Monensin is a growth promoting feed additive that has been widely used in beef cattle production; however, there is limited amount of information about the monensin effects on performance of growing, young beef calves in tropical/subtropical environments. Three studies were conducted to evaluate the effects of supplemental monensin on growth, physiological parameters and coccidiosis infestation of early-weaned calves grazing annual ryegrass (Lolium multiflorum; Exp. 1) or bahiagrass (Paspalum notatum; Exp. 2), and in calves offered creep-feding while grazing limpograss pasture (Hemarthria altissima; Exp. 3). Exp. 1 was conducted in 2018 and 2019. Thirty-two Brangus early-weaned (107 ± 18 days of age, BW = 84.6 ± 14 kg) were randomly distributed into 8 annual ryegrass pastures (4 calves/pasture/year). Treatments were control (no monensin) or 20 mg of monensin/kg of DM intake distributed in a completely randomized design with 4 replicates/treatment. Calves were supplemented with concentrate at 1% of BW (DM basis). Experiment 2 was conducted in 2 phases. During phase 1, 48 Brangus calves (92 ± 14 days of age; 97 ± 12 kg of BW) were early-weaned and randomly assigned into 1 of 8 bahiagrass pastures (6 calves/pasture). Treatments were similar to Exp. 1 and early-weaned calves were supplemented with concentrate at 2% of BW (DM basis). During phase 2, 12 calves/treatment (6 steers and 6 heifers) were randomly assigned in a drylot facility for 17 days to evaluate forage and total DM intake, and apparent DM digestibility. Calves remained on their respective treatment previously assigned in phase 1 and provided daily free-choice access to ground stargrass (Cynodon nlemfuensis) hay. In Exp. 3, 24 cows Angus x Brahman crossbreeding (BW = 488 ± 10 kg) and their respective calves (BW = 154 ± 6 kg BW; age ~ 5 months) were randomly assigned into 1 of 8 pastures (3 cows and 3 calves/pasture) of limpograss. Treatments consisted of calves creep-fed 0.4 kg/day of soybean meal with or without monensin (20 mg / kg, DM basis), allocated in a randomized complete block design with 4 replicates/treatment. In Exp. 1, monensin increased ADG (0.71 vs. 0.54 kg/day) and decreased the coccidiosis infestation (0.40 vs. 1.15 log10 eggs/g of feces) in feces compared to control treatment. Similar results were observed in Exp. 2 and monensin increased ADG (0.94 vs. 0.80 kg/d) and decreased coccidiosis infestation (0.33 vs 1.14 log10 eggs/g of feces). However, in Exp. 3, calf’ ADG and coccidiosis infestation were not affected by monensin compared to the control treatment (0.78 kg/day and 0.19 log10 eggs/g of feces, respectively). Effects of monensin were not detected for plasma concentrations of glucose, insulin, IGF-1 and urea nitrogen in Exp. 1, 2, and 3. In conclusion, the use of monensin may be a strategy to decrease coccidiosis infestation and improve the performance of early-weaned calves receiving greater levels of concentrate. Monensin may not be effective on calves receiving creepfeeding with limited levels of concentrate and reduced coccidiosis infestation.
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