The amino acid composition of protein feedstuffs before and after ruminal incubation and after subsequent passage through the intestines of dairy cows.

The amino (AA) profile of eight feedstuffs (two samples of fishmeal, soybean meal [SBM], formaldehyde-treated SBM, sopralin, cotton-seed meal [CSM], rapeseed meal [RSM], corn distillers grains [CDG], and corn gluten feed [CGF]) was determined before and after ruminal incubation for 8 and 12 h in three lactating Friesian cows using nylon bags. The profile of AA disappearing during intestinal passage was also measured by inserting the bags into the duodenum through T-piece cannulas after ruminal incubation and recovering them in the feces. The AA profile changed for all feedstuffs, except sopralin, following ruminal incubation. Changes were greater for the more degradable feedstuffs. The profile of AA disappearing during intestinal passage was generally similar to the profile after ruminal incubation, but some differences were found with feedstuffs that had low (< 84%) intestinal disappearance of AA (RSM, CDG, CGF). For the other feedstuffs, intestinal disappearance of AA was greater than 93%. The two fishmeal samples had the highest concentrations of methionine and lysine in their residues after ruminal incubation, whereas CDG and CGF had low lysine concentrations. Residues of these latter two and RSM were quite high in methionine concentration, whereas residues of SBM, sopralin, and CSM had the lowest concentrations. Corn distillers grains had 13% of its AA remaining after ruminal incubation followed by intestinal passage. These results show that feedstuffs vary in the proportion of their AA that escape ruminal degradation, in the profile of those AA, and in their intestinal digestibility. These factors should be considered in protein evaluation systems and in assessment of the protein quality of feedstuffs.