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

Objective: The objective of this research was to determine the effect of differential monensin (Rumensin, Franco 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, II.) 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 (Triticom aestiron) pasture (blocks 2, 3, and 6); or hermudagrass (Cynodon ductylon) pasture (blocks 4 and 5). There were no treatment x 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 IINV gain compared with OM steers fed mineral without monensin. Mineral intake decreased linearly (P < 0.01) as monensin concentration increased, providing monensin doses ranging from 0 to 200 mg per animal daily. Reduced monensin intake for 800M versus 1600M treatments did not affect (P > 0.55) steer BW or gain performance. 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.