Simultaneous injection of PGF2alpha and GnRH into diestrous dairy cows delays return to estrus.

Simultaneous injections of prostaglandin F2alpha (PGF) and gonadotropin releasing hormone (GnRH) or saline were given to 32 diestrous dairy cows to test the ability of GnRH to improve estrous and ovulation synchrony beyond that of PGF alone. Cows were randomly assigned to receive PGF on Day 8 or Day 10 of the estrous cycle (estrus = Day 0), and all cows were further assigned to simultaneous injection of GnRH or saline. Corpus luteum (CL) regression, return to estrus and follicular activity were monitored by plasma progesterone assay, twice-daily estrous detection and ultrasonographic examination, respectively. Plasma progesterone concentrations declined to <1.0 ng/ml at 24 hours after PGF in all cows and were not affected by GnRH. Gonadotropin releasing hormone inducted premature ovulation or delayed return to estrus in 7 of 8 cows treated with PGF/GnRH on Day 8 and 3 of 8 cows treated with PGF/GnRH on Day 10. Further, cows with premature GnRH-induced ovulations failed to develop and maintain a fully functional CL, and all returned to estrus 7 to 13 days after the induced ovulation. These data indicate that GnRH administered simultaneously with a luteolytic dose of PGF disrupts follicular dynamics and induces premature ovulation or delays normal return to estrus and, therefore, does not improve the synchrony of estrus and ovulation achieved with PGF alone.