Another factor contributing to the synchronization success is ovulatory response to GnRH in prepubertal heifers. Variations in ovulatory response are likely because of pubertal status and stage of follicular development and not the timing of GnRH administration . It is plausible progesterone supplementation improved the cyclic status of peripubertal heifers and contributed to the higher pregnancy in the artificially inseminated heifers. Lucy, et al. compared the efficacy of an intravaginal progesterone insert with that of an injection of PGF2a for synchronizing estrus and shortening the interval to pregnancy in prepubertal and cyclic beef heifers. They observed that prepubertal heifers had lower synchronization rates
and pregnancy rates during both the first 3 days of the breeding period and the entire 31-day breeding period
than did the cyclic heifers. However, the authors observed that compared with control, CIDR þ PGF2a improved rates
of estrus and of pregnancy.