Question: I have a  mare that has a history of showing aggression towards other horses in a boarding situation. The mare showed improvement being on Regumate which would suggest a hormonal imbalance. Should I give herMare Ease for its estrogen detoxifying properties. Can you please go into detail about each ingredient and how it effects the hormones?

Dr. Kellon: I wouldn't suggerst Mare-Ease. There is nothing about her behavior or history that suggests this is related to sex hormones. Regumate does not fix imbalances, it creates one - high progesterone. Most horses of both sexes respond to this by becoming "subdued" or depressed. 

She sounds to me like a typical alpha mare, a natural leader.  Her role is to establish and keep order within the herd. She worries all the time about their safety and without a stallion to stand guard may be especially stressed. Becoming unglued when new horses are added to the herd is typical.
If she can be turned out with only one or two very submissive horses that may solve the problem, but she may still be anxious if she can see other horses.
This trait is at the very core of her being and you can't expect to change who she is but moderating her intensity could work. I would try full dose Seroquine first, and if she is getting any alfalfa, to cut that from her diet. Limit sugar and grain.