Plasma Amino Acids and Nitrogen Retention by Steers Fed Purified Diets Containing Urea or Isolated Soy Protein

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Abstract
The nitrogen metabolism of steers fed purified diets containing either urea or isolated soy protein as the sole sources of dietary nitrogen was studied. Replacing glucose monohydrate with additional starch significantly altered fecal and urinary nitrogen losses and also the ratio of the ruminal volatile fatty acids. Nitrogen retention was significantly greater when the isolated soy diets were fed. Serine and glycine were detected in significantly greater quantities, whereas valine, isoleucine, leucine, and phenylalanine were detected in significantly smaller quantities in the blood plasma when steers were fed the urea diets. Evidence indicates that the lack of sufficient quantities of the branched-chain volatile fatty acids at the ruminal level may have been responsible for the lowered plasma levels of valine, isoleucine, and leucine when steers were fed the urea diets. These data also suggest that the lowered nitrogen retention may have been the result of a deficiency of these amino acids and phenylalanine or an imbalance of certain amino acids synthesize by ruminal microorganisms.