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The implications of spontaneous versus synchronized ovulations on the reproductive performance of highproducing dairy cows
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In species classified as induced ovulators, the LH surge is induced by the act of coitus thereby precisely timing insemination relative to ovulation. Lactating dairy cows are spontaneous ovulators. Thus, the LH surge is induced by a cascade of endocrine events beginning with an increase in follicular estradiol in the absence of luteal progesterone, a GnRH surge from the hypothalamus is then followed by an LH surge from the anterior pituitary, followed by ovulation. Increased estradiol in the absence of progesterone causes behavioral estrus which is manifested by increased physical activity and standing to be mounted. Detection of behavioral estrus either visually, through the use of estrus detection aids, or via automated activity monitoring systems is widely used to determine timing of AI. Cow-related factors that limit service rates include the association of high milk production and duration of estrus, ovulation failure after estrus, ovulation in the absence of behavioral estrus, and anovular conditions which affect 20% to 30% of dairy cows at the end of the voluntary waiting period. One factor that limits fertility to spontaneous estrus is the high variation among cows in the timing of ovulation relative to increased activity resulting in imprecise timing of AI relative to ovulation. Another factor is the high rate of hepatic metabolism of estradiol and progesterone associated with high feed intake in high-producing cows. Under this endocrine milieu, development of preovulatory follicles from deviation until ovulation occurs in a prolonged low-progesterone environment marked by high frequency LH pulses that overstimulate the oocyte and compromise fertility similar to a persistent follicle. Development of the Ovsynch protocol turned dairy cows into induced ovulators, which allowed for increased service rates and precise timing of AI relative to ovulation. Subsequent modifications of the Ovsynch protocol, including presynchronization strategies and complete induction of luteal regression, lead to fertility programs that yield greater fertility than that of a detected estrus in high-producing dairy cows.
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