Simultaneous injection of PGF2α and GnRH into diestrous dairy cows delays return to estrus

Abstract Simultaneous injections of prostaglandin F2α (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