Hi Robert,
The info I have is that for pigs research is inderway to develop a microchip type one to pop under the skin. Problem is that current "Dog" type microchips are glass coated and not easily detectable in the carcass - so work is underway to develop a type that can be easily found post slaughter.
As an aside, local newspapers here reported a chap who hurt a tooth at local chinese restraunt, visiting the dentist the next day who pulled out the remains of a dog-microchip from his tooth ! - anyone lost a dog ??