Effect of form of nitrogen on growth of ruminal microbes in continuous culture

A study was conducted to determine the effect of various forms of N on the growth of ruminal microbes in a continuous culture system with solids and liquid dilution rates comparable to those of a high-producing dairy cow. Nitrogen forms were isolated soy protein, soy peptides, individual amino acids (AA) blended to profile soy protein, and urea, which were fed alone and in combinations so that the total N provided was 1.6% of the diet DM. The 100% soy protein treatment resulted in reduced digestion of N and nonstructural carbohydrate compared with other N forms, and outflow of bacterial N/24 h was less than when peptides were fed. This suggested that proteolysis rather than peptide uptake was the rate-limiting step in N utilization in this study. Non-urea N forms increased ADF digestion, total VFA production and the molar percentages of isobutyrate, isovalerate, and valerate compared to urea, which reflected the contribution of carbon skeletons of AA. When combinations of N forms were used, each form contributed an equal quantity of N, 50% of the total treatment, which was .8% of the diet DM. Combinations of N forms did not enhance, and in most cases reduced, ADF and NDF digestion when compared with individual N forms, and no combinations increased microbial growth over that of the individual forms. These results confirm that N forms other than ammonia are needed not only for maximum microbial growth, and they further demonstrate a need for non-protein N for the fiber digestion. In addition, results of this study suggest a requirement for a minimum level of peptide or AA N, which was met only when individual N forms were fed.