I agree that they will over eat. I think overabundance of food available combined with not needing to hunt it down and complacent safety are the contributors to it. It's not the feed its self but a combination of those things. We watch the behavior of wild vs. not wild. They don't always find...
I think on the grasses the chickens like to eat it's the new shoots. When I move stuff in the yard around and I unearth some young grass shoots they devour them. Older tougher grass is of no interest.
I guess I was lucky with dogs, cats, and horses. All were pretty much free fed and we never had issues. But I wonder if it's because they all lived in a pasture or outside setting. Alot of things over eat or over indulge because they are board including us humans. They all got worked every day...