Effects of monensin dose from a self-fed mineral supplement on performance of growing beef steers on forage-based diets

ABSTRACT Objective The objective of this research was to determine the effect of differential monensin (Rumensin, Elanco Animal Health, Greenfield, IN) intake in a free-choice loose mineral on performance of grazing beef steers. Material and Methods Treatments consisted of offering ad libitum access to minerals (AMPT-A, ADM Animal Nutrition, Quincy, IL) supplying 0 (0M), 882 (800M), or 1,764 (1600M) g of monensin/t with targeted daily monensin intakes of 0, 100, or 200 mg of monensin per steer. Steer calves (n = 605, BW = 278 ± 27 kg) were fed hay with a hand-fed supplement (block 1); grazed wheat (Triticum aestivum) pasture (blocks 2, 3, and 6); or bermudagrass (Cynodon dactylon) pasture (blocks 4 and 5). There were no treatment × diet interactions (P ≥ 0.97); therefore, data were pooled across blocks for statistical analysis. Results and Discussion Steers offered 800M and 1600M had greater (P ≤ 0.02) final BW, ADG, and total BW gain compared with 0M steers fed mineral without monensin. Mineral intake decreased linearly (P Implications and Applications Monensin supplementation improved growth performance compared with an unmedicated control within the doses observed in this experiment and regardless of basal forage quality. Intake of a self-fed monensin-containing mineral can be decreased by at least 50% without affecting performance response to monensin of steers on forage-based diets.