Influence of Intraluminal Monensin Administration on Performance and Forage Use in Beef Cattle Grazing Early‐Summer Bluestem Range

The successful use of ionophores under extensive grazing conditions requires self‐administration systems. This study was conducted to evaluate the efficacy of monensin administration via a monensin ruminal delivery device (MRDD) for beef cattle grazing early‐summer bluestem range. Performance trials were conducted in 1986 and 1987 (76 and 75 d, respectively) with beef steers managed under three stocking rates (SR). Treatment groups within each pasture were (i) control (forage only = FO); (ii) MRDD; (iii) estradiol 17β implant (E); and (iv) MRDD + E for the 1986 trial and E and MRDD + E for the 1987 trial. Concurrent with the 1987 trial, eight beef heifers with ruminal and esophageal fistulas were assigned to the FO and MRDD treatments to determine effects of monensin administration via an MRDD on forage use characteristics. In 1986, a treatment × SR interaction was observed (P < 0.05) for average daily gain (ADG). Within the 1.75 and 1.5 SR, ADG with MRDD was similar (P > 0.10) to that with FO. In contrast, within the 1.25 SR the MRDD, E and MRDD + E groups gained more (P < 0.05) than the FO group. The E and MRDD + E treatment groups consistently displayed higher (P < 0.05) ADG than the MRDD or FO groups, regardless of stocking rate. In 1987, no treatment × SR interaction was observed for ADG. The MRDD + E steers tended (P= 0.09) to gain more than E steers; however, SR did not influence ADG. Forage organic matter intake, digestibility, liquid flow, and most fermentation characteristics were not different (P > 0.10) for heifers receiving an MRDD compared with nontreated (FO) heifers. Use of an MRDD to administer monensin to grazing cattle tended to promote increased ADG when cattle were managed under the highest SR; however, response was not consistent across years for the two lowest SR. The MRDD treatment resulted in only minor alterations of measured forage use characteristics.