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 so lids and liquid dilution rates comparable to those of a high-producing dairy cow. Nitrogen forms were isolated soy protein, soy peptides, in dividual amino acids (AA) blended to profile soy protein, and urea, wh ich were fed alone and in combinations so that the total N provided wa s 1.6% of the diet DM. The 100% soy protein treatment resulted in redu ced digestion of N and nonstructural carbohydrate compared with other N forms, and outflow of bacterial N/24 h was less than when peptides w ere fed. This suggested that proteolysis rather than peptide uptake wa s the rate-limiting step in N utilization in this study. Non-urea N fo rms increased ADF digestion, total VFA production and the molar percen tages of isobutyrate, isovalerate, and valerate compared to urea, whic h reflected the contribution of carbon skeletons of AA. When combinati ons 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. Combinatio ns of N forms did not enhance, and in most cases reduced, ADF and NDF digestion when compared with individual N forms, and no combinations i ncreased microbial growth over that of the individual forms. These res ults confirm that N forms other than ammonia are needed not only for m aximum microbial growth, and they further demonstrate a need for nonpr otein N for fiber digestion. In addition, results of this study sugges t a requirement for a minimum level of peptide or AA N, which was met only when individual N forms were fed.