Influence of puberty and antral follicle count on calving day in crossbred beef heifers.

Lifetime productivity and longevity are greater in beef cows that give birth early in their first calving season. The ability of heifers to conceive early in the breeding season is traditionally thought to be a function of pubertal status; however, recent evidence suggests that antral follicle count is positively associated with calving day in pubertal beef heifers. Additionally, there is evidence to suggest that the total number of ovarian follicles may alter response to hormonal synchronization protocols. The objectives of this study were to confirm the beneficial influence of antral follicle count on calving day in beef heifers and to determine whether response to PGF2α is associated with differences in antral follicle counts. A 2 × 2 factorial experiment was designed to investigate the interaction between calving period (early vs. late) and PGF2α (control vs. PGF) on calving day and antral follicle count in yearling beef heifers (n = 95). As intended, calving day was less in the early calving period compared to the late calving period (P < 0.01). There were no differences in calving day in response to treatment with PGF2α (P > 0.05). There was a significant interaction between calving period and PGF2α on age at first calving (P < 0.01), such that heifers treated with PGF2α that gave birth early were younger than heifers treated with saline that gave birth early. Calf weaning weights were greater in the early calving group than in the late calving group (P < 0.01). Heifers that gave birth in the early calving group possessed more antral follicles at prebreeding ultrasonographic examination than heifers that gave birth in the late calving group (P = 0.05). These findings confirm that antral follicle counts are associated with calving day in pubertal beef heifers. The use of antral follicle counts as a prebreeding phenotype provides additional utility to reproductive tract scoring for commercial production because of its association with calving day. As a prebreeding ovarian phenotype, antral follicle counts may hold additional applicability for organic beef producers looking to reduce the length of their calving season without increasing the total number of replacement heifers retained.