The effect of the form of nitrogen in the diet on ruminal fermentation andthe yield of microbial protein in sheep consuming diets of grass silage supplemented with starch or sucrose

Three experiments were conducted to investigate the influence of the type o f carbohydrate in the supplement on responses of microbial protein synthesi s (MPS, calculated from the urinary output of purine derivatives) in the ru men to dietary addition of ruminal sources of peptides and amino acids in s heep consuming grass silage. In Experiment 1, incubations of rumen liquor i n vitro indicated that bacteria fermenting sucrose grew better when ammonia was the only source of added nitrogen than did bacteria utilizing starch. These results led to the hypothesis that bacteria fermenting sucrose might be less dependent on performed amino acids and peptides for their growth th an are bacteria fermenting starch. Experiment 2 was designed to test this h ypothesis by measuring the response of MPS to inclusion of rapeseed meal, a s a source of ruminal amino acids and peptides, in supplements of sucrose o r starch given to sheep given a basal diet of grass silage, Then was no dif ference between the carbohydrate treatments in the response of MPS to the i nclusion of rapeseed meal, but also, there was no difference in MPS between the sucrose and starch supplements when given without added protein. Exper iment 3 examined the effect of the level of starch (0.20 and 0.38 of DM) in the diet on the response of MPS to isonitrogenous dietary additions of ure a or casein. The hypothesis tested was that the addition of protein would l ead to a greater increase of MPS with the higher level of starch because am ylolytic bacteria require preformed peptides and amino acids for maximal gr owth. However, this was not the case because the response of MPS to inclusi on of casein as opposed to urea was greater at the lower level of starch. T hese results question the generalization that amylolytic bacteria require p reformed peptides and amino acids for growth and highlight the need for mor e information on the influence of diet composition in determining the types of amylolytic bacteria that dominate the ruminal fermentation, there being evidence in the literature that at least one potentially dominant organism , Ruminobacter amylophilus, mainly uses ammonia for growth with only very l imited incorporation of organic nitrogen. (C) 1999 Elsevier Science B.V. Al l rights reserved.