Horses in many ways may be more compatible with wildlife than other user groups. There is no documented evidence of the horse spreading weeds. It is unknown to what extent high quality livestock forage available contains weed seeds. However, most horse owners would not buy junk hay for their horses. The days of large populations of lower-grade, inexpensive stock horses are long gone.
Horses cost money, and the purchase price is only the down payment for ownership.
Horsemen cannot afford to compromise their investments by feeding weedy hay. Responsible horse owners are concerned about getting quality feed that has been properly planted, harvested, and cured for their horse food dollar. Many horses are fed processed feedstocks. This is forage that is heated. The heat destroys weed seeds.