SYMPOSIUM: PHYSIOLOGY, LACATION, AND REPRODUCTION Mechanisms that Prevent and Produce Double Ovulations in Dairy Cattle1

This review integrates information on follicular and hormonal physiology and epidemiology into a novel physiological model for regulation of the ovulation rate in lactating dairy cows. First, the basic mechanisms that produce a single ovulation are examined. Follicular deviation is a critical new concept in our understanding of selection of a single dominant follicle. Follicular devi- ation is characterized by an abrupt deviation in the growth rates between the two largest follicles when the future dominant follicle reaches a diameter of 8.5 ± 1.2 mm (mean and SD). The mechanisms involved in this selection process are not completely defined but appear to involve acquisition of LH receptors on granulosa cells of the dominant follicle, increased estradiol production by the dominant follicle, and inhibition of circulating FSH concentrations. Second, lactation number and milk production were found to be critical epidemiologi- cal factors associated with increased ovulation rate and twinning in dairy cattle. Finally, high steroid metabo- lism is proposed as the critical link between high milk production and double ovulation. It is proposed that high milk production increases steroid metabolism due to increased blood flow to the digestive tract and subse- quently to the liver. The liver represents the primary site of steroid metabolism, and blood entering the liver is cleared of steroids. At the time of selection of the dominant follicle, the normal increase in circulating estradiol concentrations and subsequent depression in circulating FSH is blunted due to estradiol metabolism. Thus, FSH remains elevated for a time sufficient to allow follicles to undergo the physiological changes nec- essary to proceed to ovulation.