If a horse has a very large belly, it could be a sign that it is ingesting poor quality forage. The large intestine naturally retains poor quality forage longer in an attempt to break down the fiber in order to obtain as much nutrients as possible. This process will cause greater retention of fiber in the large intestine causing distension of the horse's abdomen.
The solution to this situation is to improve forage quality by using forage with greater digestibility. Since it is not possible to significantly improve the fodder you provide, we advise you to replace part of the fodder supplied with a good fibrous product, for example a lucerne-based product.