Keeping a Cornish x to point of lay
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http://www.backyardchickens.com/newsearch?search=cornish+pet - the threads on that link should help you out
How large is your run? You posted somewhere else that you weren't in a position to let your birds free range right? Limiting protein will likely help, and encouraging them to forage as much as you can inside the run. I'd limit treats, and then limit them to greens and such although an occasional meal worm is probably ok - I wouldn't give them daily (again, protein). Many people ration them to meals, but if they're housed with other chickens that may be tough to do - maybe you're able to quarantine them for meal times?
Realistically, these birds were 'born to die' at around 8 weeks old, so anything you can give them after that is a bonus. I'd just watch them and if they make it and seem healthy - let them keep going until/unless they start showing signs of illness/injury/pain that would warrant euthanization.
Edited by 0wen - 4/12/16 at 4:53pm
Some archive threads here that seem to have helpful information on this topic: