If a horse has a very large belly, it may be a sign that it is eating poor quality forage. The large intestine naturally retains poor quality forage for longer in an attempt to break down the fibre in order to obtain the maximum amount of nutrients possible. This process will cause greater fibre retention in the large intestine, causing the horse's abdomen to distend.
The solution to this situation is to improve the quality of the forage, using a more digestible forage. If it is not possible to significantly improve the forage you provide, we recommend that you replace part of the forage provided with a good fibrous product, for example a product based on alfalfa.