Really? So I guess the big chicken houses just buy straight run?
Yes - the commercial growers use Cornish Cross and as these are processed so young there is no need to bother with sexing as both genders are well suited to the intended purpose, same applies to backyard growers who use Cornish Cross for their meat birds.
I suppose it would be harder to do it they hung around for a lot longer.
One thing you can do to slow growth a little bit that I've had some luck with is to remove the food from the pen at night (leave water, obviously) and if you have the room, let them roam around a bit outside. We usually end up being able to butcher at 12 weeks instead of 7-8 weeks if we do that.
