Effects of Chlormadinone Acetate (Estrostat) on Preweaning and Postweaning Growth Rate and Breeding Efficiency of Beef Heifers

Forty-eight heifers averaging 4 months of age were divided randomly into three groups. Sixteen heifers served as controls (group 1) and groups 2 and 3 were injected with 125 mg chlormadinone acetate at 4 and 6 months of age. At 8 months of age, the heifers were weaned and moved to drylot. Group 2 was injected with 250 mg Estrostat at 0, 70 and 140 days and group 3 with 250 mg at 0 and 98 days of the 196-day growing phase. The heifers were then turned to pasture for the breeding phase of the study. The average daily gains of heifers during the preweaning phase were not influenced by Estrostat treatment. The overall gains in drylot were not affected by treatment, but differential rates of gain were observed. Estrostat injection in group 2 (250 mg Estrostat at 0, 70 and 140 days) effectively suppressed estrus throughout the 196-day growing phase. Of those heifers injected at 0 and 98 days, five of 15 heifers exhibited estrus once. The carcass traits measured at slaughter were not significantly influenced by Estrostat treatment. All heifers in group 3 were pregnant at slaughter. The heifers in group 2 exhibited estrus later in the breeding period which possibly resulted in a lower conception rate. These data suggested that growth and development of the reproductive organs were not impaired by repeated Estrostat injections.