Abstract
Five steers (690 kg) with ruminal and duodenal cannulas were fed 100% bromegrass hay or 60% bromegrass hay:40% concentrate at 90% of their ad libitum DMI. The concentrate consisted of ground corn (GC) and(or) soybean hulls (SBH): 1) 100% SBH; 2) 66% SBH/33% GC; 3) 33% SBH/66% GC; or 4) 100% GC. Increasing level of corn inclusion decreased (linear, P < .01) DM and OM digestion (corrected for microbial DM and OM flow) in the rumen. Neutral detergent fiber digestion decreased in the rumen (quadratic, P = .05) and total tract (linear, P = .06) with increasing level of corn. Ruminal NDF digestion (quadratic, P = .05) and duodenal microbial N flow (quadratic, P < .01) were greatest for 66% SBH/33% GC, whereas total N flow to the duodenum was similar (P > .10) among treatments. Microbial efficiency increased (quadratic, P = .10) as the level of corn inclusion increased. Molar percentage of ruminal acetate decreased (linear, P < .01) with increasing level of corn, whereas molar percentage of butyrate increased (linear, P < .10). Carboxymethylcellulase activity was greatest (quadratic, P = .08) for 66% SBH/33% GC and paralleled NDF digestion in the rumen. Increasing level of corn decreased (linear, P < .07) in situ DM digestibility of SBH after 4, 8, and 12 h of incubation.
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