Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2006
Publication Date: 2/20/2007
Citation: Perry, G.A., Smith, M.F., Roberts, A.J., MacNeil, M.D., Geary, T.W. 2007. Relationship between size of ovulatory follicle and pregnancy success in beef heifers. Journal of Animal Science 85:684-689. Interpretive Summary: Maternal prerequisites to successful establishment of pregnancy include ovulation of a competent oocyte, adequate progesterone production by the corpus luteum, and an adequate uterine environment. Little information is currently available on the relationship of ovulatory follicle size with pregnancy rates and embryonic/fetal mortality in heifers. Therefore, objectives of this study were to evaluate effects of size of the pre-ovulatory follicle on pregnancy rate, embryonic/fetal mortality, and circulating concentrations of estradiol following ovulation in beef heifers. Heifers were bred in one of two breeding groups: either timed insemination following estrous synchronization and induced ovulation or insemination ~12 h after detection in standing estrus. The relationship between pregnancy rate and follicle size was curvilinear and indicated a predicted maximum pregnancy rate of 64.7 ± 4.5% at a follicle diameter of 12.5 mm. Ovulation of follicles < 10.7 mm or > 14.9 mm was less likely to support pregnancy than follicles that were 12.5 mm. Ovulatory follicles < 10.7 mm were more prevalent (27% of heifers) than ovulatory follicles > 14.9 mm (7%). Thus, in beef heifers, ovulation of a small follicle is less likely to lead to successful establishment of pregnancy than if a larger follicle is ovulated. However, factors leading to production of larger pre-ovulatory follicles remain to be identified.
Technical Abstract: Previous research indicated ovulatory follicle size at time of insemination significantly influenced pregnancy rates and embryonic/fetal mortality following fixed-timed AI (TAI) in postpartum cows, but no effect on pregnancy rates was detected when cows spontaneously ovulated. Our objectives were to evaluate fertility and embryonic/fetal mortality following induced and spontaneous ovulation of different sized follicles in beef heifers. Heifers were bred in one of two breeding groups: 1) time inseminated following the CO-Synch 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 = 153). Transrectal ultrasonography was performed to determine ovulatory follicle size at time of AI and pregnancy status on d 27, 41, 55, and 68 after TAI. Interactions between breeding groups and follicle size did not affect pregnancy rate or embryonic/fetal mortality (P > 0.27). 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 64.7 ± 4.5% at a follicle diameter of 12.5 mm. Ovulation of follicles < 10.7 mm or > 14.9 mm was less likely (P < 0.05) to support pregnancy than follicles that were 12.5 mm. Ovulatory follicles < 10.7 mm were more prevalent (27% of heifers) than ovulatory follicles > 14.9 mm (7%). Embryonic/fetal mortality between d 27 and 68 was independent of ovulatory follicle size (P = 0.28). 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, respectively), concentrations of estradiol (10.1 ± 0.6 vs. 6.3 ± 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-17'. 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.