Brown midrib-3 corn silage improves digestion but not performance of growing beef steers

The brown midrib-3 (bm(3)) gene mutation has been incorporated into corn pl ants to potentially improve fiber digestibility. The objectives of this stu dy were to determine the effect of bm(3) corn silage on digestion and perfo rmance of growing beef steers and to determine whether limiting intake woul d further enhance fiber digestibility of bm(3) corn silage. A bm(3) hybrid and its isogeneic normal counterpart were harvested at three-quarters kerne l milk line. Neutral detergent fiber, ADF, and ADL were 4.5, 6.9, and 1.9 u nits lower, respectively, and DM was 5.4 units higher for bm(3) than for no rmal silage. In Trial 1, eight ruminally fistulated Angus crossbred steers (224 +/- 24 kg) were randomly assigned to a 2 x 2 factorial arrangement of treatments in a replicated 4 x 4 Latin square design. Steers had ad libitum feed access or were restricted to 80% of ad libitum intake of diets contai ning 86% normal corn silage (Control) or bm(3) corn silage (BMCS). The rema inder of the diets consisted of soybean meal, urea, monensin, vitamins, and minerals. Dry matter intake was greater (P < 0.01) for steers offered ad l ibitum access to BMCS than for those with ad libitum access to the Control diet. The BMCS treatment resulted in improved (P < 0.05) apparent total-tra ct digestibility of DM, OM, NDF, and ADF. Mean concentration of total VFA a nd molar proportions of acetate were increased (P < 0.05) by feeding BMCS. There tended to be a DMI x hybrid interaction (P = 0.16) for apparent total -tract digestibility of NDF. When diets were offered ad libitum, BMCS incre ased NDF digestibility by 10.5 percentage units compared with Control, but, when DMI was limited, BMCS increased NDF digestibility by 15.8 percentage units. In Trial 2, 128 steer contemporaries of those used in Trial 1 (245 /- 13 kg) were offered ad libitum access to BMCS or Control diets as used i n Trial 1. After a 112-d treatment period, concentrate in the diet was incr eased, and all steers were fed a common finishing diet. During the 112-d tr eatment period, steers receiving BMCS consumed 0.45 kg more DM/d (P < 0.05) and had similar ADG (P > 0.10), compared with those steers receiving the C ontrol silage. This resulted in poorer (P < 0.01) feed efficiency for steer s receiving BMCS. Finishing phase and overall performance of the steers was not different (P > 0.10) due to treatment. Although feeding BMCS in growth -phase diets resulted in increased daily DMI and improved digestibility of DM and fiber, it did not result in improved steer feedlot ADG compared with Control silage.