I have a group of dual purpose birds that are heavies for meat. How old should they be at process time? they're at 8 weeks now, so I'm thinking another 6-8 weeks? or more? Also, should I do anything to 'finish' them? I know that you put beef up for a week and feed more grain, do you do the same with chickens? They free range, spending most of the day eating from my burn pile of brush/wood.