The effect ofin saccorumen incubation of a grass silage upon the total and D-amino acid composition of the residual silage dry matter

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Abstract
Summary: Grass silage was incubated in polyester bags in the rumens of Jersey heifers for 2, 12, 24 and 48 h. The total (D + L) and D-amino acid contents of the silage and of the silage residues remaining after rumen incubation were determined. In addition, the contamination of the silage residues by rumen bacterial protein was measured by using35S as a marker of rumen bacterial protein. The amino acid profile of the residual silage dry matter differed markedly after 2 h of rumen incubation from that of the original silage; thereafter progressive changes in the amino acid composition of the residual silage dry matter occurred between 2 and 48 h of rumen incubation. The D-alanine content of the original silage was higher than that of D-glutamic acid. Both these D-amino acids disappeared almost completely from the silage after 2 h rumen incubation; between 2 and 48 h rumen incubation the quantities of D-alanine and D-glutamic acid in the residual silage dry matter increased. The residual silage dry matter contained more D-glutamic acid than D-alanine and these acids were in a similar proportion to that found in rumen bacteria; thus it was concluded that D-amino acids in the residual silage dry matter resulted from contamination of the residues by rumen bacteria. Contamination of residual silage protein by rumen bacterial protein increased with length of rumen incubation; the extent of contamination was similar for each incubation time whether assessed using35S or D-amino acids as markers of rumen bacterial protein. However, this contamination by rumen bacterial protein did not markedly alter the degradability of silage protein calculated from the disappearance of silage N incubatedin sacco.

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