Sorry to sound miserable, Steven, but my birds get no treats/dietary additives at all apart from the odd cabbage to alleviate boredom if any feather pecking breaks out. Generally I feed red cabbage because it makes their poo blue so you can see that they're all getting some.
I hold the view that layers pellets and/or breeders pellets are a well balanced diet and that additives just upset this balance.
To put it bluntly I suspect that the feeding of treats has less to do with the chickens needs and more to do with the owners need to nurture.