The control of time of oestrus and ovulation and the induction of superovulation in cattle

Abstract

In an attempt to induce superovulation at predetermined times 140 mature parous cows and 27 heifers were treated with progesterone by intramuscular injection (40 mg/day for 17–30 days) or prostaglandin F_2α (PG) by intrauterine injection on each of two consecutive days (500 µg/injection) between the 8th and the 14th day of the oestrous cycle. All animals were given pregnant mare serum gonadotrophin (PMSG) or an equine anterior pituitary extract (HAP) at one of three doses, and human chorionic gonadotrophin (HCG) at a standard dose of 1500 i.u. was given to two-thirds of the cows on either the second or third day after the final progesterone or first PG injection. Following treatment the animals were run with entire bulls and all were slaughtered within 7 days of the end of treatment.

During treatment progesterone effectively inhibited ovulation, while PG caused luteolysis in the vast majority of animals. Ninety-eight cows and 23 heifers were in oestrus within 6 days of the final progesterone or first PG injection, and the majority (87%) within 2–4 days.

All 27 heifers and 129 of the 140 cows ovulated within a few days after treatment, and there was no apparent difference in either time of ovulation or ovarian response between animals which did, and did not, exhibit oestrus.

PMSG and HAP were equally effective in inducing superovulation, but in heifers HAP gave more unruptured large follicles than did PMSG. Overall, the ovulatory response of heifers was greater than that of cows.

HCG, irrespective of when given, had no effect upon either incidence of oestrus or ovarian response.

In cows and heifers which were in oestrus, the respective percentages of eggs shed that were recovered from the reproductive tract, and the percentages of recovered eggs that were fertilized, were 73 and 79, and 78 and 95. There was no effect of any factor other than HCG on either recovery or fertilization rates. In cows but not in heifers, HCG treatment at either time appeared to reduce recovery and fertilization rates.

Where numbers of fertilized cow eggs are required, it is suggested that PMSG in conjunction with PG provides the most simple treatment. Further, it seems desirable to restrict treatment to heifers or young cows.