2 and S3, respectively) increased linearly and tended to change quadratically as CP level increased. In conclusion, a dietary CP concentration of 11–13% and DIP:TDN ratio of 0.10–0.11 were adequate for maximal total tract OM digestibility in meat goats consuming a corn-based, high concentrate diet, regardless of supplementation with a source of non-protein versus natural protein, although magnitudes of difference in digestibility with lower levels were not marked. A dietary CP concentration of 9–10% and ratio of DIP to TDN of 0.073 were sufficient for highest microbial protein production.With ample tissue N reserves, the ability of meat goats to recycle N may preclude realization of substantial benefits from supplementing high concentrate diets around 9% CP with additional DIP, and high concentrate diets appear to satisfy microbial needs for non-ammonia nitrogenous compounds. © 2003 Elsevier Science B.V. All rights reserved.