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Induction of Estrus and Ovulation in Swine during Pituitary Suppression with Methallibure¹

ARC Unit Reproductive Physiology and Biochemistry, Cambridge, England

Abstract

Normal occurrence of estrus and ovulation was inhibited in 45 mature gilts by oral administration of 100 to 200 mg. methallibure per gilt per day for 25 to 30 days. All animals received a single subcutaneous injection of PMS on the 20th day during treatment and some were also injected with 500 I.U. HCG 96 hr. after PMS. Estrus was induced in 89% of the gilts 4 to 6 days after PMS. Ovulation occurred in 79% of gilts injected with PMS alone and in 100% of gilts injected with PMS and HCG. Average ovulation rates following 1,500, 2,000 or 3,000 I.U. PMS were 19.4, 22.8 and 36.7, respectively, but there was wide variation within groups. Injection of HCG reduced significantly the number of unovulated follicles larger than 5 mm. in diameter in all groups. It is suggested that the primary mode of action of methallibure in depressing ovarian activity is to suppress endogenous release of FSH. Endogenous release of LH may not be blocked completely, however, since a large proportion of gilts ovulated following injection of PMS alone despite continued treatment with methallibure.

¹ This investigation was supported in part by U.S.P.H.S. Special Fellowship IF3 HD-28, 408 from the National Institutes of Health, and Pida Senior Fellowship from the Royal College of Veterinary Surgeons Awarded to one of us (B.N.D.) while on Sabbatical leave from the University of Missouri. The Authors are Grateful to Mr. Derek Booth for Making the progesterone determinations.