Impact of injectable long-acting progesterone and/or hCG three days after TAI on the conception rate of lactating Holstein cows

The aim of the present study was to evaluate the effects of administration of injectable long-acting progesterone (P4LA), associated or not with hCG treatment after timed-AI on conception rates of dairy cows. The study was conducted in two commercial farms (Farm A: 27.4L of milk/cow/day and free stall system and Farm B: 40.9L of milk/cow/day and corss ventilation system), both in Sao Paulo, Brazil. On a random day of the estrous cycle, 908 Holstein cows received a synchronization of ovulation treatment [D0: P4 (Sincrogest®, Ourofino Agronegocio) + EB (2 mg, i.m., Sincrodiol®, Ourofino Agronegocio); D8: device removal + PGF (0.53 mg, im, Sincrocio®, Ourofino Agronegocio) + ECP (1.0 mg, im, SincroCP®, Ourofino Agronegocio); D10 AM: TAI]. Three days after TAI, the animals were homogeneously distributed according to the daily milk yield, days in milk, parity and body condition score and divided into four experimental groups (2x2 factorial): 1. Control, cows receiving no additional treatment (n=250); 2. P4LA-900, treatment with 900 mg i.m. of P4LA (Sincrogest injetavel®, Ourofino Agronegocio; n=245); 3. hCG, treatment with 2,000 IU i.m. of hCG (Chorulon®, MSD Schering, hCG) and 4. P4LA+hCG, treatment with both treatments (n=250). Data analysis was performed with the procedure GLIMMIX of SAS 9.3®. There was no effect of hCG or an hCG-by-P4LA interaction for none of the evaluated variables. Greater conception rate 30 days after TAI (CR30) was observed in cows treated with P4LA-900 (34.3 vs. 26.6%; P=0.002), and these results were independent from hCG treatment. Both the conception rate at 60 days (CR60) and the rate of pregnancy loss between 30 and 60 days after TAI (PL) were not affected by P4LA-900 or hCG, P>0.05. There was an interaction between P4LA-900 treatment and farm for CR60 (P=0.09), being observed a positive effect of P4LA on farm A (27.9 [362] vs 37.2% [363]), but not on farm B (18.7 [123] vs 15.2% [132], without and with P4LA, respectively). Interestingly, there was an interaction between P4LA treatment and season (warm vs. not warm) on CR30 (P=0.04) and CR60 (P=0.07) after TAI. Greater CR was verified in cows treated with P4LA during the warm season of the year (CR30: 16.3 vs 30.2% and CR60: 15.2 vs 26.9%). However, this increase was not observed during cooler seasons of the year (CR30: 32.9 vs 36.7% and CR60: 31.9 vs 33.9% without and with P4LA, respectively). In conclusion, treatment with 900 mg of P4LA three days after TAI in lactating Holstein cows increased conception rate at 30 days, mainly during the hot season of the year. There was no effect of hCG on conception rate.