Hi Huxley,
The best way I know of to stretch
chicken feed, is the fermented soaking process. Essentially every day when you feed your chickens their ration, you use something like a five gallon bucked, to start soaking the next days feed ration. This is said to increase feed efficiency, and reduce overall consumption by up to 40% without reducing daily gains or health. If you want to learn more, I'm sure you could find YouTube videos on it.
Justin Rhodes does this, and I'm sure he has videos better decribing the process.
I hope that helps!