I don't advocate letting feed rot and then calling it fermented. If you are leaving feed wet and just letting any old wild yeast, mold, and bacteria grow in it, that's not "fermented" -- that's ROTTEN.
EEWWWW...
If you want to ferment feed that your chickens will find DELICIOUS, and will prefer over all other feed, you need to control exactly which organisms grow in it.
Start with a clean bucket that has a lid, a clean cloth that's big enough to cover the top of the bucket, and some ROOSTER BOOSTER or other chicken probiotics blend. I use Rooster Booster because it has the widest variety of probiotic organisms in it.
I ferment crumble-type feed. I have no idea how well this works with whole grains. I suspect they would need an additional day to absorb the water.
You will only make enough to feed your chickens for one day. Make a new batch every day, because feed gets sour and nasty if it sits too long.
This recipe makes about two gallons of feed. Adjust it for your flock size.
Fill the bucket with one gallon of clean water. Add the amount of probiotics for one gallon. (For Rooster Booster, add 1/3 teaspoon.) Mix well.
Now slowly pour your dry feed into the water, stirring as you go to prevent clumps. Keep adding feed until the mixture seems to thicken and there is only an inch or so of water on the top.
Dampen the clean cloth, stretch it over the top, and put the lid on top of it to hold it in place. (This is so the living organisms that are now growing in the feed can breathe. They need air, but you cover it to keep out nasty wild yeasts and mold and mildew.)
Check it a couple hours later. If it still looks really loose and watery, add a little bit more feed.
By the next morning, your feed will have grown BIGGER, and will be fluffy with all the trapped air from the organisms that are breathing in it. Stir it up to release the air.
Smell it. Doesn't it smell GOOD?? It's ready for your chickens to eat.
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Before all you old-timers yell at me, "That's not how you do it! I've fed my chooks rotten feed for years!"... Please. Just try it first. See if your birds don't dive head first into this fermented feed.
It's the difference between rotten juice and fine wine. Trust me on this... Your chickens will LOVE it.