Effects of Monensin supplementation on forage intake and performance of steers grazing smooth bromegrass

Abstract Text: Two experiments were conducted to determine the effects of monensin supplementation on steers grazing smooth bromegrass pastures. In Exp. 1, crossbred yearling steers (n=60; BW = 335 ± 32 kg) were utilized in a completely randomized design and assigned to one of 8 treatments. The treatments were arranged in a 2 x 4 factorial arrangement with one factor being inclusion or absence of monensin and the other factor being daily supplementation of modified distiller grains plus solubles (MDGS) at .05, 0.4, 0.6, and 0.8% of BW. Cattle were managed in an intensive rotational smooth bromegrass grazing system, and were rotated through 4 paddocks every 4 – 6 days from April 27, 2012 through August 24, 2012 (119 days). Steers were limit fed and weighed at the beginning and the end of the trial with intermediate weights taken on days 24, 60, and 90. In Exp. 2, cannulated steers (n=6; BW= 394 kg) were utilized in a switch back design experiment to determine the effects of monensin supplementation on forage dry matter intake (FDMI) of grazing steers. The steers were assigned randomly to one of 2 treatments: supplementation with or without monensin. Steers were individually supplemented with 3 lb MDGS DM (0.3% BW). Steers were bolused 200 mg monensin daily via bolus inserted into the rumen.  Titanium dioxide was used as a marker to estimate fecal output and was ruminally dosed daily at 10g. There were six, 21- day grazing periods from May 3, 2013 to September 13, 2013. At the end of each period fecal samples, rumen samples, and diet samples were collected to estimate fecal output, VFA ratios, and digestibily.  Estimated fecal output and smooth bromegrass IVDMD was used to calculate FDMI. In Exp. 1, monensin supplementation had no effect on ADG (P = 0.56). In Exp. 2, steers receiving monensin had an estimated average forage intake of 6.29 kg (SE 0.25). Steers not supplemented with monensin had an estimated average forage intake of 6.77 kg (SE 0.25). While supplementation of monensin decreased FDMI 7%, it was not detected (P= 0.19).    

Keywords: forage, intake, monensin