Cracked corn is high in sugar and will help fatten them a bit, but I personally don't offer it to my birds. I look for ways to up their protein by offering black oil sunflower seeds (like for wild birds) and cooked egg yolks. Those items help with other things, too, so I figure why not. They also eat lots of bugs!
Mine have eaten anything decent growing in their pen by July, so I'll go out into the backfields and woods and cut buckets and buckets of dandelions, plantains and clovers... they go bananas for it. I give them some grass clippings as well, as long as it hasn't been tainted by chickens and it isn't raining (you do not want it to start rotting before they have a chance to eat it).
I think the most important thing is keeping them in a pen to limit their travelling and 'exercising'. Just make sure they have food all of the time and you'll be alright. I will not buy a store bought turk ever again!!
