FEED-INTAKE AND DIGESTION BY HOLSTEIN STEER CALVES CONSUMING LOW-QUALITY GRASS SUPPLEMENTED WITH LASALOCID OR MONENSIN

Holstein steer calves were fed late vegetative bermudagrass or mature bromegrass free choice in three Latin-square experiments to determine influences of ionophore type and level, supplementation with cereal grain, and source of low-quality, grass on feed intake and digestion. In exp. 1, neither lasalocid nor monensin given at 0.5 mg kg-1 of body weight (BW) affected intake or digestion with either forage source. In exp. 2, steers received lasalocid or monensin at 0, 0.5 or 1.0 mg kg-1 of BW. Neutral detergent fiber digestibility was not depressed by ionophores and was slightly greater (P < 0.05) for ionophores at 1.0 than at 0.5 mg kg-1 of BW. Digestible organic matter intake with bermudagrass was slightly greater with ionophores at 1.0 than at 0. 5 mg kg-1 of BW, but an opposite difference between ionophore levels occurred with bromegrass (ionophore level x forage source interaction; P < 0.07). The concentration of total volatile fatty acids in rumen fluid at 4 h post-feeding was lower (P < 0.05) for monensin than for lasalocid; the acetate/propionate ratio was decreased by ionophores and was lower for monensin than for lasalocid (P < 0.05). In exp. 3, lasalocid at 0.5 mg kg-1 of BW did not affect intake or digestion or interact with supplemental com (7.2 g kg-1 of BW) or forage source. In conclusion, level of ionophore maximizing digestible organic matter intake may vary with characteristics of source of low-quality forage. However, when present, effects of ionophores on digestibility or energy intake may be relatively small.