Effect of energy and nitrogen sources on efficiency of microbial protein synthesis in the rumen of steers fed concentrate or roughage diets

A. Yusof Hamali, A. La Manna and F.N. Owens

Abstract

An experiment with ruminally and duodenally cannulated steers was undertaken to determine how the dietary energy source affected the ability of rumen fermentation to respond to peptides compared to non-protein nitrogen (N) in the diet. Four steers received diets of corn (C) or hay grass (H) supplemented with either urea (U) or casein (K) in a 4 x 4 Latin square with 2 x 2 factorial arrangement. Measurements of rumen fermentation, microbial protein synthesis and protozoa numbers were made on the third week of each 21-day period. Ruminal pH was significantly higher for steers fed hay than corn diet. Concentrations of ruminal NH3-N were higher for steers supplemented with urea than for steers supplemented with casein. Steers receiving corn or hay diets supplemented with casein had higher ruminal amino acid-N and peptide-N concentrations than those supplemented with urea. Averaged rumen peptide-N concentrations were 56.50, 1.99, 56.24 and 4.21 mg/L for treatment CK, CU, HK and HU respectively. The source of N/peptides had no influence on efficiency of microbial protein synthesis (MOEFF) in the rumen. Nevertheless, steers receiving hay had 33% greater (p <0.0001) MOEFF than steers receiving corn (18.61 vs 14.03 g of microbial N/kg of OM digested). Total N flow to the duodenum was similar among treatments but non-ammonia non-microbial N flow to duodenum was higher in corn-based diet than hay-based diet. Despite having a lower (p <0.0003) ruminal pH (6.21 vs. 6.52), steers fed the corn-based diet had greater ruminal organic matter, starch, ADF and NDF digestibilities than steers fed the hay-based diets, presumably reflecting a difference in the source of fiber with the two diets. Ruminal concentrations of total protozoa were higher for corn-fed steers than hay-fed steers but holotrichs were more prominent in hay-fed steers. Failure of peptides to increase efficiency of microbial growth in steers as noted by others with in vitro studies may reflect greater potential for nutrient crossfeeding, microbial adaptation to nutrient supply, and (or) energy limited growth and the resulting slower microbial growth rate under ruminal conditions in vivo.

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