Q: My horse has become very aggressive suddenly with others in his paddock. The yard owner wants to separate him, but he was absolutely fine until recently. What could be the cause of this?

A: The onset of, or a significant increase in, aggression towards other horses can have a range of causes, although the most common tend to be pain, feeling unsafe and competition over food.

It’s best to get your horse thoroughly checked by your vet. Aggression can be caused by anything from arthritic pain to gastric ulcers and your vet will be best placed to work out what is going on. Often in these instances, once the pain has been resolved the horse will return to ‘normal’.

Similarly, a change in availability of grazing or food can contribute to aggression between a group. Alternatively, if his food has been cut he may now be in greater competition for grass intake in the paddock to make up for this and this too could result in aggressive behaviours. In this case, if his food is increased or he is given greater access to grazing or forage the behaviour is likely to disappear.

Other changes in the environment can also have a big impact and these should be considered. Are there new horses in the group in his paddock? Are there particular horses that seem to trigger the aggression? Are the horses all male or are there mares in the vicinity that might have come into season? Is he in the same paddock with the same amount of space as before or has something changed? Has the amount of work he is doing changed? If you are unaware of any changes, it is also worth chatting to your yard manager as she may have insight into other changes in management that are not immediately obvious. Some horses are very sensitive to change and any small disruption can cause an increase in their basal stress levels, making the triggering of aggressive responses more likely. Again, once these issues have been dealt with, and your horse is made to feel more comfortable again, he is likely to return to his normal behaviours.

If, after thorough investigation, none of the above factors seem to be at play, your vet is likely to want to do more investigations into the sudden change in behaviour. There are some rare conditions that can cause rapid-onset aggression, but these are very uncommon.

Finally, if all health and environmental issues are ruled out, you will almost certainly need the assistance of an animal behaviourist to help you to tackle the issue.