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Feedlot Performance and Pregnancy Inhibition of Heifers Treated with Depot-Formulated Melengestrol Acetate¹

University of Georgia,^2,3,, Tifton 31793-0748

Abstract

In dose titration trials beef heifers received depot-formulated melengestrol acetate (DEPO-MGA^TM) in single s.c. ear injections to determine effects on performance and pregnancy inhibition. In a 112-d feedlot trial, 105 heifers were assigned to light, medium and heavy weight blocks on five treatments: dietary MGA (.5 mg · hd^–1 · d^–1), control (no MGA) or DEPO-MGA on d 1 at .5, 1.0 or 1.5 ml/hd (30, 60 or 90 mg MGA/hd, respectively). A high-energy cracked corn diet was fed to all heifers ad libitum. At d 56 and d 112, ADG and feed/gain were not affected (P > .05) by dietary MGA or DEPO-MGA treatments, although dietary MGA tended to increase 112-d ADG (1.67 vs 1.58 kg) compared with controls. In a pasture trial, 100 beef heifers were assigned by weight to five treatments: control (no MGA) or DEPO-MGA injected on d 1 at .5, 1.0, 1.5 or 2.0 ml/hd (30, 60, 90 or 120 mg MGA/hd, respectively). Heifers were pastured as a single herd and were exposed to fertile bulls from d 7 to d 181. Weights and pregnancy data were recorded at d 62, 90, 134, 181 and 225. Heifer ADG was not affected (P > .05) by DEPO-MGA treatment at any weighing date. By d 90, 90% of control heifers were pregnant, and 100% were pregnant by d 134. Pregnancy inhibition rates of 90% to 100% for each DEPO-MGA treatment at d 62, 90 and 134 were higher (P < .01) than those in control heifers. Pregnancy was inhibited in 90% of heifers on 60-mg and 90-mg DEPO-MGA treatments through d 181, and in 95% of heifers on the 120 mg DEPO-MGA treatment through d 225.

¹ Supported by State and Hatch funds allocated to Georgia Agric. Exp. Sta. and by a Research Grant from The Upjohn Company, Inc., Kalamazoo, MI, which included protocols, Lutalyse, MGA and DEPO-MGA^TM.

² College of Agric. Coastal Plain Sta., Dept. of Anim. Sci.

³ The authors gratefully acknowledge the technical assistance of C. E. Merchant, W. A. Merritt, D. A. Williams and R. C. Cook. The authors appreciate the resource contributions and cooperation of personnel of the Rogers Correctional Institute, Georgia Dept. of Corrections, Reidsville, which permitted this research to be conducted.