BEHAVIORAL ACTIVITY, SERUM PROGESTERONE AND FEEDLOT PERFORMANCE OF HEIFERS FED MELENGESTROL ACETATE AND MONENSIN
The effects of melengestrol acetate (MGA, The Upjohn Co., Kalamazoo, Mich.) and monensin (Rumensin, Elanco Division, Eli Lilly & Co., Indianapolis, Ind.) on estrous activity and feedlot performance of 96 yearling Hereford heifers (293 kg) were evaluated. Monensin was included at 33 mg/kg diet, except during the first 14 days when 11 mg/kg was fed; heifers received 0.4 mg MGA/head/day. The four treatments were: (1) no additive (control); (2) monensin; (3) MGA; (4) monensin plus MGA. The heifers were fed a finishing diet containing 66% barley and 30% brome-alfalfa hay for 98 days. Blood samples for progesterone analysis were collected at 0, 7, 14, 21, 63, 70, 77 and 84 days after the start of the experiment. Morning and evening checks for estrous activity were made daily. Organic matter and crude protein digestion coefficients were increased (P < 0.05) by monensin, MGA and monensin plus MGA. Estrous activity was suppressed (P < 0.01) in heifers fed MGA and those fed monensin plus MGA. Serum progesterone concentrations in MGA and monensin plus MGA heifers were 42 and 51% lower (P < 0.01) than control values, respectively. Monensin improved (P < 0.05) feed efficiency by 6%. MGA increased (P < 0.05) daily gain by 12% but had no effect on feed utilization. Heifers fed monensin plus MGA gained 17% (P < 0.05) faster and utilized feed 9% more efficiently than control heifers. These results show that MGA suppressed estrous activity in feedlot heifers, and that improvements in daily gain and feed efficiency were greatest with MGA and monensin, respectively.