Whether or not to have a mare as a roping or performance horse is a controversial topic. I believe most mares present no significant behavioral problems, and some of the great roping horses I've seen have been mares. However, if you talk to someone who has suffered with a difficult mare, they'll say they never want another one.
Mares typically have a 21-day cycle. When cycling normally, the mare will be in heat (estrus) for five to seven of those days. Those are usually the troublesome days. However, the time of year impacts that cycle. In the late fall with decreasing daylight hours, or photoperiod, most mares go into a dormant period. As the photoperiod increases in early spring, mares start cycling again.
There is a transitional period with a lot of mares in early spring, when they may be showing signs of estrus for two weeks or more at a time. This phenomenon is due to the mares not completing the heat cycle with an ovulation. They are therefore are under the influence of estrogen produced by many immature follicles on the ovaries this time of year. Time will solve this phase, once they start ovulating.
As a veterinarian, I'm often asked what can be done to minimize the distractive, or disruptive, behavior associated with some mares in heat. There are several possibilities. Putting the mare on a progesterone supplementation program will usually mask the signs of estrus. Progesterone has been referred to as the hormone of pregnancy.
Progesterone is naturally produced by the ovary after an ovulation, and is produced by the placenta during pregnancy. Sources of progesterone supplements can be in the form of daily oral administration, long acting (repository) injections that last for weeks, and off label use of cattle implants. Another method of suppressing estrus that's evolved is the placement of a glass marble (about one inch in diameter) in the mare's uterus.
One other thing I'd like to mention regards the mare that experiences behavioral changes not on a cyclical basis. The mare that becomes aggressive and even stallion-like in attitude should be examined by a veterinarian for the possibility of an ovarian tumor. In this case, surgical removal of the affected ovary is the only treatment recourse.