If they are truly Cornish x, they need to be fed a very restricted diet. While they are chicks, feed them a lower protein grower with a niacin supplement if you can in the day time. Take the feed away at night. When they are about four weeks old, feed them in the morning and before bed, only what they will eat in twenty minutes. At eight weeks, feed them in the evening, a layer food. This is all if it is warm outdoors and they are free ranging all day. If you plan on keeping them confined, make sure they got lots of greens, fruits, and foraging opportunities. In the winter, go back to feeding them in the morning as well an the evening. If they are starting to gain too much, cut back, but if they are doing really poorly, give them more. I have five pullets that will be four months old in a week, and they are doing really well considering they were fed like a meat bird and kept closely confined by the person who had them the first three months of their life. They can run, jump up two feet, roost on a low pole, and actively scratch around now that I have had them on a diet free ranging. They seem pretty happy, and they do not pant like they did when I got them. Fermenting their food helps, too, with keeping their crap from getting as nasty as it is prone to become. Just know that they are not much like "regular" chickens, regardless of what everyone else says. I love my darlings, though.
