Plants can still use the nutrients even after incineration temperatures, animals can't - and I doubt any prions would be incorporated into plant tissue in any case, but you'd have to be careful if land was fertilized with material containing prions.

Prions can be destroyed by temperatures significantly above normal sterilization (above 140°C/280°F) and by chlorine or lye.

California poultery growers are really into contamination elimination right now. They're using techniques more frequently associated with electronic clean rooms and employees are forbidden to have pet birds.