Estrous Cycle Regulation in the Bovine

Abstract

Three methods of regulating the estrous cycle in Hereford cattle were studied in three separate trials. In Trial 1 an attempt was made to bring 27 Hereford heifers into estrus during a target period of 3 days by altering the naturally occurring cycles, where necessary, by recycling by means of oxytocin injections or by a combined oxytocin and progesterone treatment. Twenty-six of the 27 heifers were brought into estrus and artifically inseminated within an 8-day period. The major cause of this dispersion was the relatively long interval (6–8 days) between the cessation of treatment and estrus in the oxytocin-progesterone treated group, which resulted in 14 animals being in estrus on the 3 days following the target period. Fifty percent of the 26 heifers conceived to the first service. In Trial II a progestational compound, 6-methyl-17-acetoxyprogesterone, was fed in soybean meal to 32 Hereford cows for 20 days. None of these cows came in estrus during the feeding period. Half (16) of the cows came in estrus and were inseminated 3–4 days after cessation of the treatment. An additional 13 cows ovulated without showing estrus. All cows were inseminated, and 25% conceived to the first service. Some of the cows which had “silent” ovulations were inseminated after they had ovulated. Half of the cows in this trial were injected with estradiol at the time of insemination, but the conception rate was not improved by this treatment. In Trial III the oral administration of 17α-ethynyl-17-hydroxy-estr-5(10)-en-3-one in soybean meal did not prove effective in inhibiting estrus and ovulation.