Synchronization of estrus in virgin beef heifers using melengestrol acetate and PGF2alpha: an efficacy comparison of cloprostenol and dinoprost tromethamine.

This study compared the efficacy of two sources of PGF2α on the reproductive performance of virgin beef heifers, after synchronization of estrus using melengestrol acetate (MGA) and PGF2α. Angus-based heifers (n=1002) in five herds were fed 0.5mg per head per day of MGA for 14 days. Nineteen days after the last day of MGA feeding, heifers were randomly assigned to receive (i.m.) either 0.5mg cloprostenol (n=504; Estrumate®, E) or 25mg dinoprost tromethamine (n=498; Lutalyse®, L) as a source of exogenous PGF2α. Heifers were observed twice daily for 5 days for signs of estrus and artificially inseminated 8–12h later, except in herd A, wherein animals not detected in estrus by 80h after PGF2α were mass-mated and no longer monitored for signs of estrus. Estrumate and Lutalyse were equally (P>0.1) effective among all response variables evaluated, including estrus response (E, 89% and L, 86%), conception rate (E, 67% and L, 67%), and synchronized pregnancy rate (E, 61% and L, 57%). Synchrony of estrus was not affected (P>0.1) by PGF2α source, and peak estrus response occurred 60h post-PGF for both treatments. Conception rate to timed insemination was not different (P>0.1) among Estrumate- and Lutalyse-treated heifers within herd A (14%, 4/28 and 7%, 2/29, respectively). Herd had a significant (P<0.05) effect on all indicators of reproductive performance. Conception rates within herds A and D were influenced by technician (P<0.05), however, this effect was balanced across treatments and no treatment by technician interaction was detected. In conclusion, when administered 19 days after a 14-day MGA feeding period, cloprostenol and dinoprost tromethamine are equally efficacious for synchronous induction of a fertile estrus in virgin beef heifers.