Mechanisms that prevent and produce double ovulations in dairy cattle

This review integrates information on follicular and hormonal physiology an d epidemiology into a novel physiological model for regulation of the ovula tion rate in lactating dairy cows. First, the basic mechanisms that produce a single ovulation are examined. Follicular deviation is a critical new co ncept in our understanding of selection of a single dominant follicle. Foll icular deviation is characterized by an abrupt deviation in the growth rate s between the two largest follicles when the future dominant follicle reach es a diameter of 8.5 +/- 1.2 mm (mean and SD). The mechanisms involved in t his selection process are not completely defined but appear to involve acqu isition of LH receptors on granulosa cells of the dominant follicle, increa sed estradiol production by the dominant follicle, and inhibition of circul ating FSH concentrations. Second, lactation number and milk production were found to be critical epidemiological factors associated with increased ovu lation rate and twinning in dairy cattle. Finally, high steroid metabolism is proposed as the critical link between high milk production and double ov ulation. It is proposed that high milk production increases steroid metabol ism due to increased blood flow to the digestive tract and subsequently 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 conc entrations and subsequent depression in circulating FSH is blunted due to e stradiol metabolism. Thus, FSH remains elevated for a time sufficient to al low follicles to undergo the physiological changes necessary to proceed to ovulation.