(Editor’s note: This article was written by Denise Brusveen, University of Wisconsin Extension Agriculture Agent for Sauk County and originally appeared in El Companero.)

There are two structures that grow on the ovaries of cows: the follicle and the corpus luteum (CL). The follicle is filled with fluid and holds the egg that will eventually be released down into the uterus when the cow ovulates. When the follicle gets very large, it produces a lot of estrogen, and this is what causes cows to show signs of estrus or heat. When you give a cow an injection of GnRH, you can cause a cow to ovulate this follicle without ever being in heat. GnRH is usually given to cows during synchronization protocols such as Ovsynch. Timing and compliance to these protocols is critical in helping Ovsynch to work.

The other structure that grows on the ovary is the CL.The CL forms in the place from where the cow ovulated. Another name for the CL is “yellow body” because of its yellow color inside. The CL begins producing progesterone after the cow ovulates. If the cow becomes pregnant, this CL will be responsible for producing progesterone, which will maintain the pregnancy. If you accidentally give PGF to a pregnant cow, you will cause her to abort her pregnancy. If you are giving PGF injections to a group of cows as part of a synchronization protocol, be very careful about making sure you are giving the right injections to the right cows!

Source: El Companero