Q: I have an 8-year-old gelding who was castrated when he was 6 months old, and he seems to still be overly aggressive. We just got a very sweet 10-year-old mare, and we were hoping to pasture them together; however, they are not safe together. Even when we have them both on lead ropes, he lunges at her while attempting to bite. He was previously pastured with his dam and another mare. He tormented them almost continuously, but they put him in his place. Then he was on his own for a while and seemed fine–he liked not having to share his food. What can we do with him to prevent injury to the mare? Is there a drug we can give to make him less aggressive? Are there behavior modifications we could try or training methods to deal with this?

Alice, via e-mail

A: First, we should determine what you mean when you say the gelding “tormented” his dam and the other mare. If he is herding, protecting, and responding sexually as a stallion would, it is pretty difficult to suppress that behavior in a pasture setting.

Because you mention this gelding’s difficulty with sharing his food, I wonder whether some of the aggressive behavior might be food-related. Food-urgency and food-related aggression is not necessarily stallionlike, in that it probably occurs as often with geldings and mares as with stallions. This behavior, too, is especially challenging to handle in group-pastured animals. The safest and most humane approach to handling food-related aggression is probably to arrange the paddock so that the food-urgent or food-aggressive horse(s) can be separated individually whenever they are fed anything that incites the aggression