Effect of ovulatory follicle size and expression of estrus on progesterone secretion in beef cows.
Induced ovulation of small dominant follicles (SF, < 12 mm; CO-Synch protocol) in postpartum beef cows resulted in formation of corpora lutea (CL) that exhibited a delayed rise in progesterone (P4) compared with CL from large dominant follicles (LF, > 12 mm). Experiment 1 characterized P4 concentrations from ovulation to subsequent estrus among GnRH-induced or spontaneously ovulated SF (<or= 11 mm) or LF (>or= 12 mm) to determine if P4 secretion by CL formed from GnRH-induced SF remains lower postovulation in nonlactating beef cows. Nonlactating beef cows were induced to ovulate 48 h after PGF(2alpha) (CO-Synch; GnRH on d - 9, PGF(2alpha) on d - 2, and GnRH on d 0) or exhibited estrus and spontaneously ovulated after PGF(2alpha). Follicle size was measured at the second GnRH in cows induced to ovulate or approximately 3 h after the onset of estrus for cows that ovulated spontaneously. Cows were classified into 1 of 4 groups: 1) GnRH-induced ovulation-SF (<or= 11 mm; Ind-SF; n = 9); 2) GnRH-induced ovulation-LF (>or= 12 mm; Ind-LF; n = 16); 3) spontaneous ovulation-SF (<or= 11 mm; Spon-SF; n = 8); 4) spontaneous ovulation-LF (>or= 12 mm; Spon-LF; n = 22). Serum concentrations of P4 from d 3 to 15 were reduced in the Ind-SF compared with the Ind-LF (P = 0.05), Spon-SF (P = 0.07), and Spon-LF (P = 0.03). Experiment 2 characterized P4 concentrations (0 to 60 d postAI) among GnRH-induced or spontaneously ovulated SF (<or= 12 mm) or LF (>or= 13 mm) to determine if P4 secretion by CL formed from GnRH-induced SF remained lower during early gestation. Ovulation was induced with GnRH 48 h after PGF(2) (CO-Synch) or occurred spontaneously, and ovulatory follicle size was measured at AI. Lactating cows were classified into 1 of 3 groups: 1) GnRH-induced ovulation-SF (<or= 12 mm; Ind-SF; n = 10); 2) GnRH-induced ovulation-LF (>or= 13 mm; Ind-LF; n = 43); or 3) spontaneous ovulation-LF (>or= 13 mm; Spon-LF; n = 27). The increase in P4 concentrations was greater (P = 0.06) in pregnant (d 2 to 12) compared with nonpregnant cows. Also, the increase in P4 from d 2 to 12 was greater (P = 0.01) in the Ind-LF compared with the Ind-SF groups, but there was no difference (P = 0.94) among groups in P4 from d 14 to 60 in pregnant cows. Follicle size at AI influenced the increase in P4 in cows that failed to conceive (P = 0.007), but not among cows that became pregnant (P = 0.32) to AI. In summary, P4 secretion after GnRH-induced ovulation of SF was decreased from d 2 to 12 compared with that of LF, but was similar among pregnant cows from d 14 to 60 postAI (d 0).
D000818:Animals; D002417:Cattle / Q000502:physiology*; D004958:Estradiol / Q000097:blood / Q000502:physiology; D004971:Estrus / Q000502:physiology*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage; D007315:Insemination, Artificial / Q000662:veterinary; D007774:Lactation / Q000502:physiology; D006080:Ovarian Follicle / Q000000981:diagnostic imaging / Q000502:physiology*; D010060:Ovulation / Q000502:physiology; D010062:Ovulation Induction / Q000662:veterinary; D011247:Pregnancy; D011374:Progesterone / Q000097:blood / Q000378:metabolism / Q000502:physiology*; D013997:Time Factors; D014463:Ultrasonography
D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone; D004958:Estradiol