I am not overly concerned about this. I think its disconcerting but realistically the law will change. The AVMA and the AAEP have much to much money, business, vets and ethical dilemmas to not have controlled substances on the truck. You have a vet go out to deal with a painful colic or euthanize a horse with a broken leg and they can't use anything to make that pain better? That's an ethical issue as is stated by the guidelines on euthanasia and the ethical guidelines oath that vets take. The euthanasia guidelines specifies that pentobarbital (the euthanasia agent of choice) is the number one choice for equine euthanasia as a gunshot to the head is a. Illegal in some areas and b. Must be performed by someone with experience to avoid causing unnecessary trauma. The same goes for a captive bolt. This may mean that mobile vets are required to carry small secure lock boxes but it will be resolved. They are not taking away controlled substances. They just might have to figure a way to control them better which lets face it should probably happen.
I heard of a case where an equine insurance company required a veterinary specialist to diagnose a leg fracture. The owners loaded the horse into our trailer (borrow with our permission, safe fiberglass solid trailer)and traillered the horse to the local speciality hospital about an hour away. They reached the hospital and the vet on staff looked through the door and asked for euthanasia solution. During the drive the horse had opened the fracture, bled all over the trailer and its hoof was literally hanging by a strip of flesh. The doctor subsequently wrote a long and harsh letter to the insurance company, urged the owners to seek damages for the horses suffering over the fact that it had to go for that trailer ride from hell. The insurance company changed its requirement. I can see a few legal charges from rightly upset owners regarding pain and suffering changing this law as well.