Encouraging Ovulation

Ovulatory agents and drugs that synchronize estrus affect the size and development of the follicles on the ovary. For those of you that don’t know, some basic physiology: a follicle will develop for every egg that is released by the horse. When a mare ovulates, this means that the follicle bursts and releases an egg into the ovuduct. The egg can them be fertilized in one of the uterine horns and then finally the embryo moves into the body of the uterus. When the follicle bursts, it leave a corpus luteum, which the veterinarian can also see on the ultrasound and this body produces progesterone, a hormone that helps maintain pregnancy. Progesterone is given intra muscularly to mares that receive embryos from the recipient herd, and also to mares that have a low progesterone level in their blood.

Sometimes mares will not ovulate their large follicles and the follicle can actually regress in size. In order to encourage a mare to ovulate, veterinarians can implant an Ovuplant which is made of desorelin, an analog of GnRH which is a hormone leads to follicle release. The Ovuplant is supposed to cause the release of an egg within 36-48 hours of use (if there is an appropriate follicle). This makes it possible to know that if you breed a mare before ovulation, that the chances are high the mare will ovulate before the semen is dead. It is also easier to know when the mare will ovulate to breed after ovulation. Ovuplant is also useful in estrus synchronization because if one mare has already ovulated, then by making the other ovulate ASAP, the gap between days of the donor and recipient will be small.

Other methods of causing a mare to ovulate include shots of desorelin, and HCG injections. GnRH is produced by the body and causes the release of LH, the hormone that directly causes the follicle to erupt. HCG acts in the same way as LH, causes release of the follicle and desorelin, acting as GnRH, causes the natural increse in production of LH. GnRH will help a mare that is not building follicles to begin to do so. All of these injections and implants can cause a change in the mare’s hormonal level and make her ovulate sooner than she would naturally.