Cornish Cross on Scratch grains?
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Medicated chick starter is fine to feed to Cornish X, right up to butchering. The 'medication' is actually a thiamin blocker, and it does not linger in the tissues of the bird. It's a very small amount, just enough to stave off a potential coccidia outbreak, and lets the birds develop a natural resistance to the protozoa. If it still bothers you, switch to a grower, flock raiser, or all flock feed.