Effects of supplemental alfalfa hay on the digestion of soybean hull-baseddiets by cattle

We evaluated the optimal level of alfalfa inclusion in soybean hull-based d iets. In Exp. 1, 20 Holstein steers (319 kg of BW) were used in a complete block design. Treatments included a soybean hull mix (95.7% soybean hulls, 3% molasses, 0.5% urea, 0.8% mineral mix; DM basis) fed alone (100:0) or wi th 10.4, 20.7, or 30.9% (DM basis) coarsely chopped alfalfa hay (90:10, 80: 20, and 70:30, respectively) or alfalfa alone (0:100). Diets were fed once daily at 1.75% (DM basis) of BW. In some cases, orts were present, which ca used DM, OM, and NDF intakes to decrease (linear, P < 0.05) as alfalfa was added to the diets. Digestibilities of DM, OM, and NDF decreased linearly ( P < 0.05) as alfalfa was added,to the diets, but quadratic responses (P < 0 .05) indicated that positive associative effects occurred between soybean h ulls and alfalfa. Liquid dilution rates increased (linear, P < 0.05) with a lfalfa additions to the diets and also demonstrated positive associative ef fects between soybean hulls and alfalfa (quadrat;ic, P < 0.05). Solid passa ge rates were similar for 100:0 and 0:100 but were increased (quadratically and cubically, P < 0.05) when combinations of soybean hulls and alfalfa we re fed. In Exp. 2, in vitro NDF digestibilities were measured for soybean h ulls, alfalfa, and a blend of 85% soybean hulls and 15% alfalfa, each with no N source or supplemented with casein or urea to ascertain the effects of protein from alfalfa on digestibility. Disappearances were increased (P < 0.05) by addition of urea or casein, but no interactions between substrate and N supplement were observed. Addition of 30% alfalfa to diets consisting primarily of soybean hulls led to positive associative effects on diet dig estibility, but alfalfa additions led to increased liquid and solid passage rates, suggesting that the benefit was not a result; of slower passage of soybean hulls from the rumen.