Effect of estradiol cypionate on estrus expression and pregnancy in timed artificially inseminated beef cows

Abstract

Several studies have shown that cows displaying estrus before timed artificial insemination (TAI) are more likely to become pregnant. However, the most frequently used ovulation-inducing agents by South American beef operations, estradiol esters, may artificially induce estrus in treated cows. The hypotheses tested in the present study were: 1) estradiol cypionate (EC) treatment will induce estrus in ovariectomized cows, and 2) cows that exhibit estrus following administration of EC and eCG will have lower P/AI compared to those that do not receive EC. In Experiment 1, 14 ovariectomized cows were used in a cross-over design with an interval of 30 d between replicates. On Day 0, all cows received 2 mg of estradiol benzoate and a CIDR device and 12.5 mg of Dinoprost (PGF2α analogue) on Day 7. On Day 9, the CIDR devices were removed and cows were placed at random into one of two experimental groups and administered either 0.5 mL of canola oil (Control, CTL group, n = 14) or 1 mg of EC (EC group, n = 14). Estrotect estrus detection patches were used and cows were observed 4 times daily to determine the occurrence of estrus. More EC-treated cows showed estrus (12/14, 85.7%) than canola oil-treated cows (0/14; P < 0,001). The interval from EC treatment to estrus was 46 ± 3.7 h. In Experiment 2, 804 postpartum Nelore cows were treated as in Experiment 1, except that all cows received 300 IU of equine chorionic gonadotrophin at CIDR removal, and were randomly divided into 3 groups: 1) EC group (n = 269) received 1 mg i.m. of EC on Day 9; 2) EC-GnRH group (n = 269) received EC on Day 9, and were inseminated on observation of estrus, while those not showing estrus received GnRH on Day 11; and, 3) Estrus-GnRH group (n = 266) did not receive EC on Day 9 and were inseminated on observation of estrus. Those in Groups 2 and 3 that did not show estrus received GnRH on Day 11 at the time of TAI. The proportion of cows showing estrus was greater (P < 0.0001) in EC (68.7%, 185/269) and EC-GnRH (57.2%; 154/269) groups than in the Estrus-GnRH group (35.7%, 95/266). However, no difference in P/AI was observed among the groups (P = 0.87). These results show that EC will induce estrus in ovariectomized cows, and that EC treatment increased the proportion of intact cows exhibiting estrus in a TAI protocol without reducing P/AI.