I had that issue too until I took a two pronged approach to feeding. I feed fermented feed outside which they do a good job cleaning up as the day goes on, and dry feed inside only. The dry feeder is removed at night so any dry feed that's spilled gets picked up the next morning before they're let out.They would bill it out, not clean it up, and it got ground into the dirt and created sheets of stinking feed. It smelled terrible and once they did that, it was wasted.
I tend to underfeed my birds verses overfeed. I watch what is left over pretty carefully. If a lot is left over, I feed less, if it is bare clean - I feed more. Spilled of billed feed is cleaned up by the girls. Thing is, there is a lot of variance day to day.
So keeping your feed dry, keeping it from being spilled, from collecting in the mud is also important.
They should primarily be eating their feed or foraging if they free range. I only hand out treats to get them to come back to the run, and I mix in feed to further reduce how much "junk food" they get.They like corn a lot, and go for mealworms regularly, but they do serm to ignore the crumbles.....I will cut back on treats and see what happens.