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
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.
the mare does not become pregnant, the uterus releases hormones
("prostaglandins") which break down the CL, allowing the cycle to be
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.
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.
option for prolonging luteal function and suppressing oestrus is oxytocin
treatment. Dr Dirk K. Vanderwall, describes two protocols:
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.
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