Effect of ovulatory follicle size and expression of estrus on progesterone secretion in beef cows

¹ Division of Animal Science, University of Missouri, Columbia, Missouri 65211
² USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, Montana 59301

^* To whom correspondence should be addressed. E-mail: Smithmf{at}missouri.edu.

	Abstract

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). Exp. 1 characterized P4 concentrations from ovulation to subsequent estrus among GnRH-induced or spontaneously ovulated SF ( 11 mm) or LF ( 12 mm) to determine if P4 secretion by CL formed from GnRH-induced SF remains lower post ovulation in non-lactating beef cows. Non-lactating beef cows were induced to ovulate 48 h after PG (CO-Synch; GnRH on d -9, PG on d -2, and GnRH on d 0) or exhibited estrus and spontaneously ovulated after PG. Follicle size was measured at the second GnRH in cows induced to ovulate or ~3 h after onset of estrus for cows that ovulated spontaneously. Cows were classified into one of four groups: 1) GnRH-induced ovulation-SF (11 mm; Ind-SF; n = 9); 2) GnRH-induced ovulation-LF ( 12 mm; Ind-LF; n = 16); 3) spontaneous ovulation-SF ( 11 mm; Spon-SF; n = 8); 4) spontaneous ovulation-LF ( 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). Exp. 2 characterized P4 concentrations (0 to 60 d post AI) among GnRH-induced or spontaneously ovulated SF ( 12 mm) or LF ( 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 PG (CO-Synch) or occurred spontaneously and ovulatory follicle size measured at AI. Lactating cows were classified into one of three groups: 1) GnRH-induced ovulation-SF ( 12 mm; Ind-SF; n = 10); 2) GnRH-induced ovulation-LF ( 13 mm; Ind-LF; n = 43); or 3) spontaneous ovulation-LF ( 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 LF, but similar among pregnant cows from d 14 to 60 post AI (d 0).

Key Words: beef cows, estradiol, follicle size, ovulation, progesterone

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