If the feed has been produced anytime in the last 15-20 years there will be absolutely no animal by-products in them since it is now illegal to do so after the BSE and FMD scares.
as far as adding it to the mash, it depends on what is in it. If it is a TMR that you or a neighbor is producing you should easily be able to figure out what is in it. If it is a commercially produced feed you may be able to call the manufacturer, as the analysis tag may or may not actually list ingredients.
What I would watch for are the coccidiostats (like monensin, decoquinate, and amprolium) which are added to commercially produced feeds to help reduce parasite loads and incidents of "scours" in your cattle (esp. calves) Many producers will also go ahead and add coccidostats to their TMR's as well. I don't know if these drugs will do anything to a human, but I know monensin is toxic to horses, and decoquinate is not approved by the FDA to be uses in milk producing cattle and laying chickens (for what it is worth...)