INFLUENCE OF MONENSIN ON HOLSTEIN STEERS FED HIGH-CONCENTRATE DIETS CONTAINING SOYBEAN-MEAL OR UREA

We conducted two growth trials to evaluate the effects of monensin on amino acid sparing. When Holstein steers were fed a 90% concentrate di et supplemented with soybean meal (13.5% CP), the DMI, ADG, and effici encies of feed and nitrogen utilization were greater than with urea (P < .10). Monensin improved ADG with both nitrogen supplements (P < .01 ), but the positive effects of monensin on efficiencies of feed (P = . 12) and nitrogen (P = .26) utilization were greater for soybean meal t han for urea. Increasing amounts of monensin (0, 11, or 22 mg/kg of DM ) caused a linear increase in DMI with urea. Diets with soybean had gr eater intakes than diets with urea (P < .01); the greatest intake was of a soybean diet with monensin at 11 mg/kg of DM. Holstein steers fed soybean meal at 13.5% CP had lower DMI and greater efficiencies of fe ed and nitrogen utilization than steers fed 16.7% CP (P < .10). Crude protein level had no effect on ADG (P > .10). Monensin always increase d the efficiencies of feed and nitrogen utilization (P < .05), but the se trends were greater for diets with 16.7 than for those with 13.5% C P. Overall, monensin decreased DMI (P < .01), but this effect was grea ter for 16.7% than for 13.5% CP. Because the positive effects of monen sin on diet NEg (P = .16) and efficiency of nitrogen utilization (P = .26) were greater for soybean meal than for urea, it seemed that monen sin was sparing amino acids.