THE SIGNIFICANCE OF SILAGE PROTEIN DEGRADATION AND PLASMA AMINO ACID RATIOS IN THE CONTROL OF FOOD INTAKE BY LAMBS FED ENSILED AND FRESH ALFALFA

Abstract

Alfalfa (30% dry matter) was harvested with a particle length of 0.6 cm, conserved fresh (frozen) or as silage and fed to eight lambs during two 27-d periods. The ensiling process resulted in substantial degradation of lysine, threonine, histidine, arginine, tryptophan and tyrosine in the original herbage. Isoleucine, leucine and valine resisted degradation during ensilage. As a consequence of these changes, the ratios of tyrosine: large neutral amino acids and tyrosine: phenylalanine (TYR:LNAA and TYR:PHE) were markedly lower in ensiled compared to fresh alfalfa (0.035 vs. 0.051 and 0.235 vs. 0.313, respectively) but these ratios in plasma were higher (P < 0.05) with silage than with fresh forage (0.114 vs. 0.095 and 1.020 vs. 0.997). Ensiling of alfalfa resulted in a 19% reduction in silage intake but the response did not correlate with changes in plasma ratios of TYR:LNAA and TYR:PHE. Nitrogen retention was 27% lower with ensiled than with fresh alfalfa. Plasma levels of branched chain and other essential amino acids tended to be higher with fresh than ensiled forage but the difference was not significant. It is concluded that the reduction in food intake with alfalfa silage was unrelated to changes in specific ratios of amino acids in plasma but could be related to the supply of essential amino acids to the intestine of lambs. Key words: Silage intake, lambs, nitrogen balance, plasma amino acids, alfalfa