Current understanding of factors influencing antral follicle count and applications to reproductive management in cattle

In recent years, an increasing amount of research has focused on variation in numbers of antral follicles in
the bovine ovary. This measurement is used as a proxy for the number of microscopic primordial follicles
in the ovaries (e.g., the ovarian reserve), because antral follicles can be visualized by ultrasonography and
are correlated positively with the number of primordial follicles. Questions remain about whether number
of antral follicles is predictive of fertility or reproductive longevity, about what genetic parameters are
associated with this trait, and about how knowledge associated with this trait can be applied to improve
reproductive management of cattle. Beef heifers with increased numbers of antral follicles at a pre-
breeding examination give birth earlier in their first calving season. This suggests an association with
reproductive longevity, because giving birth early as a heifer is associated with a longer herd life. It
remains possible, however, that heifers with increased numbers of antral follicles are actually depleting
their reserves more rapidly and will have a shorter reproductive life. Furthermore, there could be
biological variation among females in rate of depletion of the ovarian reserve. Design of experiments to
measure events occurring at the microscopic level is the greatest challenge to investigating rate of
depletion of follicles. External factors such as age of dam and maternal nutrition during early pregnancy
influence antral follicle numbers in heifers; however, there is considerable genetic control as well,
because heritability of antral follicle count is moderate (0.25 to 0.49). This indicates that genetic
selection could be applied to alter follicle number. Genomic correlation of antral follicle count with heifer
pregnancy is unfavorable (-0.22 to -0.55), indicating that increased numbers of follicles impact fertility
negatively in heifers. This directly contradicts phenotypic data, but implies that once we understand
relationship to fertility we should be able to apply genomics to select for follicle reserve markers that
benefit reproduction. Superovulatory response is correlated favorably with follicle number, but follicle
number may be correlated unfavorably with response to synchronization protocols, because indirect
evidence indicates that cows with increased numbers of follicles are less likely to exhibit estrus and
conceive to fixed-time artificial insemination. Clearly, significant progress has been made over the last
two decades in understanding how antral follicle number influences reproductive performance. Further
investigation of the ovarian reserve will provide more applications to improve reproductive management
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