An alternative strategy to control the time of ovulation is the ability of exogenous estradiol to induce ovulation in a low-progesterone environment during late diestrus and proestrus. Estradiol cypionate (ECP), an esterified form of estradiol-17b, can be used as part of a TAI protocol. Lactating cows are presynchronized with two injections of PGF2a given 14 days apart with Heatsynch beginning 14 days after the second injection of PGF2o!. Cows are then injected with GnRH followed by PGF2a 7 days later. The ECP (1 mg, i.m.) is injected 24 h after PGF2a and cows are inseminated 48 h later (Fig. 3). Pregnancy rates did not differ between Heatsynch (35.1%) and Ovsynch® (37.1%) protocols.1-4-1 Cows detected in estrus after ECP had a higher fertility than those not detected in estrus before the TAI. Cows in estrus during the first 36 h after ECP injection should be inseminated at detected estrus, and all remaining cows inseminated at 48 h. The elevation of estradiol following ECP injection appears to compensate for a lactational-induced deficiency in estradiol concentrations, and cows expressing estrus are fertile. If cows are anovulatory, the Heatsynch protocol may not be as effective as the GnRH-based Ovsynch® protocol in which GnRH causes the direct secretion of LH. Greater uterine tone, ease of insemination, and occurrence of estrus with the use of the Heatsynch protocol are well received by inseminators.
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