121 Synchronisation of ovulation in lactating dairy cows following a lengthened proestrus in an estradiol/progesterone-based protocol

Two experiments evaluated treatments for synchronisation of ovulation in lactating dairy cows following the conventional oestradiol/progesterone (P4)-based protocol in which oestradiol cypionate (ECP) is administered at the time of removal of a P4 device and cows undergo timed AI (TAI) 48 to 60 h later (short proestrus, Control) and two prolonged proestrus protocols (J-Synch), in which ECP is not administered and cows undergo TAI 72 to 84 h later. Multiparous Holstein cows with a body condition score of 2.9 ± 0.1 (1 to 5 scale), 146.7 ± 4.4 days in milk, producing 41.2 ± 0.6 kg of milk per day and housed in a dry-lot system with total mixed ration feeding were randomly allocated to one of three synchronisation treatments. All cows had a CL or a follicle >10 mm in diameter, detected by ultrasonography. On Day 0, all cows received 2 mg oestradiol benzoate (Oestradiol, Over) and a vaginal device containing 1 g of progesterone (P4; Sincrover, Over). The P4 device was removed on Day 6 in the J-Synch 6 group, on Day 7 in the J-Synch 7 group, and on Day 8 in the Control group. All cows received 150 µg of D-cloprostenol (Prostal, Over) and 400 IU of equine chorionic gonadotrophin (eCG; Novormon, Zoetis) at the time of P4 device removal, and cows in the Control group also received 1 mg of estradiol cypionate (ECP; Estrocinc, Over) at the same time. All cows were tail painted for oestrus detection. In experiment 1, cows (n = 14 per group) were monitored by ultrasonography every 12 h from P4 device removal to determine the time of ovulation. In experiment 2, cows (n = 573) with >30% of the tail-paint rubbed off by 72 h (J-Synch groups) or 48 h (Control group) after P4 device removal, were inseminated at that time, whereas those not in oestrus received 10 µg of Buserelin (gonadotrophin-releasing hormone, Gestar, Over) and underwent TAI 12 h later. Pregnancy per AI (P/AI) was determined by ultrasonography 30 days later. Data were analysed using the GLM mixed procedure for continuous data in experiment 1 and for binary data in experiment 2 (InfoStat, 2019). In experiment 1, the interval from P4 device removal to ovulation was longer (P < 0.05) in the J-Synch groups (J-Synch 6: 102.0 ± 3.7 h, and J-Synch 7: 99.0 ± 3.7 h) than in the Control group (78.0 ± 4.1 h), but the diameter of the ovulatory follicle did not differ (P > 0.2) among groups (13.4 ± 1.0, 11.9 ± 1.0 and 12.1 ± 1.1 mm for the J-Synch 6, J-Synch 7, and Control groups, respectively). In experiment 2, overall oestrus expression was 69.8% (400/573) and P/AI did not differ (P > 0.2) between cows in oestrus (32.2%, 129/400) and those not in oestrus (36.9%, 64/173) at the time of TAI. However, neither oestrus expression nor pregnancy rates differed among groups (J-Synch 6: 64.7%, 119/184 and 33.2%, 61/184; J-Synch 7: 70.0%, 133/190 and 32.1%, 61/190; and Control: 74.4%, 148/199 and 35.7%, 71/199). In conclusion, the three TAI protocols were equally efficacious in lactating dairy cows.