I received some cornish X culls from the commercial houses when they were approx. 3 weeks old. I put them in the brooder with my layer chicks of the same age; outside on our screened porch. We were experiencing a heat wave at the time - high 90s daytime, 70s at night. Since my layer chicks were already off their heat, I didn't add heat for the meaties, even though they were much less feathered out than my layer chicks. They did just fine. It did take them weeks longer, like until they were 10 weeks old, to completely feather out. Their metabolisms are faster and they are putting up their intake of protein into growing their bodies, not feather growth. On several occasions I did have to take a meatie and wet them down due to overheating. If you feel their bellies you can tell that they are super hot.