Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/2/2002
Publication Date: 6/2/2002
Citation: PERRY, G.A., GEARY, T.W., LUCY, M.C., SMITH, M.F. EFFECT OF FOLLICLE SIZE AT TIME OF GNRH-INDUCED OVULATION ON LUTEAL FUNCTION AND FERTILITY. WESTERN SECTION OF ANIMAL SCIENCE PROCEEDINGS. 2002. v. 53. p. 45-48. Interpretive Summary: GnRH-induced ovulation of smaller dominant follicles (> 10 mm in diameter) among beef cows receiving the CO-Synch protocol did not affect subsequent CL formation or circulating concentrations of progesterone. The induced ovulation of smaller follicles, however, may lead to increased embryonic death near the time of embryo attachment. Whether other current management practices affect ovulatory follicle size is not known; however, management practices that reduce the size of the ovulatory follicle may have a large impact on pregnancy rates during defined breeding seasons.
Technical Abstract: GnRH in fixed-time AI protocols results in the ovulation of a wide range of follicle sizes. Multiparous beef cows were assigned to one of three treatments: Cyclic or anestrous cows treated with the CO-Synch protocol and anestrous control cows. CO-Synch treated cows received GnRH (100 mcg; im) on d 0 of treatment, and all cows received PG (25 mg; im) on d 7. Forty-eight hours following PG injection, cows received GnRH (100 mcg; im) and were A.I. Blood was collected daily from d 0 to 31 and once weekly from d 34 to 69. Ovarian follicles were measured by ultrasonography on d 7 to 9 to determine growth rate and ovulatory size. Ultrasonography was used to determine pregnancy rates on d 34 and fetal viability on d 41, 48, 55, 62, and 69. Cyclic cows were further postpartum than anestrous cows (P<0.05) on trial start date. Follicular growth rate (d 7 to 9) and size of the ovulatory follicle did not differ among treatments (P>0.05). CO-Synch treated cows had longer luteal lifespan than control cows (19.7 ± 0.9 d and 9.6 ± 1.8 d; P<0.05). Cows that were pregnant on d 41 had higher (P<0.05) concentrations of progesterone than nonpregnant cows from d 19 to d 31. On d 25 post breeding, there was no difference (P>0.05) in pregnancy rates among follicle size groups of CO-Synch cows [4/7(57%), 8/10(80%), 10/13(77%), and 4/6(67%) for 10 to 12, 12.5 to 14, 14.5 to 16, and > 16 mm; respectively]; however, by d 60 post breeding, cows that ovulated 10 to 12 mm follicles maintained fewer pregnancies [2/7(29%); P<0.05] than cows that ovulated 14.5 to 16 [9/12(75%)] mm follicles. In summary, ovulation of smaller follicles does not affect CL formation and progesterone secretion, but may lead to increased embryonic death.