How many pounds of grain does a horse needs daily?

Depends on the weight of the horse, the age of the horse, how hard the horse is working, the type of grain, and how well they keep their weight on just hay. The nutritional value of the hay your horse is being fed can also affect how much grain your horse needs.

If you are trying to put weight on, the horse generally needs more grain. If you're trying to reduce the weight of your horse, they generally need less or none at all.

If a horse is not being worked at all, they do not need grain unless they cannot keep weight on with out it.