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Effect of ruminally degraded nitrogen source and level in a high concentrate diet on site of digestion in Boer x Spanish wethers

S. A. Soto-Navarro1, A. L. Goetsch1, T. Sahlu1, R. Puchala1, and L. J. Dawson1,2

1E (Kika) de la Garza Institute for Goat Research, Langston University, Langston, OK
2College of Veterinary Medicine, Oklahoma State University, Stillwater, OK

Eight yearling Boer x Spanish goat wethers (35.3 6.6 kg average initial BW) with ruminal and duodenal cannulas were used in an experiment with two simultaneous 4 x 4 Latin squares to study effects of supplemental ruminally degraded N (DIP) source and level on sites of digestion. Diets were (DM basis) 9.2% CP, without inclusion of urea (U0) or soybean meal (S0); 11.3% CP achieved with 0.73% urea (U1) or 4.48% soybean meal (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). Microbial OM and N flows to the duodenum linearly decreased as (P < 0.05) CP level increased (N: 8.8, 7.6, 7.8, 6.7, 7.4, 6.0, 6.7, and 6.7 g/d for U0, U1, U2, U3, S0, S1, S2, and S3, respectively). Apparent ruminal OM digestibility increased linearly as CP level increased, and there was an interaction between the quadratic effect of CP level and source in total tract OM digestibility (P < 0.05). With urea diets, total tract OM digestibility plateued at U1, while the peak with SBM was at S2 (71.3, 78.0, 77.7, 77.8, 71.5, 73.1, 74.7, and 75.0% for U0, U1, U2, U3, S0, S1, S2, and S3, respectively; SE = 0.14). There were interactions between linear and quadratic effects of CP level and source in true ruminal and postruminal N digestibilities (P < 0.08). With urea diets, true ruminal N digestibility linearly increased and postruminal N digestibility linearly decreased as CP level rose, whereas there were marked effects of SBM inclusion but no differences among S1, S2, and S3. Ruminal and total tract NDF digestibilities (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 (P < 0.05) with increasing CP level, although differences tended to be greater for urea or SBM inclusion than among U1, U2, and U3 or S1, S2, and S3 (quadratic, P <0.13). In conclusion, N recycling in yearling goats appears adequate to support high microbial growth with a high concentrate diet, although OM and NDF digestion may be enhanced by additional DIP for a DIP:TDN ratio of 0.10 to 0.11. Supported by USDA project No. 98-38814-6240.


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