A Comparison of Melengestrol Acetate Fed at Two Dose Levels to Feedlot Heifers

A randomized complete block design was used to compare the effects of feeding melengestrol acetate (MGA) at 0.4 (0.4M) and 0.5 (0.5M) mg/d per head on feedlot performance, estrus activity, and carcass characteristics of commercial beef heifers (n = 1,418; 10 pens/ treatment). Within 48 h of arrival at the research site, heifers were assigned to treatment, processed according to feedlot protocol, and administered an abortifacient. After adaptation to a 95% concentrate diet, MGA was supplied at either 0.4 or 0.5 mg/head daily. Estrus activity was monitored twice daily and summarized as a count of heifers showing estrus within a pen over each 21-d interval throughout the study. Dry matter intake, ADG, G:F, and all other carcass measurements were not different (P > 0.10) between treatments. Overall treatment effects were different only for estrus activity (P = 0.03; 3.2 vs. 2.1% for 0.4M and 0.5M, respectively) and tended to be different for the percentage of dark-cutting carcasses (P = 0.10; 3.0 vs. 1.7% for 0.4M and 0.5M, respectively). Results of this study showed little difference between treatment groups for performance and carcass characteristics of feedlot heifers. The decrease in estrus activity and percentage of dark-cutting carcasses, however, may suggest an economic advantage of feeding a higher level of MGA to finishing heifers.