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Its just because they don't have an pathway there for the poop to go through their fluff so it gets stuck to their but. If it gets really bad, or looks like it plugging the chicks rectum then you can take some scissors and cut the fluff the poop is stuck to.
Edited by chickenweirdo1 - 4/8/16 at 9:21am
The above advice is all spot on. When I started my fist flock in January I obsessed about it for a day or so after reading a lot about it, but as long as it's not plugging the chick up it's not generally an issue. Just try to keep the temps about right, maybe give some probiotics at 1/2 strength, and keep them as stress-free as possible. After a week or so most pasty butt issues disappear. That was the case with mine anyway. I'm raising 30 broilers that are a week old as of yesterday and only noticed one with dried fertilizer on his butt but it's not causing any problems so I left him alone. As much as broilers create fertilizer he'd have been belly-up in 10 minutes if he was having any problems.