When I fed commercial feed I tried this (in 2018 to be precise) and can confirm some chemical changes took place, because it stank to the rafters. I even took a photo of it
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but I have yet to find a decent source on what exactly happens to processed feed when wetted and left for days
would you care to share a source for this?
Processing to make commercial chicken feed does not just grind up whole grains and make their contents available; it also involves heating, sometimes micronizing, sometimes various other processes, some of which destroy nutrients in the grain, rather than make them available to whatever eats it. That's why they have to add back some of the vitamins and minerals, just like with kids' cereals, which manufacturers then claim are 'fortified' with this, that or the other. They are required by law - here at least - to put back what their processing procedures destroyed.