Abstract
Fertility of lactating dairy cows has declined during the past 40 years yet fertility of heifers has remained constant. Lactating dairy cows have larger follicles and corpora lutea compared to heifers yet significantly less estrogen and progesterone concentrations in circulation. Follicles must attain greater age and size to have sufficient concentrations of estrogen to induce estrus and an LH surge. This likely compromises fertility of cows that are artificially inseminated following a detected estrus. New Ovsynch programs are designed to initiate Ovsynch near day 6 or 7 of the estrous cycle to control follicle growth, increase number of corpora lutea, and subsequent concentrations of progesterone in vivo prior to the PGF of Ovsynch. Cows must be pre-synchronized in order initiate Ovsynch on day 6 or 7 of the cycle in the greatest percentage of cows possible. The 14/11 Presynch/Ovsynch program improves the percentage of cows that are near day 6 or 7 of the estrous cycle at the start of Ovsynch, and thus enhances ovulation to the first GnRH of Ovsynch and progesterone concentrations at time of PGF of Ovsynch seven days later.
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