I have already asked about the wild mustang?

See, now you're dealing with a basic ecological principle. When feeding animals, either wild or domestic, the rule is to supplement the quality of the food, never the quantity. Otherwise, you can never stop feeding them or they will starve. The foliage present is being limited by the number of horses. This in turn provides a limiting factor for the horses, so if you give them twice the food and water, you'll have twice as many horses. In the meantime, remember what Aldo Leopold said, "The first rule of Intelligent tinkering is to save all the pieces." When the horses, an introduced species, destroy native plants and cause erosion and range degradation, it may be difficult to remove the horses, but in the long run it is a much better answer. Your question is a good one though, and it shows you are thinking. Also, it has been a problem, but nobody addressed it or looked for a solution.