Farmers know when their livestock is sick, and they know how to treat the animals they care for in almost every case. They can give an animal a shot of penicillin.
But if antibiotics are by prescription only then that means every time an animal needs antibiotics we will need to call the veterinarian. Vets are expensive. A vet call to the farm is serious business, because most animals, sheep, hogs, chickens, etc. are monetarily worth less than the cost of a visit by a vet. It is difficult to justify a $150 vet bill to save an animal that will sell for a hundred dollars or less… IF the animal even survives.
Farmers care about their animals… but they must survive or they lose their way of life. We WANT to care for our animals. It is in our best interests to take good care of our animals, both emotionally and financially.
But this law will kill a lot of animals that would otherwise be saved easily… over a concern that doesn't exist.