ARS | Publication request: EFFECT OF OVULATORY FOLLICLE SIZE AT TIME OF GNRH INJECTION OR STANDING ESTRUS ON PREGNANCY RATES AND EMBRYONIC/FETAL MORTALITY IN BEEF CATTLE

Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 1, 2003
Publication Date: June 1, 2003
Citation: PERRY JR, G.A., SMITH, M.F., LUCY, M.C., ROBERTS, A.J., MACNEIL, M.D., GEARY, T.W. EFFECT OF OVULATORY FOLLICLE SIZE AT TIME OF GNRH INJECTION OR STANDING ESTRUS ON PREGNANCY RATES AND EMBRYONIC/FETAL MORTALITY IN BEEF CATTLE. WESTERN SECTION OF ANIMAL SCIENCE PROCEEDINGS. 2003. v. 54. p. 281-284.

Interpretive Summary: Gonadotropin releasing hormone is capable of inducing ovulation of dominant (> 10 mm) follicles at the time of insemination. However, GnRH-induced ovulation of follicles <_ 11 mm in diameter resulted in decreased pregnancy rates and increased incidence of embryonic mortality near the time of embryo attachment. Whether this decrease in pregnancy rate and increase in embryonic mortality is due to induced ovulation of a follicle before the oocyte it contains is completely competent, or if it is due to the response of the animal to a surge of GnRH before other physiological events have occurred is not known. However, when cows were detected in standing estrus, follicle size did not affect pregnancy rates or embryonic mortality. Therefore, GnRH-induced ovulation of follicles £ 11 mm at the time of insemination may have an important impact on pregnancy rates during defined breeding seasons.

Technical Abstract: Use of GnRH in AI protocols results in ovulation of a wide range of follicle sizes. Our objective was to determine the effect of ovulatory follicle size at GnRH-induced ovulation or standing estrus on pregnancy rates and embryonic/fetal mortality. Lactating beef cows (n = 273) received the CO-Synch protocol (100 ug GnRH, i.m. on d -9; 25 mg PG, i.m. on d -2; and 100 ug GnRH, i.m. on d 0 with timed AI) or were inseminated following detection of estrus using Heatwatch (electronic mount detectors). Ovulatory follicle size was determined by transrectal ultrasonography on d 0 (timed AI) or 12 h after detection in estrus. Pregnancy rates and fetal viability were determined by transrectal ultrasonography on d 27, 41, 55, and 68 after timed-insemination. On d 27 following GnRH-induced ovulation, there was a tendency (P = 0.07) for follicle size to effect pregnancy rates [13/45 (29%), 13/22 (59%), 18/39 (46%), 11/28 (39%), 13/20 (65%), and 7/19 (37%) for <_ 11, 11.5 to 12, 12.5 to 13, 13.5 to 14, 14.5 to 15, and >_ 15.5 mm follicles; respectively]; however, by d 68 embryonic loss in cows that were induced to ovulate <_ 11 mm follicles resulted in lower (P < 0.01) pregnancy rates (8/45; 18%) than cows in each of the other groups, which were unchanged from d 27. When ovulation occurred following standing estrus (37 d AI breeding season) there was no effect of follicle size (P = 0.18) on pregnancy rates at d 25 to 39 after insemination [11/14 (79%), 14/20 (70%), 28/35 (80%), 26/41 (63%), 21/25 (84%), and 25/38 (66%) for <_ 11, 11.5 to 12, 12.5 to 13, 13.5 to 14, 14.5 to 15, and >_ 15.5 mm; respectively], nor were embryonic/fetal mortalities affected by ovulatory follicle diameter (P = 0.66). In summary, embryonic/fetal survival was decreased following GnRH-induced but not spontaneous ovulation of small (<_ 11 mm) ovulatory follicles in beef cows.