Factors influencing rumen microbial growth rates and yields: effect of amino acid additions to a purified diet with nitrogen from urea.

Effects of isonitrogenous urea and amino acid additions upon microbial growth in rumen contents from a cow fed a purified diet in which urea was the sole nitrogen source were studied. Incorporation of amino acids into microbial cells, volatile fatty acids, and carbon dioxide was estimated. Rates of microbial growth, volatile fatty acid production, and effects of amino acids upon microbial nitrogen yields were highest right after feeding and decreased with time after feeding. Microbial growth and amounts of amino acids incorporated into microbial cells, volatile fatty acids and carbon dioxide were related closely to quantity of starch remaining in the rumen. High amounts of starch increased microbial protein synthesis from carbon-14 labeled amino acids and reduced amounts of amino acid fermentation. Estimated microbial protein yields per day were 326.0, 444.4, 497.3, and 527.3 g when 0, 15, 30, and 45 mg amino acid nitrogen replaced urea nitrogen during incubation. Respective values for microbial cells per mole estimated adenosine triphosphate were 15.2, 19.2, 21.0, and 24.5. Microbial cell yields per kg carbohydrate digested were 139.0, 189.5, 212.0, and 224.8 g for 0, 15, 30, and 45 mg amino acid nitrogen. Addition of small amounts of amino acids to a diet containing urea as the sole nitrogen source improved considerably rumen microbial protein yields.