Consistent
increases in pregnancy rate are only achieved when administration of progesterone
to mated cows is initiated during the first week of pregnancy (Mann and Lamming,
1999). Furthermore, in a large field trial, Starbuck et al. (2001) identified a close relationship
between the concentration of milk progesterone on day 5 after mating and
the outcome of early pregnancy indicating that the post-ovulatory progesterone rise
may be the critical factor of maternal progesterone secretion controlling early embryo
development.