Effects of monensin inclusion into increasing amount of concentrate on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses

Abstract Two experiments evaluated the effects of concentrate amount and monensin inclusion on growth and physiological parameters of early-weaned beef calves consuming warm-season grasses in drylot (Exp. 1) and pastures (Exp. 2). In both experiments, treatments consisted of two concentrate DM amounts (1 or 2% of BW) and two inclusion rates of monensin (0 or 20 mg of monensin/kg of total DM intake). In Exp. 1, 48 Angus × Brahman crossbred early-weaned (EW) beef calves (initial age = 90 ± 13 d; initial BW = 83 ± 12 kg) were distributed in 12 drylot pens (four calves per pen; three pens per treatment) and provided stargrass (Cynodon nlemfuensis) hay (9% CP and 52% IVDOM) at amounts to ensure 10% DM refusals for 56 d. In Exp. 2, 36 Angus × Brahman crossbred EW heifer calves (initial BW = 171 ± 15 kg) were randomly allocated into one of 12 bahiagrass (Paspalum notatum) pastures on a continuous and fixed stocking rate (1 ha and three heifers per pasture; three pastures per treatment) and received daily supplementation of their respective treatments for 84 d. In both experiments, effects of monensin inclusion × concentrate amount were not detected for any variable (P ≥ 0.14), but overall ADG and plasma IGF-1 concentrations were greater (P ≤ 0.05), whereas fecal coccidia egg counts tended (P = 0.09; Exp. 1) or were less (P = 0.05; Exp. 2) for calves offered concentrate with vs. without monensin inclusion. Calves offered concentrate at 2% of BW had greater (P ≤ 0.05) overall ADG (Exps. 1 and 2), herbage mass (Exp. 2 only), in vivo apparent digestibility, total DMI and plasma concentrations of glucose and IGF-1 (Exp. 1 only), less forage DM intake (Exp. 1 only), and no effects on fecal coccidia egg counts (Exps. 1 and 2) compared to calves offered concentrate at 1% of BW. Increasing concentrate amount is an effective management practice to increase ADG and decrease forage DMI in early-weaned calves consuming warm-season grasses, whereas the decrease in fecal coccidia egg count and additional ADG provide evidence that monensin should be supplied to early-weaned calves grazing warm-season pastures and receiving concentrate at 1% of BW or above.