Try the following: 1) Mix baking soda with cornmeal or cake mix (50/50). Place in containers that will keep the contents dry (and out of reach of the chickens). Rats can’t fart and burp like other mammals, resulting in rats that explode from the inside.
I don't know whether rats can burp or not.
But we know they can poop, so why wouldn't they fart? What could possibly keep air in while letting poop out?
A quick google search turns up lots of sources saying that rats can indeed fart. I found pest-control sites that say it, myth-busting sites that say it, and even discussions among people that keep pet rats, about the stinky farts their pets produce (to be honest, I found that the most convincing evidence of all, that pet owners were trading stories as if this is a perfectly normal thing.)