Intestinal supply of amino acids in steers fed ruminally degradable and undegradable crude protein sources alone and in combination.

The objective of this study was to examine the effect of combining ruminally degradable and undegradable CP sources on ruminal microbial protein synthesis and postruminal N and amino acid (AA) flows in steers. Six steers fitted with ruminal, duodenal, and ileal cannulas were fed diets containing corn silage and high-moisture corn supplemented with urea, soybean meal (SBM), dry corn gluten feed (DCGF), a combination of corn gluten meal and blood meal (CB), or SBM and DCGF in combination with CB. Estimated ruminal N escapes for SBM, DCGF, and CB were 32, 25, and 68%, respectively. Supplemental CP sources supplied 35 to 40% of diet CP (12.5% CP diets). Dry matter intake was adjusted to 2.3% of BW for each steer in each period. Total N flow at the duodenum decreased (P < .01) when the diet was supplemented with urea vs other proteins due to decreased (P < .01) flow of nonmicrobial N. However, microbial N and AA flows were greater (P < .05) for urea than for other protein supplements. Disappearance of OM and NDF in the stomach decreased (P < .07) or was numerically lower but nonmicrobial N at the duodenum increased (P < .08) as CB replaced SBM or DCGF in the diet. Protein source had little effect on ruminal fermentation characteristics except that ruminal ammonia N (NH3N) concentration was higher (P < .05) for urea than for other treatments. Total AA and essential AA flows to and disappearance from the small intestine increased (P < .06) as CB replaced DCGF. However, substituting CB for SBM had little effect on intestinal flows and disappearance of AA. These data suggest that source of ruminally degradable CP can influence the efficacy of feeding ruminally degradable and undegradable CP in combination. In general, source of supplemental CP had a greater effect on the quantity than on the profile of absorbable AA supplied to the duodenum.