It's unlikely that bugs and greens alone will be able to supply all the nutritional needs that good laying dual purpose birds like Barred Rocks will need. Eventually at some point (it may take a while before it becomes noticeable), their health will begin to decline. As far as the additives in...
Welcome to BYC! Glad you decided to join our flock. A lot of us were lurkers before joining. :o) You should consider supplying some layer crumbles instead of scratch to supplement their free range diet as scratch will not adequately meet their dietary needs. Also, I would suggest supplying them...