Norgestomet and gonadotropin-releasing hormone enhance corpus luteum function and fertility of postpartum suckled beef cows

In Exp.1, 101 suckled beef cows were administered two injections of prostaglandin F2 alpha (PGF2 alpha) 11 d apart. Cows in Group I received no additional treatment. Cows in Groups 2 and 3 received 250 micrograms of GnRH plus 47-h calf removal. Cows in Group 3 also received a norgestomet ear implant that was inserted 4 d after the first PGF2 alpha injection and left in situ for 8 d. The second PGF2 alpha injection was administered approximately 28 h before the time of norgestomet implant removal. The GnRH was administered approximately 30 h after the time of norgestomet implant removal (or 58 h after the second PGF2 alpha injection). Calf removal was the period from the time of implant removal to AI. All cows were artificially inseminated once 75 h after the second PGF2 alpha injection (47 h after implant removal). Reproductive status before the time of GnRH treatment (presynchronization) was determined by multiple sampling (2, 13, and 23 d before GnRH treatment) for blood progesterone concentrations (62% anestrous and 38% cyclic). The GnRH treatment increased (P < 0.01) the ovulation response and norgestomet reduced (P < 0.01) the incidence of short luteal phases in the presynchronization anestrous cows. Norgestomet treatment increased (P < 0.05) the timed breeding pregnancy rates for both presynchronization anestrous and cyclic cows. In Exp. 2, 174 suckled beef cows were administered Syncro-Mate B (which includes norgestomet). Cows in group 1 received no additional treatment, whereas cows in Group 2 received 250 micrograms of GnRH (30 h after norgestomet implant removal ). Cows were inseminated approximately 47 h after Syncro-Mate B implant removal. Reproductive status before the Syncro-Mate B treatment (presynchronization) was determined by multiple sampling for blood progesterone concentrations (49% anestrous and 51% cyclic). The GnRH treatment increased (P < 0.05) the timed breeding pregnancy rates for both presynchronizatio.