Animal abuse on factory farms?

I am writing an essay on animal abuse/cruelty in factory farms (dairy too) and slaughter houses. I need some legit sources!! I have googled lots of things but most are things like PETA or GoVeg and they are biased sources, also they don't say where their facts come from so I can't use them. I found a short National Geographic article but thats about it. Anything else you know of??? Thanks a ton!