EFFECT OF OVULATORY FOLLICLE SIZE AT TIME OF GnRH INJECTION OR STANDING ESTRUS ON PREGNANCY RATES AND EMBRYONIC/FETAL MORTALITY IN BEEF CATTLE

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 μg GnRH, i.m. on d -9; 25 mg PG, i.m. on d -2; and 100 μg 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 affect 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.