The effect of estradiol benzoate on synchrony of estrus and fertility in cattle after removal of a progesterone-releasing intravaginal device

Abstract

The aim was to determine the effect of estradiol benzoate (EDB) given after removal of a progesterone-releasing intravaginal device (PRID) at either emergence or dominance of a follicle wave, on the interval to estrus, variation in its onset and pregnancy rate in heifers. Heifers (n=186) were assigned randomly to four treatments in a 2 × 2 factorial design; emergence or dominance of a follicle wave at PRID removal, with or without 0.5 mg EDB 24 h after PRID removal. Ovarian ultrasonography was performed to confirm follicular status; data from heifers of undeterminable follicular status were excluded (n=36). Mean size of the largest follicle of the new wave at PRID removal was smaller (P < 0.01) in heifers given EDB at emergence (6.3 ± 0.09 mm) compared with those given it at dominance (10.9 ± 0.30 mm). The onset of estrus was earlier (P < 0.01) in heifers given EDB at dominance (median 42 h, range 13 h) compared with those not given EDB at dominance (median 43 h, range 42 h). The median interval to estrus was decreased (P < 0.01) in heifers given EDB at emergence (median 48 h, range 73 h) compared with those not given EDB at emergence (median 66 h, range 45 h). Variation in onset of estrus was reduced (P < 0.05) in heifers given EDB compared with those not given EDB. The pregnancy rate was not affected when EDB was given at dominance, however, it was decreased (P < 0.05) when given at emergence (23 of 40 vs 26 of 32, respectively). To determine the effect of EDB on follicular dynamics in heifers treated with EDB at emergence, heifers (n=37) were assigned to two treatments: at emergence with or without EDB and their ovaries were examined daily using ultrasonography. Follicular dynamics were not different (P > 0.05) in EDB-heifers compared with untreated controls. Mean serum estradiol was greater (P < 0.01) in EDB-treated heifers compared with controls. In conclusion, 0.5 mg EDB given 24 h after PRID removal to heifers decreased the interval to estrous onset at emergence or dominance, decreased variation in onset of estrus and decreased pregnancy rates when given at emergence of a follicle wave.