Influence of diets supplemented with different nitrogen sources on rumen microbial protein synthesis and protozoa numbers in steers

An experiment with ruminally and duodenally cannulated steers was undertaken to determine how type of diets affected the ability of rumen fermentation to respond to different sources of nitrogen in the diet. Four steers received diets of corn or hay grass supplemented with either urea or casein in a 4 x 4 Latin Square Design with a 2 x 2 factorial arrangement. Measurements of protozoa numbers, rumen fermentation and microbial protein synthesis were made on the third week of each 21-day period. Ruminal pH was higher (P<0.01) 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. Nevertheless, steers receiving hay had 33% greater (P<0.01) 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 (P<0.0002) in corn-based diet than hay-based diet. Despite having a lower (P<0.01) ruminal pH (6.21 vs. 6.52), steers fed the corn-based diet had greater ruminal organic matter digestibility than steers fed the hay-based diets, presumably reflecting a difference in the source of fiber in the two diets. Ruminal concentrations of total protozoa were higher (P<0.01) for corn-fed steers than hay-fed steers but holotrichs were more prominent in hay-fed steers. The source of N or peptides had no influence on protozoa numbers and MOEFF in the rumen. No benefit was noted in terms of ruminal or total tract digestion of organic matter or starch when a dietary source of peptides (casein) replaced dietary urea. Protozoa population in greater in corn based diets but their role in reducing rumen microbial synthesis is unclear in this experiment. Nevertheless, microbial efficiency was 32% greater with a hay-based than a corn-based diet.