Evaluation of different eCG doses in resynchronization of ovulation in lactating nelore cows subjected to fixed-time insemination

The aim was to evaluate the use of eCG in resynchronization (RS) in lactating Nelore cows, understand the interaction between dose, presence or absence of estrus, presence or absence of the corpus luteum (CL) and pregnancy rate. Were used 775 cows in first TAI and that not pregnant (449) were resynchronized 30 days after. Ultrasound (US) to observe the presence or absence of CL was performed before first TAI and when the RS. On day 0 (D0) an intravaginal device (IVD) with 1 g of progesterone and intramuscular (IM) 2 mg estradiol benzoate was administered. On D8 devices were removed and applied IM 1mg estradiol cypionate; 0.53mg of sodium cloprostenol, Group - 0 (G0) didn´t received eCG, G200 received 200 IU and G300 300 IU. Estrus was observed and insemination (AI) performed 48 hours after the removal of the IVD. US was performed 30 days after AI. The treatments didn´t affect the occurrence of pregnancy in estrus. In cows that didn´t show estrus, there´s no difference in the occurrence of pregnancy between the G200 and G300 but with G0. The cumulative pregnancy rate after 40 days of the breeding season was (77.54%). The cumulative pregnancy rate in 40 days enables the RS as a way to optimize reproductive management on beef cattle farms.