Relationship between size of ovulatory follicle and pregnancy success in beef heifers

¹ USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301; Department of Animal Sciences, University of Missouri, Columbia, MO 65211
² Department of Animal Sciences, University of Missouri, Columbia, MO 65211
³ USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301

^* To whom correspondence should be addressed. E-mail: tom{at}larrl.ars.usda.gov.

	Abstract

Previous research indicated ovulatory follicle size at time of insemination significantly influenced pregnancy rates and embryonic/fetal mortality after fixed-timed AI in postpartum cows, but no effect on pregnancy rates was detected when cows spontaneously ovulated. Our objective was to evaluate relationships of fertility and embryonic/fetal mortality with preovulatory follicle size and circulating concentrations of estradiol after induced or spontaneous ovulation in beef heifers. Heifers were inseminated in one of two breeding groups: 1) time inseminated after an estrous synchronization and induced ovulation protocol (TAI; n = 98), or 2) inseminated ~12 h after detection in standing estrus by electronic mount detectors during a 23-d breeding season (spontaneous ovulation; n = 110). Ovulatory follicle size at time of AI and pregnancy status 27 d, 41 d, 55 d, and 68 d after TAI (d 0) were determined by transrectal ultrasonography. Only 6 heifers experienced late embryonic or early fetal mortality. Interactions between breeding groups and follicle size did not affect pregnancy rate (P = 0.13). Pooled across breeding groups, logistic regression of pregnancy rate on follicle size was curvilinear (P < 0.01) and indicated a predicted maximum pregnancy rate of 68.0 ± 4.9% at a follicle size of 12.8 mm. Ovulation of follicles < 10.7 mm or > 15.7 mm was less likely (P < 0.05) to support pregnancy than follicles that were 12.8 mm. Ovulatory follicles < 10.7 mm were more prevalent (28% of heifers) than ovulatory follicles > 15.7 mm (4%). Heifers exhibiting standing estrus within 24 h of TAI had greater (P < 0.01) follicle diameter (12.2 ± 0.2 mm vs 11.1 ± 0.3 mm) and concentrations of estradiol (9.9 ± 0.6 vs 6.6 ± 0.7, respectively) and pregnancy rates (63% vs. 20%, respectively) than contemporaries that did not exhibit behavioral estrus. However, when accounting for differences in ovulatory follicle size, pregnancy rates were independent of expression of either behavioral estrus or circulating concentration of estradiol. Therefore, the effects of serum concentrations of estradiol and behavioral estrus on pregnancy rate appear to be mediated through ovulatory follicle size, and management practices that optimize ovulatory follicle size may improve fertility.

Key Words: Cattle, Follicle Size, Pregnancy Rate

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