In the meantime I do believe it’s a good advice to feed chickens what is healthy for them in confinement. But what is healthy for an individual chicken? It means you need to give ( ex)-bats different feed than roosters , heritage breeds and roosters.
In other words: impossible!
If you have a flock with all kind of chickens you probably shouldn’t give them 90% layer, without choice to eat what nature supplies in a green environment.
Anyway. After 8,5 years keeping chickens I don’t believe its best to give 90% layer to my flock with now 4 old Dutch who haven’t layed an egg since October. 90% layer probably isn’t a must either for the 2 bantams laying since the start of January again.
My believe is that optimised feed is important for factory farmers and people who want to make a profit.
My back yard chickens that free range, can eat
Always available organic/vegan layer with 15-17% proteins and 0,35-0,4% Ca + a handful scratch + oyster and grit + whatever they find in the run with good soil with lots of organic material in it from autumn leaves and chipped wood
whatever they find free ranging in the garden + leftovers ( the yummie and healthy ones) + some old fruit

+ crushed eggshells + a few mealworms + songbird seeds (still hoping for the chickens to sing like a sparrow or a nightingale).
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2 broodies in February

And Kraai didn’t even lay an egg this year.
What optimised feed do they need?
Edited typo and minor change