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

The quantity and quality of protein reaching the small intestine are influenced by ruminally undegraded intake N or protein and microbial protein synthesized in the rumen. There have been few determinations of microbial protein synthesis in goats. Similarly, ruminal degradability of feed CP in goats has not been extensively studied, with some estimates based on in situ disappearance and assumed or measured ruminal digesta passage rate. Although differences between goats and other ruminant species in ruminal digesta passage rates are not well agreed upon, potential for differences with some diets raises the possibility that extent of ruminal digestion of protein differs between goats and cattle or sheep. Results of this experiment support suggestions that goats have considerable ability to recycle N to the rumen. For goats with ample tissue protein stores available for mobilization, this permits high microbial protein production and efficiency of microbial growth with high concentrate diets containing as little as 9.3-9.6% CP and with a ruminally degraded intake protein(DIP) to TDN ratio of 0.073. In such instances, only small increases in ruminal and total tract OM and NDF digestibilities can be achieved by supplying additional DIP, such as with a dietary CP concentration of 11.5-13.5% and a DIP to TDN ratio of 0.104-0.113. When ruminal ammonia availability is not limiting, with a high quality, high concentrate diet, it is unlikely that benefits in microbial growth or digestion will occur with use of a true protein source compared with a source of non-protein N such as urea.