Effect of estradiol cipyonate on time of ovulation, luteal function and pregnancy rate in protocols for FTAI in beef cattle

The objective was to determine the effect of different doses of estradiol cypionate on ovulation time, progesterone concentration (P4) and pregnancy rate in fixed-time AI (FTAI). Experiment 1, was carried out in 45 multiparous Hereford cows, with 60-70 days postpartum, body condition score (BCS) 3.3 ± 0.4 (1 to 8 scale) in anestrus. All females received an intravaginal progesterone device with (0.5g DIB, Syntex, Argentina) for 7 days and 2 mg of estradiol benzoate (Gonadiol, Syntex) im at began. One dose of 500 μg of sodium cloprostenol (Ciclace DL, Syntex) and 400 IU equine chorionic gonadotropin (Novormon, Syntex) im at the device removal. In that moment were divided into three experimental groups (n = 15 per group) according to BCS and randomly distributed, 1 mg or 0.5 mg to receive ECP (Cipiosyn, Syntex) at DIB removal or not, to receive ECP. Ovaries were monitored by ultrasonography,(WED-9618V, Well.D, China), every 12 h from removal DIB until ovulation, and every 24 h after ovulation and for 14 days. Daily blood samples were taken from DIB removal at the Day 14 after ovulation. Calves received nose plates for 10 days. In Experiment 2, pregnancy rate was determined as compared to only two experimental groups treated with ECP described in Experiment 1. Multiparous crossbreed Hereford and Angus cows (n = 4.156), with ≈ 60-90 days postpartum, 92.2% (3835/4156) in anestrus were divided into two groups to receive 1 mg (n = 2044) or 0.5 mg of ECP (n = 2112) at DIB removal. Insemination was done at 46-50 h or between 52-56 h of DIB removal (FTAI AM and PM, respectively) in both groups, blocked by animals, insemination time, technicians, bull and semen. Statistical analysis was performed by GLMM. In Experiment 1, ovulation rate (ovulated/treated) was similar between the groups, however, there was a tendency (P = 0.06) between cows receiving (93.3%, 28/30) vs. those did not receive ECP (73.3%, 11/15). The interval between DIB removal and ovulation was higher with 0.5 mg or not received ECP (66.7 ± 2.5h and 69.1 ± 2.9h, respectively) than 1 mg (58.7 ± 2.7h) (P < 0.05). Cows that received ECP and ovulated (ECP 0.5 mg; n = 15 and ECP 1 mg; n = 13) had higher P4 concentrations during first two weeks after ovulation (6.3 ± 0.2 and 6.5 ± 0.2 ng/mL) compared to cows that did not receive ECP and that ovulated (n = 11) (5.3 ± 0.3 ng/mL) with a significant effect on days 13 and 14 (P < 0.05). In Experiment 2, pregnancy rate was greaterin cows that received 0.5 mg compared to 1 mg ECP (60.4% vs. 50.4%, 1227/2112, 1031/2044 respectively; P < 0.01). There was interaction between dose of ECP and insemination time, with 1 mg pregnancy rate was higher with FTAI in AM (54.0%, 564/1045) vs. PM (46.7%, 467/999; P < 0.01). However, with 0.5 mg of ECP no differences were found in performing FTAI AM (61.1%, 645/1056) vs. PM (59.8%, 632/1056). The administration of ECP enhances ovulatory rate in Bos taurus beef cattle in anestrus. When early weaning by nose plates was used, administration of 0.5 mg instead of 1 mg ECP resulted in higher pregnancy rate for inseminations all day along.