Amino acid profile and intestinal digestibility in dairy cows of rumen-undegradable protein from various feedstuffs.

Three lactating cows fitted with rumen cannulas and three cows fitted with proximal duodenal cannulas were used to determine the effect of in situ rumen degradation on the AA profile of rumen-undegraded protein of 12 feedstuffs. Intestinal digestibility of rumen-undegraded protein was determined using the mobile bag technique. The absorbable AA profile of rumen-undegraded protein for each feedstuff was compared with profiles of the original feedstuff and the rumen-exposed undegraded protein. Branched-chain AA in particular seemed to be rather resistant to degradation in the rumen, as was Phe. Lysine concentrations decreased in the undegraded protein fraction in 9 of 12 feedstuffs; the degradation of Met depended on the feedstuff. The absorbable AA profiles of undegraded protein, in general, closely reflected the AA profiles of the rumen-exposed residues, which suggests that rumen degradation had a greater influence than postruminal digestion on the postruminal provision of specific absorbable AA. Intestinal digestibility of undegraded protein varied from 37.8% for Eragrostis curvula hay to 98% for soybean meal; the constant digestibility factor used by most protein systems should be reconsidered.