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Influence of the Branched-chain Volatile Fatty Acids and Phenylacetate on Ruminal Microorganisms and Nitrogen Utilization by Steers Fed Urea or Isolated Soy Protein

United States Department of Agriculture, ARS, Animal Science Research Division, Beltsville, Maryland 20705

The purpose of this study was to determine if dietary additions of the branched-chain volatile fatty acids and phenylacetate will improve the nitrogen retention of steers fed a protein-free and a protein-containing diet. Purified diets containing either urea or isolated soy protein and either with or without 0.27% 2-methylbutyrate, 0.31% isovalerate, 0.25% isobutyrate and 0.20% phenylacetate (Na salts) were fed to growing steers in a replicated 4 x 4 Latin square using 42-day adaptation and 7-day excreta collection periods. Prefeeding blood and postfeeding (4 hr) ruminal samples were obtained following each collection period. Nitrogen retention was less (P < 0.01) when steers consumed the urea diets. The dietary acids decreased (P < 0.05) urinary nitrogen and increased (P < 0.05) nitrogen retention but most of this change was observed in steers fed isolated soy protein. Ruminal ammonia concentration was greater (P < 0.01) in steers fed the urea diets but it was reduced when acids were included in the diet. The additions of the acids increased the molar percentage of ruminal isovaleric + 2-methylbutyric acids (P < 0.01) and isobutyric acid (P < 0.10). Acid addition did not change the rumen protozoa numbers or the total, viable or cellulolytic bacteria numbers. Bacteroides sp. comprised the largest group of bacteria presumptively identified regardless of treatment. The molar percentage of amino acids in bacterial hydrolysates obtained from the steers did not differ with diet and were very similar to the pattern found in a steer fed a natural diet. Blood plasma levels of valine, isoleucine, leucine and phenylalanine were less (P < 0.01) in steers fed the urea diets than steers fed the isolated soy diets. An additional study indicated less diurnal variation in free blood plasma amino acid concentrations when steers were fed a natural diet compared to the purified diets.