First Service Pregnancy Rates Following Post‐AI Use of hCG in Ovsynch and Heatsynch Programmes in Lactating Dairy Cows

Lactating dairy cows (n = 667) at random stages of the oestrous cycle were assigned to either ovsynch (O, n = 228), heatsynch (H, n = 252) or control (C, n = 187) groups. Cows in O and H groups received 100 μg of GnRH agonist, i.m. (day 0) starting at 44 ± 3 days in milk (DIM), and 500 μg of cloprostenol, i.m. (day 7). In O group, cows received 100 μg of GnRH (day 9) and were artificially inseminated without oestrus detection 16–20 h later. In H group, cows received 1 mg oestradiol benzoate (EB) i.m., 24 h after the cloprostenol injection and were artificially inseminated without oestrus detection 48–52 h after the EB injection. Cows in C group were inseminated at natural oestrus. On the day of artificial insemination (AI), cows in all groups were assigned to subgroups as follows: human Chorionic Gonadotrophin (O‐hCG) (n = 112), O‐saline (n = 116), H‐hCG (n = 123), H‐saline (n = 129), C‐hCG (n = 94) and C‐saline (n = 93) subgroups. Cows in hCG and saline subgroups received 3000 IU hCG i.m. and or 10 ml saline at day 5 post‐AI (day 15), respectively. Pregnancy status was assessed by palpation per rectum at days 40 to 45 after AI. The logistic regression model using just main effects of season (summer and winter), parity (primiparous and pluriparous), method₁ (O, H and C) and method₂ (hCG and saline) showed that all factors, except method₁, were significant. Significant effects of season (p < 0.01), hCG and parity (p < 0.01), and a trend of parity and season (p < 0.1) were detected. A clear negative effect of warm period on first service pregnancy rate was noted (p < 0.01). The pregnancy rate was the lowest in the H protocol during warm period (p < 0.05). Treatment with hCG 5 days after AI significantly improved pregnancy rates in those cows that were treated with the H protocol compared with saline treatments (41.5% vs 24.8%; p < 0.01). O and H were more effective in primiparous than in pluriparous cows (46.1% vs 29.9%; p < 0.1 and 43.6% vs 24.6%; p < 0.01). First service pregnancy rates were higher in primiparous hCG‐treated than in pluriparous hCG‐treated cows (57.9% vs 32.3%; p < 0.01). The pregnancy rate was higher for the hCG‐treated cows compared with saline‐treated cows during warm period (37.9% vs 23.6%; p < 0.001).