Ruminal degradation, amino acid composition, and estimated intestinal digestibilities of four protein supplements

Blood meal, canola meal, corn gluten meal, and menhaden fish meal were weighed into dacron bags for incubation in the rumens of two ruminally cannulated Holstein cows on 3 d for 0, 3, 6, 12, 18, and 24 h. Both the original feeds and the residues remaining after 12 h were analyzed for amino acid (AA) content. Canola meal was degraded the most extensively in the rumen, and blood meal was degraded the least extensively. Intestinal digestibilities estimated using an enzymatic in vitro technique were all high; canola meal was estimated to have the lowest intestinal digestibility, and corn gluten meal was estimated to have the highest intestinal digestibility. The AA profile of the 12-h residues differed only slightly from the AA profile of the original protein supplements. A comparison of the AA profiles of feed residues with milk protein showed that isoleucine was the first-limiting AA in blood meal, canola meal, and fish meal, and lysine was the first-limiting AA in corn gluten meal. Although canola meal was extensively degraded in the rumen, its 12-h residue still provided an estimated AA profile to the intestinal tract that was closest to the AA profile of milk protein. Blood meal and corn gluten meal are good sources of ruminally undegradable protein but are deficient in some AA and should probably be fed only in combination with other protein sources that complement their AA profiles.