Studies on the underlying physiology and treatment of anovulatory condition in dairy cattle

Many mammals, including cattle, can develop ovarian follicular cysts but the physiological mechanisms leading to this condition remain undefined. In the physiological studies, we hypothesized that follicular cysts can develop because estradiol will induce a GnRH/LH surge on one occasion but progesterone exposure is required before another GnRH/LH surge can be induced by estradiol. These hypotheses were tested by using a single estradiol injection to induce an LH surge in cows that did not have an ovulatory follicle. We further tested whether the lack of an estradiol-induced GnRH/LH surge was due to a lesion at the hypothalamus or pituitary and whether progesterone exposure could eliminate this lesion. Control and progesterone-treated cows had a similar LH surge and ovulation when treated with GnRH. In contrast, only progesterone-treated cows had an LH surge following estradiol. Thus, an initial GnRH/LH surge can be induced with high estradiol but estradiol induction of a subsequent GnRH/LH surge requires exposure to progesterone. This effect is mediated by the hypothalamus and not the pituitary gland as evidenced by similar LH release in response to exogenous GnRH. In the practical study, we evaluated percentage cows with anovulatory condition and examined follicular sizes in anovulatory cows in one commercial dairy herd (n = 316 cows) by using ultrasonography and circulating progesterone concentration. Of 64 anovulatory cows, 20% had follicles ≥25 mm that might be considered cystic, 58% had 15- to 24-mm follicles, and 22% had 9- to 14-mm follicles. We further compared synchronized ovulation (Ovsynch) vs. detection of estrus on fertility of anovulatory and ovulatory cows. Conception rates (CR) were greater for ovulatory than anovulatory cows regardless of treatment but CR between Ovsynch and estrous detection groups for ovulatory or anovulatory cows were similar. The reduced CR was not due to reduced response to Ovsynch as evidenced by a high-synchronized ovulation rate after Ovsynch. Thus, anovulation in lactating dairy cows was mainly characterized by follicle growth to larger than ovulatory size with an underlying physiology of lack of estradiol response in the hypothalamus. Although progesterone or Ovsynch could resolve this anovulation, CR remained low in anovulatory cows bred after Ovsynch.