Theriogenology Causes for Decreased Fertility in Out-of-season Mated Ewes

The interval from onset of estrus to preovulatory luteinizing hormone (LH) release, conception and fertilization rates, and number of accessory spermatozoa per ovum at 48 hr postmating in untreated cyclic ewes and fn progestogen-pregnant mare serum gonadotropin (PMSG) treated, anestrous ewes were compared in efforts to identify sources of lowered fertility for matings induced in anestrous ewes with exogenous hormones. Blood samples for LH determination were collected at 0, 2, 4, 6, 8, 10, 12, and 24 hr after the onset of estrus. Conception and fertilization failure rates were determined at 48 hr, 12 days, or parturition. The progestogen-PMSG treated ewes had a shorter interval from onset of estrus to preovulatory LH release, lower conception rates, and fewer accessory spermatozoa than cyclic ewes had. Conception failure, rather than embryonic mortality, was the major cause of reduced fertility for the out-of-season mated ewes and apparently resulted from insufficient viable spermatozoa in the oviducts to fertilize the ova. INTRODUCTION Many studies have been conducted to determine the feasibility of inducing fertile matings in seasonally anestrous ewes by the use of exogenous hormones. The treatment of anestrous ewes with progestogens followed by pregnant mare serum gonadotropin (PMSG) has resulted in a high percentage of ewes showing behavioral estrus but only moderate conception rates (l-3). The objective of this experiment was to identify the source and physiological basis for the lowered conception rates of anestrous ewes after hormone therapy.