I brood outdoors. The chicks are 2-3 days old when I get them and are immediately exposed to the outdoor environment. That could be replicated by bringing in some of the outside run dirt in, but I find it easier to integrate the chicks ASAP with the flock so the chicks go out instead.
The feed makes no difference, begin introducing them to the outdoors when you're ready.
The reality is you're already past the period of building up immunity by early exposure (general recommendation is to start before 2 weeks or so... in my case I start at 2-3 days old). If you haven't had...