What are sources for reliable support material for a persuasive speech about additives in animal feed?

Hi :) I am writing a persuasive speech about the additives given to animals particularly about how they are harmful to people. My teacher requires us to follow a rigorous outline and I'm having trouble finding reliable sources that support my claim. If anyone has any information about harmful effects of additives given to animals and eaten by people I would appreciate any help or suggestions. Unfortunately it is too late to change my topic so I'm stuck with this one. I think that it is a relevant issue but I'm having trouble supporting my claims. Thank you in advance!