I'm not sure if you're joking, but opiates are grown from opium poppy plants, so that would be a good place to start if you actually wanted an abundance to feed to animals. Tramadol, however, is an opioid, which is a term that encompasses both opiates and synthetic opiate-like drugs -- of which Tramadol falls in the latter.

that's the point. synthetic opiates are not cheap. and those farmers are feeding herd with it.

Tramadol seems to be relatively cheap to produce, and has all sorts of uses beyond just pain medication.