Here's the exact wording:
The Oregon Board of Chiropractic Examiners in consultation with their legal counsel at their 1996 September meeting determined that "treating animals is not within the scope of chiropractic in Oregon....even with a referral from a veterinarian. The Board advises that chiropractic physicians refrain from the treatment of animals.". The unqualified veterinarians attempting equine chiropractic are practicing within the law, but their lack of chiropractic knowledge leaves question as to their abilities.
This is from, Understanding Chiropractic Care for Your Horse, booklet by Michal Partington DVM, Certified Veterinary Chiropractor.
For awhile he was one of a tiny handful of equine chiro's. The stories he told of regular chiro's working on horses and the bad results would curl your toes. Now thankfully there are more equine certified.