Abstract Beef heifers that give birth early in their first calving season have greater reproductive longevity and productivity. This suggests that management systems that increase the percentage of heifers conceiving early in the breeding season can increase profitability for cow calf producers. Such management systems include applying hormonal synchronization of estrus at the start of the breeding season to increase the percentage of heifers that conceive during the first week of the breeding season, estimating fetal age at pregnancy diagnosis to identify heifers that conceived early and sell the heifers that conceived later or that did not conceive, or reducing the length of the breeding season to select heifers that conceive early. While hormonal synchronization increases the percentage of heifers that conceive very early, there is no evidence that it increases the reproductive longevity of these heifers. This would only be true if the major component of early conception is age at puberty. Calving day is lowly heritable, but may indirectly select for age at puberty and for antral follicle count, because heifers must be pubertal to become pregnant and heifers with increased numbers of ovarian follicles may conceive earlier in the breeding season. Antral follicle count is moderately heritable in mammalian females, but there is disagreement as to whether an increase in ovarian follicle number is beneficial or antagonistic to fertility and reproductive longevity in beef cows. The objective of the current study that began in 2011 was to establish a production system that incorporated a thirty day breeding season for Angus heifers and monitor the change over time in age at puberty, antral follicle count, fertility, and reproductive longevity. This was identified as the best strategy to determine how antral follicle count influences fertility and reproductive longevity while focusing on an economically important trait, early calving. Direct selection for antral follicle count was rejected because it had the potential to decrease fertility and negatively impact the economics of the production system. Each year, a subset (n = 100) of the heifers are evaluated by ultrasonography for reproductive tract development and antral follicle count for three months before the start of the breeding season. This design allows for the determination of the relationship between age at puberty and antral follicle count, and whether these traits are beneficially influenced by selecting for calving day. The generation interval in cattle is approximately six years, and, therefore, the long-term benefits of such a production system are still unclear, but this may be the best way to determine the relationship of antral follicle count to fertility and reproductive longevity in beef heifers while maintaining an efficient production system. USDA is an equal opportunity provider and employer.
Robert A. Cushman (Wed,) studied this question.