Ovulation and reproductive hormones following steroid pretreatment, calf removal and GnRH in postpartum suckled beef cows.

Two experiments were conducted to determine the effect of calf removal, steroid pretreatment and treatment with gonadotropin releasing hormone (GnRH) on subsequent reproductive hormone secretion and ovulation. In the first experiment, GnRH was administered to postpartum beef cows at two levels (100 and 250 microgram) and in two carrier vehicles (saline and carboxymethylcellulose [CMC]). Although peak luteinizing hormone (LH) concentrations were similar for all cows, the interval from treatment to peak LH release and the duration of the LH release was prolonged (P < .05) in the cows that were given GnRH in CMC. In the second experiment, 30 postpartum suckled beef cows were assigned to a 2 X 3 factorial experiment, with calf removal (none or short-term) and steroid pretreatment (none, norgestomet implant or Synchro-Mate B) as main effects. Short-term calf removal consisted of a 36-hr nonsuckling period beginning at the time of implant removal. GnRH (250 microgram in CMC) was injected subcutaneously 24 hr after implant and calf removal. An increased GnRH-induced LH release due to short-term calf removal (P < .05) and steroid pretreatment (P < .10) was detected. The interaction was not significant, however, (P < .25). Therefore, the suckling-induced suppression of pituitary LH release to exogenous GnRH can be at least partially eliminated within 24 hr of calf separation of norgestomet implant removal. The GnRH-induced LH release was correlated with pretreatment concentrations of estradiol-17 beta for cows that were not separated from their calves (r = .61; P < .05) but not for cows that were subjected to short-term calf removal (r = .28; P > .10). Ovulation occurred in all cows (100%) that were separated from their calves and given GnRH, whereas only 67% of cows not subjected to calf removal ovulated following GnRH treatment. Although the ovulation responses differed (P < .025) between cows that were and were not separated from their calves, ovulation response was not affected by steroid pretreatment (P > .10).