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.
Horses are physiologically incapable of vomiting or regurgitating.
As a consequence, horses must be fed carefully to avoid the common and potentially fatal condition of colic. If horses were a vector of seed spread, these grasses could be prevalent in our open spaces and parks, but they are not. Nor are these forage feeds listed on the California Exotic Pest Plant Council web site at this time. Tamarisk, knapweed, thistles, etc., are the big culprits. Horses do not eat these species, many of which are actually toxic to the equine.