EFFECTS OF ZINC-TREATED SOYBEAN-MEAL ON RUMINAL FERMENTATION AND INTESTINAL AMINO-ACID FLOWS IN STEERS FED CORN SILAGE-BASED DIETS

The objectives of this study were to examine the effects of feeding zi nc-treated soybean meal (Zn-SBM) on ruminal fermentation patterns and duodenal AA flows in steers fed diets based on com silage and corn. Si x steers (385 kg) fitted with ruminal, duodenal, and ileal cannulas we re used in a replicated 3 x 3 Latin square design experiment with 14-d periods. Diets were supplemented with solvent-extracted soybean meal (SBM), Zn-SBM, or a 50:50 combination (CP basis) of SBM:Zn-SBM. Rumina l escape N content of SBM and Zn-SBM were 30.0 and 57.0%, respectively , based on 12-h Dacron bag incubation. Protein sources provided approx imately 30% of total CP in diets containing 12.6% CP (DM basis). Dry m atter intake was equalized throughout the study at 2.2% of average ini tial BW. Total N flow at the duodenum was similar (P = .47) among trea tments, but a trend (P = .15) for increased nonmicrobial N flow occurr ed when SBM and Zn-SBM were fed in combination. Microbial N flow and t rue efficiency of microbial CP synthesis were not affected by treatmen t (P = .87 and .37, respectively). Ruminal fermentation characteristic s generally were unaffected ( P > .10) by protein source. A positive q uadratic response ( P < .06) was observed for total and essential AA f lows to the small intestine because flows of total and essential AA fr om ruminally undegraded dietary protein tended (P = . 12) to increase when SBM and Zn-SBM were fed in combination. Absorption of AA from the small intestine also showed a positive quadratic (P < .06) response f or SBM:Zn-SBM. Microbial AA flow to the small intestine was similar (P = .87) among treatments. Although estimated ruminal escape N content of Zn-SBM was twice that of solvent-extracted SBM, supplementing diets with Zn-SBM or with SBM alone resulted in similar flows of N and AA t o the small intestine. However, feeding ruminally degradable and undeg radable SBM in combination improved the flow of absorbable AA to the s mall intestine compared with feeding either protein source alone.