Not sure about up in Canada, but certain dewromers can be used for "organic" cattle here in the states. Fenbendazole, ivermectin, and moxidectin are allowed to be used if conventional organic remedies fail. There is a 90 day moratorium on the milk produced from these cattle, and they cannot be used as organic slaughter stock either.
I couldn't find any info on wether the resulting poop (composted or not) can be marketed as organic though, so I don't feel too comfortable using it moving forward. It wiped out an entire bin of mine in a matter of a couple weeks. Bin full of dead worms
edit: shit, I meant to quote DP's message above about cattle raised on organic farms