truth be told, I don't think there's any way to enforce this. Most responsible owners have their animals spayed/neutered anyway. Irresponsible owners aren't gonna care what you tell them. If the vet insists that they have thier animals fixed then eventually they'll just stop going to the vet.

A Memphis councilman is trying to do a mandantory spay/neuter of pitbulls because the local shelters are absolutely FULL of them. Problem is a lot of the dogfighters' dogs are in hiding. They have never ever seen a vet and probably never will. These dogs are bred like crazy and the non-aggressive pups either get used as bait or dumped on the streets.

Mandantory spay/neuter laws might make a small dent in pet overpopulation, but it's far from a real solution unless everyone complies.