First of all, horse feed is NOT exotic weed seed laden, and there is no scientific basis for using this term. Horse owners do 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 are not going to compromise their investments by feeding weedy hay. If there is one thing responsible horse owners are concerned about, it is getting quality feed that has been properly planted, harvested, cured, and marketed for their horse food dollar. The 6.9 million horses in the United States today are in better health because of a broad spectrum of preventive immunization and vermifuge programs, sport horse medical applications, higher quality feed and supplements, and ultra sophisticated medical technology. This, of course, results in healthier horses out on the trails and on public lands. The number one exotic weed menace in California, yellow star thistle, is toxic to horses, as are many other plants such as fiddle neck, jimsonweed, common groundsel, tansy ragwort, and Russian knapweed.