In situ disappearance of amino acids from grass silages in the rumen and intestine of cattle.

Nineteen grass silages were evaluated using the in situ rumen and mobile nylon bag techniques to determine the amino acid (AA) composition of rumen-undegradable protein and the possibility of predicting the concentrations of individual AA presented to the duodenum from the dietary AA profiles. All feeds and residues from the nylon bags were analyzed for diaminopimelic acid to correct for contamination by microbial proteins. All essential AA behaved similarly; the initial feed had the highest concentrations, and the material remaining in the mobile nylon bag had the lowest concentrations. The reduction in the concentration of methionine between the 12-h rumen residue and the residue in the mobile nylon bag was significant. With the exception of arginine (r2 = 0.76) and serine (r2 = 0.82), the relationship was poor between the concentrations of AA in the grass silage and those in the residue in the nylon bag following 12 h of rumen incubation. The lack of reliable relationships between concentrations of individual AA in the silages and concentrations of AA in the 12-h rumen residue indicated that degradability characteristics of AA in grass silage were not alike. This poor relationship was likely the reason that prediction equations could not be developed between the AA composition of the initial feed and the pattern of AA presented to the duodenum following 12 h of rumen incubation. The AA composition of the rumen-undegradable portion of grass silages differs from the AA composition of grass silages.