Fertility programs for lactating dairy cows, their physiological basis, and the factors that are critical for their success

Lactating dairy cows have unique reproductive responses compared to when they were heifers that result in distinctly different reproductive measurements and pregnancy outcomes that can be partially overcome with pharmacological strategies. These parameters include circulating progesterone and estradiol concentrations, ovulatory follicle and corpus luteum diameter, incidence of anovulation and double ovulations, time in estrus, pregnancies per artificial insemination and pregnancy losses. Circulating concentrations of progesterone during diestrus are approximately half that in cows compared with heifers. This marked difference in progesterone is likely the explanation for an increased size in diameter of the ovulatory follicle and incidence of double ovulations in cows compared with heifers. Differences in diameter of the ovulatory follicle may explain why cows have greater corpora lutea diameters compared to heifers. The increase in double ovulations appears to be a key driver in the increase in twinning and pregnancy loss as dairy heifers transition to primi-and multiparous cows. Reduced estradiol concentrations in cows at time of estrus helps to explain the decreased duration of estrus in cows compared with heifers. Concentrations of progesterone during growth of the ovulatory follicle may be a key driver in differences in pregnancies per AI in cows compared with heifers. The difference in circulating progesterone may be related to LH overstimulation of the oocyte/cumulus complex in cows compared to when they were heifers. Pharmacological strategies have been developed in lactating dairy cows to manipulate ovarian development to create a hormonal environment similar to that of heifers. Three primary strategies, Presynch-11, G6G and Double Ovsynch appear to enhance fertility of dairy cows. This review discusses how these three strategies manipulate ovarian development similar to that of heifers and why compelling data indicate these programs should be referred to as "fertility programs."