Feedlot Performance of Heifers Fed Melengestrol Acetate and Oxytetracycline Separately and in Combination

Abstract

Twenty-eight crossbred yearling heifers were allotted at random to four groups to study the physiological effect of melengestrol acetate (MGA) and oxytetracycline (OTC), fed separately and in combination, on feedlot performance. All heifers were full-fed a ground snapped corn basal diet calculated to contain 11.6% crude protein, 10.3% crude fiber and 67.5% total digestible nutrients. Lot I served as a negative control; lot II received 75 mg OTC per head per day; lot III received 0.4 mg MGA per head per day; and lot IV received 75 mg OTC plus 0.4 mg MGA per head per day. Treatment did not significantly (P>.05) affect feedlot performance or carcass characteristics; however, the two groups of heifers fed MGA gained 6% faster and required about 9% less feed per unit of gain than non-MGA fed heifers. Estrus was suppressed in MGA treated heifers as reflected by visual observation during the feeding trial, absence of corpora lutea and the presence of greater amounts (P<.05) of follicular fluid. With the exception of a slight improvement in feed efficiency, a combination of OTC and MGA did not appear to be additive for any response criteria studied.