I clean my coop out weekly. Just use pine shavings. I see some people say they use sand & I have wanted to add sand to an area of my yard for my dog to dig up instead of the grass. However I have been afraid to do this because I do let my chickens out to free range from time to time & this is an area they would then have access too. Anyway my question is: Is it safe for the chickens??? I've heard that its bad for their digestive track. Is that True or False???