The effect of progestin and GnRH treatments on ovarian function and reproductive hormone secretions of anestrous postpartum suckled beef cows

Eighteen anestrous crossbred suckled beef cows were assigned to one of three treatment groups. Treatments were as follows: Group 1 cows (n = 3) were untreated and served as controls, Groups 2 cows (n = 6) were intramuscularly administered 250 mug GnRH, and Group 3 cows (n = 9) were subcutaneously administered a progestin ear implant for eight days prior to the administration of 250 mug GnRH. The GnRH was given to cows in Group 3 24 h after the time of progestin implant removal. Cows were 21 to 31 days postpartum at the time of GnRH treatment. The percent of cows that ovulated after the time of GnRH treatment was 0%, 83% and 100% for Groups 1, 2 and 3, respectively. For the cows that ovulated, more (P < 0.05) cows in Group 2 (80%) had abnormal luteal phases than in Group 3 (33%). The GnRH-induced LH release and peak LH concentrations were greater (P < 0.01) in the cows in Group 3 (214.3 +/- 37.1 ng/ml) than in the cows in Group 2 (142.7 +/- 19.0 ng/ml). The LH concentrations of the control cows remained very low throughout the sampling period. Although prostaglandin metabolite (PGFM) concentrations were not significantly (P > 0.10) different among groups, mean concentrations were higher and more variable for cows in Groups 1 (39.2 +/- 5.2 pg/ml) and 2 (39.4 + 6.1 pg/ml) than for cows in Group 3 (25.1 + 1.4 pg/ml).