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The Upjohn Company, Kalamazoo, Michigan
Abstract
Three experiments were conducted to study the effects of oral melengestrol acetate (MGA) on feedlot performance of heifers and steers. While all levels of MGA that were studied in intact heifers provided improvements in both weight gains and feed efficiencies, best results were obtained within an approximate daily dosage range of 0.35 to 0.50 mg. Within this range weight gain improvements from 6.2 to 9.5% over controls were not significantly different, while an improvement of 18.1% in one trial was significant (P<.05).
Heifers which were found to be sexually mature and then spayed grew more slowly and had poorer feed efficiency than mature intact heifers. The MGA treatment of spayed heifers had no significant effect on growth or feed efficiency. These results, along with data on follicular size, substantiate the hypothesis that MGA caused increased weight gain by allowing continual endogenous estrogen secretion.
A comparison of the weight gain response of MGA-treated heifers for the first and second period of a 198-day experiment indicated that the response to MGA was more pronounced, relative to control, (P
.05) during the final period (107 days). On this basis it was concluded that a greater response was obtained as heifers became more mature.
Heifers which received MGA throughoul an entire 198-day feeding period showed as much growth response during the final 107 days as did heifers which received a similar level of MGA during only this final period. The MGA treatments of heifers had no significant effect on carcass weights, grades or dressing percent.
Steers which received an average daily dose level of 0.35 mg. of MGA during a 198-day experiment had a slight but nonsignificant decrease in growth, while at a level of 1.74 mg. the decease approached significance (P.05).
1 Agricultural Products Division, Animal Research and Development.
3 6-dehydro-16-methylene-6-methyl-17a-acetoxyprogesterone.
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