|Perry, George - SOUTH DAKOTA STATE UNIV|
|Smith, M - UNIV MISSOURI COLUMBIA|
Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: May 1, 2004
Publication Date: July 1, 2004
Citation: Perry, G.A., Smith, M.F., Roberts, A.J., Macneil, M.D., Geary, T.W. 2004. Effect of ovulatory follicle size on pregnancy rates and fetal mortality in beef heifers. Journal of Animal Science Supplement 82(1):101. Technical Abstract: Ovulatory follicle size at time of GnRH/TAI significantly influenced pregnancy rates and fetal mortality following timed AI (TAI) in postpartum cows. Our objective was to assess the relationships between ovulatory follicle size and pregnancy rates or fetal mortality in beef heifers bred in one of two breeding groups: 1) heifers (n = 108) were inseminated following the CO-Synch protocol (TAI), subsequently 2) heifers (n = 154) were inseminated after detection in standing estrus by electronic mount detectors during a 23-d breeding season (EAI). Transrectal ultrasonography was used to determine ovulatory follicle size at AI and pregnancy status on d 27, 41, 55, and 68 after TAI. The relationship between follicle size and pregnancy status was quadratic for both TAI (P < 0 .01) and EAI (P = 0.04), indicating predicted maximum pregnancy rates of 69.3 ± 5.8% at a follicle size of 12.7 mm and 73.6 ± 4.8% at a follicle size of 13.3 mm, respectively. Interactions between breeding group and follicle size did not affect pregnancy rate or fetal mortality (P > 0.20). Pooled across breeding groups, the relationship between pregnancy rate and follicle size was quadratic (P < 0.01) and indicated an intermediate predicted maximum pregnancy rate of 71.2 ± 3.8% at a follicle size of 13.1 mm. Follicles <11.5 mm or >16 mm were less likely (P < 0.05) to support pregnancy than an optimal sized follicle (13.1 mm). Fetal mortality between d 27 and 68 was independent of follicle size (P > 0.20), but was only 2% in these data. Ovulation of smaller than optimal sized follicles are a more serious problem than ovulation of larger than optimal sized follicles, as 47% of heifers ovulated follicles ' 11.5 mm but only 3% ovulated follicles > 16.0 mm. Ovulatory follicle size is a strong indicator of fertility and management practices that influence ovulatory follicle size may significantly influence fertility.