Increasing of the P/AI with the use of a long-acting injectable progesterone 4 days after TAI in suckled multiparous and primiparous Nelore cows

Long-acting progesterone (P4) treatment four days after TAI (named P4-14 TAI protocol) increase the conception rate in Nelore cows with post-partum anestrus (Pugliesi et al., 2016). However, there is little information about the results of P4-14 TAI protocol in lactating-cows with unknown cyclicity status at the beginning of the TAI protocol. By this reason, this field trial aimed to evaluate the pregnancy per AI (P/AI) of P4-14 TAI protocol in Nelore lactating-cows. At the beginning of protocol (D0) 422 Nelore lactating-cows (326 multiparous and 96 primiparous), between 30 and 60 days post-partum, with an average BCS = 2.27 ± 0.04 and from two different commercial farms (Rondonia and Para, states of Brazil) received a new intravaginal P4 device (Sincrogest®, Ourofino Saude Animal) and 2 mg intramuscular (IM) of estradiol benzoate (Sincrodiol, Ourofino Saude Animal). After eight days (D8), the Sincrogest® were removed and the cows received 1 mg IM of estradiol cypionate (SincroCP, Ourofino Saude Animal), 0,530 mg IM of sodium cloprostenol (Sincrocio®, Ourofino Saude Animal) and 300 IU IM of eCG (SincroeCG, Ourofino Saude Animal). All the cows were inseminated 48 hours after Sincrogest® has been removed (D10). Four days after insemination (D14) the cows were homogeneously allocated by BCS, inside each cohort, in two different groups: 1) Control group [n = 230 (172 multiparous and 58 primiparous); BCS = 2.27 ± 0.05; no additional treatment] and 2) P4-14 TAI protocol [n = 192 (154 multiparous and 38 primiparous); BCS = 2.28 ± 0.06; 150 mg IM of long-acting injectable progesterone (Sincrogest® injetavel, Ourofino Saude Animal)]. The pregnancy status of all cows was checked 30 days post-insemination by transrectal ultrasonography and no other ultrasound evaluation was performed. The statistical analysis was performed using the GLIMMIX procedure of SAS® (version 9.3). All explanatory variables were sequentially removed from the statistical model following the Wald criteria if P > 0.20. P < 0.05 was considered as significant. There was no interaction (P = 0.34) between the different farms and treatments and it was not observed effect (P = 0.15) among different farms on P/AI [Farm A = 49.1% (155/316) vs. Farm B = 57.5% (61/106)]. The P/AI of the Control group was 45.6% (105/230) and of the P4-14 TAI protocol was 57.8% (111/192, P = 0.01). In conclusion, the P4-14 TAI protocol increased 26.7% the conception rate in lactating Nelore cows between 30 and 60 days post-partum with unknown ciclicity on D0. Pugliesi et al. (Theriogenology, v.85, Issue 7, 2016) didn’t find such difference when the cyclicity status on D0 was not considered, probably due to the higer BCS of the females (average of 3.7), which could indicate higher proportion of cyclic cows. However, when they considered only cows without a CL at D0, it was observed a 20% increase on the P/AI when females were treated with a long acting progesterone 4 days after TAI.