Summary of the way PG works - Dr Adrian Owen Equine Veterinarian
After a mare ovulates, the area of the ovary that released the egg becomes a structure called a corpus Luteum (CL). This take about 5 days. The CL releases progesterone. If the mare does not become pregnant, her uterus releases prostaglandin at about day 14. This causes destruction of the CL, which then allows the ovary to start producing a follicle. The follicle releases oestrogen, and the mare will come into season.
Importantly, the CL will not respond to PG until about 6 days after ovulation.
Therefore, a PG injection will not work if the mare is coming out of Winter, and not ovulating regularly. It will not work if she has ovulated less than six days previously. And if she already has a large follicle, she may come into season, and ovulate, much more quickly than expected.
If given at the correct time in her cycle, a PG shot will cause her to come into season within 3 to 4 days, and normally (if using artificial insemination) be inseminated at about day 6.
In summary, although PG injections are a very useful tool for taking some of the guesswork out of breeding it is recommended firstly ultrasound scanning the mare. Then you can see where she is in her cycle. Otherwise, giving PG can be very much hit and miss