This is NOT a stupid question...I know. I asked the same one myself a few weeks back. The old boy at the feed store told me to give my birds either broiler ration or turkey ration. Whatever the case they need to have at least 22% or greater protein. My birds grew off in 6 weeks and dressed out (the males) at 5 pounds without necks or giblets. Those were some big birds. They took a long time to roast and could have been butchered at 5 weeks and done just fine. The hens I let go a few days longer and they still weighed in around 4.5 pounds. Chick starter is too low in protein and they won't get the amount of protein they could really use to get growing fast. I used a bit of electrolytes in their water for three days every 2 weeks and that helped out with their legs---didn't have any problems there. I did have one (out of 10) that had some toes try to curl a bit. 
