Manipulation of the proestrous by exogenous gonadotropin and estradiol during a timed artificial insemination protocol in suckled Bos indicus beef cows

Abstract

The objective of this study was to evaluate the effects of an exogenous administration of equine chorionic gonadotrophin (eCG) and eCG plus estradiol cypionate (eCG+ECP) during the proestrous period on the occurrence of estrus (based on the activation of a self-adhesive heat detection patch), on ovarian responses, and on pregnancy per AI (P/AI; 30 d after artificial insemination) in suckled beef cows subjected to timed artificial insemination (TAI) protocols. A total of 386 suckled cows received an intravaginal device containing 1.9 g progesterone (P4) and 2.0 mg estradiol benzoate IM on the first day of the synchronization protocol. Eight days later, the P4 device was removed, and PGF2α (0.15 mg of D-cloprostenol) was administrated, followed by TAI, plus 100 μg of GnRH IM 48 h later. At the time of the removal of the P4 device, the cows received one of three treatments: 300 IU of eCG (eCG; n=134); 300 IU of eCG plus 1 mg of ECP IM (eCG+ECP; n=123) or no additional treatment (Control; n=129). A subset of the cows (n=96) were evaluated according to the occurrence of estrus between the P4 device removal and the TAI. Their ovarian follicles were evaluated using ultrasound at the time of P4 device removal and at TAI, and the corpus lutea (CL) formed from follicles 5 days after TAI. A greater P/IA (P=0.002) was noted in the cows receiving the eCG+ECP treatment (50.4%; 62/123) than in the control group (29.5%; 38/129). Whereas the eCG group (41.8%; 56/134) was intermediate. More cows in the eCG+ECP treatment (56.3%; 18/32) displayed estrus (P=0.002) compared to those cows in the eCG treatment (24.2%; 8/33) or the cows from the control group (16.1%; 5/31). In addition, the cows receiving the eCG+ECP treatment had a greater ovulation rate (90.6%; 29/32) than the control group (64.5%; 20/31), with the eCG group (84.9%; 28/33) remaining intermadiate. The eCG+ECP group also tended (P=0.08) to have a larger CL diameter 5 days after the TAI (16.3±0.43 mm) than the cows that received only GnRH (14.7±0.57 mm) and eCG (15.4±0.44 mm). In conclusion, exogenous estradiol supplementation at the time of P4 device removal increased the proportion of cows that displayed estrus and an ovulatory response after the synchronization protocol for TAI. The manipulation of the proestrous period by exogenous gonadotropin plus estradiol improved the occurrence of estrus, the ovarian response and the pregnancy outcomes of suckled beef cows subjected to an estradiol/P4-based synchronization protocol for TAI.