Where can I find reasons that the USDA humanely slaughters livestock?

I'm writing a research paper and I need reasons why people don't want to change the way slaughterhouses are now. It needs to be from a magazine or newspaper or scholarly journal. I'm against the way animals are treated, but I need to explain to the audience why people don't make change in the slaughter houses. I need links because I must site these sources. I have looked everywhere but alls I can find is how the cruelty must be stopped. I need resources as to why the way we slaughter animals today is OKAY. HELP!