Calving rate of cows following insemination after a 12-day treatment with silastic coils impregnated with progesterone.

Farm trials were conducted to determine if the calving rate was different for control cows inseminated with frozen semen at estrus in comparison to cows treated with progesterone for 12 days and bred at the ensuing estrus. Intravaginal silastic coils impregnated with progesterone were used and an injection of 5 mg estradiol benzoate and 50 mg progesterone in corn oil was given at the time of insertion of the coils. Calving rate was not different when 315 Friesian dairy cows were allocated to control or to 12-day progesterone treated groups or when 253 Friesian cows were treated with progesterone and calving rate compared to that obtained for 228 control cows in the same herds. The calving rate for 156 heifers treated and inseminated at estrus was 58%. To determine if animals could be bred on a fixed-time basis, Hereford cross heifers were allocated at random to a control group and three 12-day progesterone treated groups. Animals in the control and one progesterone treated group were inseminated at estrus. Animals in the other two progesterone treated groups were inseminated 48 hr after treatment, and animals in one group received an injection of GnRH 36 hr after the end of treatment. There were no differences in pregnancy rate at slaughter for controls and progesterone-treated heifers bred at estrus or after GnRH injection. Fertility was reduced following a single insemination 48 hr after treatment (P<.05). Finally, 100 Hereford cross heifers were allocated at random to a