I have raised many chicks in my life and still have yet to find out why chicks get pasty bottoms. I have read several different opinions on the internet, but does anyone know for a fact why this happens?
Runny pasty poop that dries on their bottoms and blocks them from further pooping. It can make them very sick and even cause death. Wash their little bottoms off well, apply some vaseline and keep a check. it will usually clear up on its own.
Giving new chicks sugar water can cause pasty butt.
If ever I have a chick that is showing signs of pasteing up I take a pair of hair sissors and very carefully trim all the fluff on the bum and put vaseline or antibiotic ointment and it has always worked.
You will know it when you see it. Goopy poop that dries and stick to their butts and won't come off. It blocks them like a big nasty scab all stuck and dried in the fluff and on their skin back there. If you get pasty butt you'll know it - that's a promise.