Induced corpora lutea in the postpartum beef cow. II. Effects of treatment with progestogen and gonadotropins.

Seventy-three multiparous beef cows (21 to 35 d postpartum) were used in an experiment with a 2 X 4 factorial design to test the effects of pretreatment with norgestomet and preovulatory gonadotropic stimulation on the life span of corpora lutea induced by human chorionic gonadotropin (hCG). Cows were implanted subcutaneously for 9 d with norgestomet (3 mg; 35 cows) or sham-implanted (38 cows). HCG (1,000 IU) was administered in to all cows 36 h (time = 0) after implant removal. Each cow received one of the following treatments: (1) pregnant mare's serum gonadotropin (PMSG; 750 IU) in a single subcutaneous injection 48 h prior to hCG (-48 h); (2) follicle stimulation hormone (FHS-P) in saline (10 mg) in a series of four subcutaneous injections (2.5 mg/injection) at -48, -36, -24 and -12 h; (3) FSH-P in 5% beeswax-sesame oil (10 mg) administered as in 2; or (4) saline, administered as in 2. Samples of serum taken at the time of implantation, at the time of implant removal and daily on d 5 to 20 after administration of hCG were assayed for progesterone. Samples taken at 12-h intervals from -48 to +12 h were assayed for estradiol-17 beta. Cows were observed for estrus at 6-h intervals from 24 to 60 h after implant removal and twice daily until d 22. Seventy-one percent of the cows (25/35) were in standing estrus from 29 to 60 h after removal of a norgestomet implant, compared to 16% of the sham-implanted cows (six of 38). As determined by progesterone in serum and by palpation of the ovaries, 88% of the cows (64/73) formed corpora lutea in response to hCG, with a tendency (P less than or equal to .10) for a positive effect of pretreatment with norgestomet (94 vs 82%). The life span of corpora lutea was longer (P less than or equal to .01) in cows treated with norgestomet (19.6 d) than in sham-implanted cows (13.4 d). Level of function of the induced corpora lutea as indicated by the pattern of daily concentrations of progesterone in serum was increased by pretreatment with either norgestomet (P less than .01) or gonadotropin (P less than .05), but there was no interaction of these treatments. The mean level of estrogen before ovulation was increased (P less than .01) by pretreatment with norgestomet or any regimen of gonadotropin, and, again, there was no significant interaction. Pretreatment with progestogen was associated with a higher pattern of concentration of estrogen over time (P less than .01).