I go to college in New Mexico and can tell you that it is not by any means easy to get your horses back
The Piro. makes things really difficult, at least for us heading back to Alberta it did. With the Piro. you have to get blood taken, and be back across the border within 15 days (I believe this is the right number) from WHEN THE BLOOD WAS TAKEN. After the blood is taken the vets have to send the blood off to a couple different places, I believe to the federal vet, but this may be wrong, either way they blood is sent away. When I had this done, we recieved the paperwork back, giving us five days to get to the border. With this, we had to be to the border on a week day, as the vet is not in on the weekends. Contact the vet at the crossing you plan on passing through. It helps a lot to stay in touch with the vet!