I am so glad I found this thread! I usually use straw but was warned that if it gets wet, you have to get it away from the chickens ASAP because it will mold and cause respiratory problems. I am in So Cal and we are expecting a wet winter, already have had three days of rain and the run is so gross and stinky as I have no substrate down currently. Can any of you straw users tell me if mold is really a problem?