Gelding Aggression

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.

So, for example, if you are putting out hay, you might be able to get by with just placing the portions well apart, at least 50 feet or greater if possible. Putting out the hay can stir up the aggression, so I still find it unnerving and somewhat risky to go into the paddock to deliver hay. When doing so you often become the part of the guarded resource, and it is not easy to avoid getting in the middle of spats between horses. In that situation it is also easy to become the target of displaced aggression. And it is also challenging not to inadvertently reinforce aggressive behavior by quickly releasing the feed and making a run for it.

When feeding grain, it really is best to have a separate enclosure such as a small round pen in which to separate the food-aggressive animal. Food-related aggression seems worse in horses with gastric ulcers, so you might also consider having your veterinarian check and treat him for ulcers, if necessary.

Does this gelding lunge at your mare at any other time than feeding? If he does, the next question would be to figure out the motivation. It may or may not be sexual.

You asked about behavior modification. Because the gelding's aggression toward the mare is occurring even when you have them on leads, one strategy I often recommend is to just put them out into a very large open space with no man-made ¬obstacles they could run into or places they could become cornered. Let them work it out, as your gelding apparently did with his dam and the other mare pasturemate in the past. However, this is easy to recommend until you've had horses injured in the process.

So I would first try to get a good history on how he behaved on a lead with his dam and the other mare once they put him in his place. In other words, did the lessons he learned from the mares in the pasture carry over to other hand-held situations? When this works, it's great. Things can often settle within a few hours or even less, but usually within a few days. You would still have to be very careful when taking them in and out of the pasture. For example, depending on the gelding's motivation for the aggression, taking the mare from the pasture might stir up his aggression.

You asked if there is a drug to suppress aggression. If the behavior is sexual in nature (e.g., herding and protecting the mares from other males; responding sexually when the mare is in estrus; or fighting with other males) there are some medications veterinarians have tested to specifically subdue stallionlike behavior. The drugs' effectiveness varies, depending no doubt on many factors, including how motivated the behavior seems to be, the herd members' individual temperaments, the group's stability, etc. Probably the most common medication approach to subduing sexual and aggressive behavior in stallions and residual stallionlike behavior in geldings is administering progesterone hormone products such as Regu-mate. These are usually administered to mares to control estrous behavior.

Other medication approaches behaviorists, vets, and owners might try when desperate are sedation or tranquilization. When a highly motivated male has direct access to a mare, usually nothing short of fairly heavy sedation subdues the problematic behavior effectively. But for many reasons, long-term sedation/tranquilization would not be recommended as a safe or humane management tool.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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