Effect of dietary nitrogen source on microbial protein synthesis, dietary protein degradation and nutrient digestion in steers

Abstract

Effects of dietary nitrogen source on microbial protein synthesis, dietary protein degradation and nutrient digestion were examined with mature Holstein steers fitted with a permanent rumen cannula and “T” type duodenal and ileal cannulae. Animals were fed 40% hay and 60% concentrate mixtures containing urea, soya bean meal (SBM), canola meal (CM) and dehydrated alfalfa + urea (Dehy-U) as a supplementary protein source. Nitrogen source in the diets did not alter duodenal or ileal flow rates of dry matter (DM) or organic matter (OM). Partial digestibility of DM and OM prior to the duodenum (mean 34.0 ± 3.24 and 45.5 ± 2.81), and in the small intestine (mean 46.9 ± 3.31 and 47.8 ± 2.72), and total digestibility in the whole digestive tract (mean 69.3 ± 1.89 and 70.4 ± 1.95) were similar across all dietary treatments. Steers fed SBM and Dehy-U had higher ( P < 0.05) concentrations of total nitrogen and non-ammonia nitrogen in duodenal digesta than those fed urea and CM. Dehy-U-fed steers had a lower ( P < 0.05) concentration of microbial nitrogen than the other groups. Although numerically higher flow of total nitrogen and lower flow of microbial nitrogen and, therefore, lower ratio of microbial to non-ammonia nitrogen was observed with decreasing protein degradabilities of the diets containing the natural protein sources, the differences were not statistically significant. The degradabilities of dietary protein, in vivo, were 66.3, 57.9, 63.3 and 41.0% for urea, SBM, CM and Dehy-U supplemented diets, respectively.