What is the role of progesterone in the reproductive cycle of the horse?

Progesterone is the hormone that maintains pregnancy in the cow. This is a sort of "barrier" that prohibits any hormonal reaction that would trigger the cow to begin parturition. If the progesterone "barrier" breaks down, estrogen levels increase promoting the consequential reaction to begin labour.

Progesterone is still an important hormone when the cow is not pregnant. Progesterone levels increase to allow the corpus luteum to degrade and the ovary to start development of new oocytes or ovums. Progesterone levels begin to increase soon after ovulation (which occurs after the standing heat period), and last for around 20 days until the ovum reaches maturity and estrogen levels increase again.