The British Equestrian magazine Horse and Hound published an interesting article on how to deal with moody mares.
Some mares become difficult to manage when in season, but there are veterinary techniques which can help, as H&H explains
Mares frequently come to the vet with a history of behaving badly or being difficult to train or manage when they are in season (oestrus). Occasionally, mares may show stallion-like or nymphomaniac behaviour.
Some mares experience pain and/or colic during oestrus around ovulation. In these animals, medicines to alter their cycle can help, such as treatments to encourage the follicle to ovulate.
Vets may be asked to supply a hormonal product to help. The only hormone that can effectively suppress the behavioural signs of heat is progesterone. In this country, only the synthetic progesterone–like drug Equine Regumate, which is an oil solution for oral use, is approved for use in horses.
To suppress signs of heat effectively, Regumate needs to be either given in the feed or syringed into the mare's mouth every day. It may take several days to work and the oily solution can be tricky to administer, so owners must wear gloves whenever handling the product. Although it is the most commonly used method to suppress oestrus, behavioural problems may well recur following cessation of treatment.
Although Regumate has a product licence for use in horses, it is important to remember that it is regarded as a “prohibited substance” by the governing bodies of organisations such as the Jockey Club. This means that any horse must be off the medication for at least eight days before racing. Other organisations, such as the FEI, do permit mares to compete on Regumate.