Administration of gonadotropin-releasing hormone (GnRH) induces a surge of luteinizing hormone and ovulation in a variety of species, including human beings. Our objectives were to determine the effect of follicle size at the time of ovulation on corpus luteum function and establishment and maintenance of pregnancy in cows in which ovulation was either spontaneous or induced with GnRH. GnRH-induced ovulation of follicles Շ11 mm in diameter resulted in decreased pregnancy rates and increased late embryonic mortality. This decrease in fertility was associated with lower circulating concentrations of estradiol on the day of insemination, a decreased rate of increase in progesterone after insemination, and, ultimately, decreased circulating concentrations of progesterone. In contrast, ovulatory follicle size had no apparent effect on fertility when ovulation occurred spontaneously. Follicles undergoing spontaneous ovulation do so at a wide range of sizes when they are physiologically mature. Therefore, administration of GnRH to induce ovulation likely initiates a preovulatory gonadotropin surge before some dominant follicles attain physiological maturity. GnRH-induced ovulation of follicles that are physiologically immature has a negative impact on pregnancy rates and late embryonic͞fetal survival. These observations in cattle may have implications for assisted reproductive procedures in human beings.ovulation ͉ cattle ͉ artificial insemination ͉ embryonic mortality ͉ fertility P rocedures that control the timing of ovulation in human beings and other mammals are of enormous value in advancing the use of assisted reproductive technologies. In cattle, several protocols are effective at controlling the estrous cycle and reducing the time required to detect estrus (1-3), but the timing of ovulation is imprecise, which makes it difficult to inseminate cows at a fixed time. When fixed-time insemination protocols (protocols that synchronize ovulation) are attempted, gonadotropin-releasing hormone (GnRH) is used to induce ovulation. In some synchronization protocols, GnRH is administered 9 days before insemination to induce ovulation and corpus luteum (CL) formation and to initiate a new follicular wave. Two days before insemination, prostaglandin F 2␣ is administered to induce luteolysis, and 48 h later, GnRH is administered to induce ovulation of the preovulatory follicle (4, 5). Insemination is performed at the time of the second GnRH injection (4) or 16-24 h after the second GnRH injection (5).Bovine follicles achieve ovulatory capacity at Ϸ10 mm in diameter. However, a larger dose of luteinizing hormone is required to induce ovulation of a 10-mm follicle than to induce ovulation of larger follicles (6). In cattle, the efficiency of a single injection of GnRH to induce ovulation and thereby synchronize the initiation of the subsequent follicular wave is only 66% when evaluated across all stages of the estrous cycle (7). Because of this variation in ovulatory response, we hypothesized that considerable variation would e...

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