Just don't overfeed them. They grow to a certain weight no matter what or how much you feed them, I have found. I fed them once a day along with my layer flock, fed laying mash and later added cracked corn. They free ranged from 2 wks. of age and they ALL had grass in their gizzards and not just a little.
They all had reached approx. 10 lbs live weight~give or take a few pounds~ by the 8-9 wks process time. I held mine over until 11 wks due to time constraints in processing but they never seemed to gain any weight after reaching that 8 wk. mark. Just leveled out and maintained.
No ascites or organ failure noted in these birds, no sores on their chests, no birds that couldn't walk. I started with 25 and ended with 25. My freezer runneth over!