Supplementing progesterone
reduced the proportion of cows that lost pregnancy only
among those that ovulated at the initiation of the timed AI
program. It is possible that the level of progesterone supplemented
was insufficient to influence follicle and oocyte
characteristics involved in processes related to early stages
of embryo development, thereby resulting in no benefit for
P/AI. On the other hand, pathways implicated with subsequent
embryo development and placentation after Day 32
could have been altered by progesterone supplementation
during the growth of the ovulatory follicle, leading to a
legacy effect on pregnancy loss. In general, the effect of
progesterone supplementation during timed AI protocols
on pregnancy loss is inconsistent; some studies report
numerical reduction [8,32,36], whereas others report numerical
increases in pregnancy loss. Therefore, a
more comprehensive evaluation of this concept is necessary
before conclusions can be drawn.