Progesterone is a Hormone that is released into the blood stream from a active corpus luteum on the ovary. A corpus lute is formed after a follicle has ovulated from the ovary during estrus. The CL will regress around 18 days in the estrus cycle unless the cow conceives following breeding. Progesterone will be low for the 6 days surrounding estrus (day 20-day 4) until; the new CL produces enough progesterone to measure in the blood. Another reason for low progesterone is cows that are anestrus and don't have a active CL. Knowing where progesterone should be high in the blood allows the producer and veterinarian to test and see if shot programs are in compliance and working.
If blood is pulled on those cows being bred off a ovsync then the progesterone should be low(<1ng/ml). It is a good idea to pull bloods on 10-12 cows to get a good representation of the shot compliance. If animals are low at breeding then they should be high in progesterone 10 days later. If they are low at breeding and low 10 days later they are probably anestrus. If progesterone is high at breeding then there is a active CL which will make ovulation of the follicle improbable. When testing progesterone in the blood it is important to get the samples pulled and chilled as quick as possible . Once the serum is pulled from the clot the samples can be frozen and stored for extended periods of time . If any questions on how to handle the samples contact the lab.