Here's a cut/n/paste from a website:
Arsenic is no longer used in cotton defoliants and thus is no longer a concern. A study conducted in Georgia evaluated cotton gin trash for evidence of residues from pesticides used in cotton production. Only one residue, tribufos (DEF) – a commonly used defoliant, was found. The average concentration found was 4.5 ppm, which is low. While tolerance levels of DEF for two other cotton by-product feeds (whole cottonseed and cottonseed hulls) have been established, a tolerance level for cotton gin trash has not been established. Therefore even if chemical residue(s) are not detected in the gin trash, it is still illegal to feed the gin trash if DEF or other, non-cleared chemicals were used during the production of the cotton crop. This restriction is in place because of the potential for chemical residues to end up in beef products