What I would do: If it's in a confined area, yes worry. If it's out in the open, like in an outside run that is muddy/flooded, then the sunshine and constant use will take care of any mold issues. The chickens aren't confined with the mold in an enclosed area, there's plenty of fresh air. Just make sure they're out of the run when you spread it. And, if there's definitely mold - a lot of it, I might not use it. If it's minimal or you just don't know, spread it, and stir it (lol), turn it so it all gets exposed to sunshine and wind. It will be kind of like compost.I wonder if folks could weigh in on the issue of wet straw getting moldy? I have the same question- I'm dealing with a flooded out run, and have three "ruined" bales of straw I could open to try to absorb it. But if they were walking on wet straw for a couple of weeks, is that as much of a health problem as walking in standing water?
I have bales stacked on the side of the run under the chicken house to block the wind in the winter. Come spring, I take those bales and spread them in the run because we always get too much rain. Those bales are rained on, snowed on, have grass growing out of them by spring and usually have lots of creeply crawlies that the chickens and ducks love but that make me shudder.
Sometimes the rain is so bad that I just separate flakes and put a layer of flakes over the entire area, the compact form of the flakes keeps the chicken feet from getting as wet as they would if I separated and spread them.