Effects of a Single Dose of Direct-Fed Microbials on Performance of Stocker Calves Grazing Annual Cool-Season Grasses1

Previous research has shown that beef calves have less ADG during the first 3 wk of grazing winter wheat pastures. Three experiments were conducted to determine the impact of a single oral dose of direct-fed microbials (DFM) on the overall performance of stocker calves grazing annual cool-season grass pastures. Calves received 0 g (control) or 15 g (DFM) of a gelatinous paste that contained 10 million cfu of bacteria/g of product before initiation of grazing. Within an experiment, control and DFM calves were grazed together in the same pasture. In Exp. 1, steer calves (n = 241) grazed wheat pastures in the spring for 77 d. Calves in the DFM group gained more (P=0.02) BW than did calves in the control group. Experiments 2 and 3 were conducted in the fall. Heifers (n = 53; Exp. 2) and steers (n = 61; Exp. 3) grazed pastures containing a mixture of wheat, triticale, and annual ryegrass for 73 d (Exp. 2) and 83 d (Exp. 3). A single dose of DFM did not increase BW gains during the fall and winter grazing periods. Comparison of stocker performance during the spring and fall grazing periods was confounded by breed of calf, forage source, and seasonal differences in forage chemical composition. A single dose of DFM may improve performance of stocker calves grazing cool-season grasses in the spring, but daily feeding of DFM may be needed during the fall grazing period to overcome the greater plant concentrations of digestible DM and N.