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

The aim was to compare the estrous response in heifers given either gonadot ropin-releasing hormone (GnRH) or estradiol benzoate (EDB) at the start of a progesterone treatment initiated at emergence or dominance of the first o r second follicular wave of the estrous cycle. Cross-bred beef heifers (n=1 34) 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 dur ation (10dE(2)), 0.75 mg EDB at insertion of a PRID) treatment of 8 days du ration with 15 mg luprostiol (PGF), a luteolytic agent, given 1 day before PRID removal (8dE(2)) or 250 mug GnRH at insertion of a PRID treatment of 8 days duration with 15 mg PGF given 1 day before PRID removal (8dGnRH). Tre atments 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 ta ken, from 2 days before PRID insertion until detection of estrus. The propo rtion of heifers detected in estrus was higher (P < 0.05) for heifers in th e 8dE(2) treatment group (40/40) compared with those in the 8dGnRH group (3 8/42) 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). Su bmission 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 g iven at PRID insertion, with PGF given 1 day before PRID removal, was more effective at synchronizing estrus than was GnRN at PRID insertion. Decreasi ng the length of treatment and the use of PGF 1 day before the end of an ED B and progesterone treatment improved estrous synchrony. (C) 2001 by Elsevi er Science Inc.