The effect of estradiol benzoate or a synthetic gonadotropin-releasing hormone used at the start of a progesterone treatment on estrous response in cattle

Abstract

The aim was to compare the estrous response in heifers given either gonadotropin-releasing hormone (GnRH) or estradiol benzoate (EDB) at the start of a progesterone treatment initiated at emergence or dominance of the first or second follicular wave of the estrous cycle. Cross-bred beef heifers (n=134) were assigned to 1 of 3 treatments; 0.75 mg EDB given at insertion of a progesterone-releasing intravaginal device (PRID) treatment of 10 days duration (10dE 2), 0.75 mg EDB at insertion of a PRID treatment of 8 days duration with 15 mg luprostiol (PGF), a luteolytic agent, given 1 day before PRID removal (8dE 2) or 250 μg GnRH at insertion of a PRID treatment of 8 days duration with 15 mg PFG given 1 day before PRID removal (8dGnRH). Treatments were initiated on Days 2, 5, 10 or 13 of the estrous cycle. Estrous detection was conducted six times daily. Twice daily blood samples were taken, from 2 days before PRID insertion until detection of estrus. The proportion of heifers detected in estrus was higher (P < 0.05) for heifers in the 8dE 2 treatment group 40/40) compared with those in the 8dGnRH group (38/42) and tended to be higher (P = 0.08) than heifers in the 10dE 2 group (38/41). The onset of estrus was earlier (P < 0.05) for heifers in the 10dE 2 treatment group (Median 41 h, range 92 H) compared with either the 8dE 2 (median 49 h, range 64 h) or 8dGnRH groups (median 49 h, range 92 h). Submission rate at 72 h was higher (P < 0.01) in the 8dE 2 (95%) group than for those in the 10dE 2 (74%) and 8dGnRH (69%) groups. In conclusion, EDB given at PRID insertion, with PFG given 1 day before PRID removal, was more effective at synchronizing estrus than was GnRH at PRID insertion. Decreasing the length of treatment and the use of PGF 1 day before the end of an EDB and progesterone treatment improved estrous synchrony.