Some mares are difficult to manage when in season and perform poorly as a result.
Most methods of controlling this behaviour are based on suppressing the oestrus cycle. The cyclic events of oestrus are associated with the development of follicles in the ovary. In the normal cycle, one of these follicles will develop, releasing oestrogen, which makes the mare receptive to the stallion as ovulation approaches. It is during this phase that some mares become uncooperative.
Once ovulation has occurred, a "corpus luteum" (CL) forms at the site of the recently ovulated ovarian follicle. This secretes progesterone that prepares the uterus for pregnancy.
If the mare does not become pregnant, the uterus releases hormones ("prostaglandins") which break down the CL, allowing the cycle to be repeated.
If the mare is pregnant, the embryo reaches the uterus about 6 or 7 days after ovulation. It then spends over a week migrating around the uterus, releasing molecular signals that prevent the uterus releasing prostaglandin. This ensures that the CL continues to produce progesterone, preventing a return to oestrus, and allowing the pregnancy to proceed.
To prevent behavioural signs associated with the mare coming in season it is necessary to maintain adequate levels of progesterone. This can be done by getting the mare pregnant. But pregnancy itself may interfere with the mare's ability to perform. Other methods include progesterone by injection or the daily administration of synthetic progesterone-like drugs ("progestagens") in the food.
Another option for prolonging luteal function and suppressing oestrus is oxytocin treatment. Dr Dirk K. Vanderwall, describes two protocols:
The first involves administering 60 units of oxytocin intramuscularly (IM) once daily on days 7 to 14 after ovulation. This induces prolonged CL function in 60% to 70% of treated mares.
Alternatively, by extending the duration of oxytocin treatment to 29 days, administration of 60 units of oxytocin IM can be initiated randomly at any point in the estrous cycle with no loss in efficacy (i.e., over 70% response).
Vanderwall suggests that, given these options, it may be time to end the practice among veterinarians of using intrauterine glass balls for oestrus suppression in mares.