183 Ovulatory follicle size, time of ovulation, and pregnancy rates to AI in lactating dairy cows treated with a new gonadotrophin-releasing-hormone-based protocol with lengthened proestrus

Two experiments evaluated ovulatory follicle size, time of ovulation, and pregnancy rates to AI (P/AI) in lactating Holstein cows synchronised with an oestradiol-based protocol or with a new gonadotrophin-releasing hormone (GnRH)-based protocol with lengthened proestrus named Web-Synch. Lactating Holstein cows (n = 39 in Experiment 1 and 256 in Experiment 2) 160.0 ± 7.1 days in milk, producing 35.6 ± 0.8 kg of milk per day, 2.8 ± 0.3 lactations, body condition score 3.1 ± 0.1 (1–5 scale; mean ± s.e.m.) and managed in a confinement system, were randomly allocated into one of two treatment groups. On Day 0, cows in the Control group received 2 mg oestradiol benzoate (Oestradiol, Over) and a vaginal device containing 1 g of progesterone (P4, Sincrover, Over). On Day 6, cows received 150 µg D (+) cloprostenol (PGF, Prostal, Over). On Day 7, P4 devices were removed, and cows received a second PGF, 140 IU recombinant equine chorionic gonadotrophin (reCG, FoliRec, Zoovet) and 1 mg oestradiol cypionate (Estrosinc, Over). Cows in the Web-Synch group were treated with PGF and a P4 device on Day 5, and 10 µg buserelin (GnRH, Gestar, Over) on Day 0; P4 device removal, PGF, and reCG was done on Day 6, and a second dose of PGF was administered on Day 7. Cows in both groups were tail painted for oestrus detection. In Experiment 1, cows were scanned ultrasonographically (Mindray M7) twice daily from P4 device removal to ovulation. In Experiment 2, all cows with >30% of the tail paint rubbed off on Day 9 (48 h after P4 device removal in the Control group and 72 h after P4 device removal in the Web-Synch group) were AI at that time, and cows without the tail paint rubbed off in both groups received 10 µg GnRH and were AI 12 h later. Cows in Experiment 2 were examined for pregnancy 30 days after AI. Data were analysed using the GLM mixed procedure for continuous data in Experiment 1 and for binary data with a logit link in Experiment 2 (InfoStat, 2021). In Experiment 1, the mean (± s.e.m.) interval from P4 device removal to ovulation was longer (P < 0.05) in the Web-synch group (101.6 ± 2.9 h) than in the Control group (78.3 ± 3.1 h), but the diameter of the ovulatory follicle did not differ (P = 0.3; 19.7 ± 0.8 and 18.5 ± 0.8 mm for the Web-Synch and Control groups, respectively). In Experiment 2, expression of oestrus did not differ between groups (Web-Synch: 74.0%, 94/127 and Control: 74.4%, 96/129, P = 0.9), but P/AI was greater (P = 0.01) in the Web-Synch group (54.3%, 69/127) than in the Control group (39.5%, 51/129). In summary, the GnRH-based synchronisation protocol (Web-Synch) resulted in a longer proestrus period and greater P/AI than the conventional oestradiol-based protocol in lactating dairy cows.

Research was supported by Fondo Nacional de Ciencia y Tecnología (FONCYT PICT 2017-4550) and Instituto de Investigación, Universidad Nacional de Villa María.