Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/17/2007
Publication Date: 3/1/2008
Citation: Busch, D.C., Atkins, J.A., Bader, J.F., Shafer, D.J., Patterson, D.J., Geary, T.W., Smith, M.F. 2008. Effect of Ovulatory Folicle Size and Expression of Estrus on Progesterone Secretion in Beef Cows. Journal of Animal Science 86(3):553-563. Interpretive Summary: Previous research has demonstrated that GnRH-induced ovulation of small follicles resulted in decreased fertility in beef cattle. This study demonstrated that one mechanism by which GnRH-induced ovulation of small follicles reduces pregnancy rates may be through reduced progesterone concentrations during early pregnancy. We observed no effect of ovulatory follicle size on serum concentrations of progesterone from d 2 to 60 of gestation in cows that were diagnosed pregnant by ultrasonography on d 28 post-insemination. In contrast, serum concentrations of progesterone from d 2 to 12 post-insemination among non-pregnant cows was decreased following GnRH-induced ovulation of small follicles. Thus, induced ovulation of small dominant follicles (SF, < 12mm; CO-Synch protocol) in postpartum beef cows resulted in formation of corpora lutea (CL) that exhibited a delayed rise in progesterone (P4) compared to CL that formed from large dominant follicles (LF, > 12mm).
Technical Abstract: The objectives of this study were to determine the effect of ovulatory follicle size at GnRH-induced or spontaneous ovulation on serum concentrations of progesterone in nonlactating beef cows (Exp. 1), and to determine the effect of ovulatory follicle size at GnRH-induced or spontaneous ovulation on serum concentrations of progesterone (d 2 to 60) and estradiol (d -2 to 0) in lactating beef cows through the time of placental attachment (Exp. 2). 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 nonlactating beef cows. 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 were allowed to exhibit estrus and spontaneously ovulate 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 (</= 11mm; 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 to 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 whether 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 in postpartum suckled cows. Follicle size was measured at AI in cows induced to ovulate or 12 h after onset of estrus for cows that ovulated spontaneously. 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 higher (P = 0.06) in pregnant (d 2 to 12) compared to nonpregnant cows. Also the increase in P4 from d 2 to 12 was higher (P = 0.01) in the Ind-LF compared to 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 secretions following GnRH-induced ovulation of SF was decreased from d 2 to 12 compared to LF, but similar among pregnant cows from d 14 to 60 post AI (d 0).