In the mare there is a fluctuation of hormones that directly affect her behavior. As her heat cycle progresses she naturally becomes more aggressive, irritable, distracted and domineering. Individually, each mare may vary in the intensity and type of behavior In the horse's natural environment this can be quite important. "These behavior changes benefit her by temporarily moving her position up in the herd status and increasing her chances of being bred.
Our mares had to show respect even when being handled during their cycles. We have never had a problem with our mares. We always knew when they would be moody, and dealt with it.
For two Mares that came to us for breeding on the other hand was a different story.
Not well behaved and had no form of real training to speak of, but the owners wanted a foal from them.