Effects of peptide source on microbial protein synthesis and nitrogen digestion in steers fed high concentrate diets

Six steers fitted with rumen and duodenal cannulas were used in a replicated 3 x 3 latin square to determine the effects of peptide source on the efficiency of microbial protein synthesis and n digestibility. steers were fed corn-based diets supplemented with three different sources of nitrogen. the three treatment diets consisted of 1) corn + casein (cc), 2) corn + soybean meal (csbm), and 3) corn + urea (cu). mean ruminal nh3 -n concentrations for cc, csbm and cu diets were 7.13, 5.93 and 9.27 mg/dl, respectively. steers supplemented with sbm had higher ruminal peptide-n concentration (p <0.02) and amino acid-n concentration (p <0.03) than steers supplemented with casein. mean ruminal concentrations of peptide-n for cc, csbm and cu diets were 56.67, 66.82 and 1.66 mg/liter respectively. efficiency of microbial protein synthesis was not altered by ruminal peptide-n concentrations and averaged 13.89 g of microbial n/kg of organic matter (om) digested for all treatments. supplementation with sbm and casein tended to increase microbial n and nanmn flow but the difference between them was not significant (p <0.08). mean microbial n entering duodenum averaged 65.0, 67.2 and 59.0 g/d for treatments cc, csbm and cu respectively. steers receiving sbm had 7.1% more microbial n flow to duodenum than steers receiving urea. faecal n excretion was greater for steers receiving urea or casein than for steers receiving sbm (p <0.02, p <0.04) but total tract n digestibilities were similar among treatments. efficiency of microbial protein synthesis was not increased by feeding diets that increased the concentration of peptides in the rumen.