I see a lot of people talking about "fermented" feed but I don't understand the process really. Is it really healthy to feed my girls "rotten" food?
There's a big difference between rotten and fermented foods. Rotten foods are likely full of bad bacteria. Fermented foods are full of healthy bacteria. Rotten is more likely to happen with animal proteins. Grains and vegetables are naturally covered in beneficial yeasts and bacteria (probiotics). If you give them the proper environment, they will multiply those probiotics, aka fermentation.
Most fermentations related to veggies, seeds and grains results in them smelling and tasting sour. This sour is a sign of a fairly acidic environment, which is good, because such an environment is inhospitable to bad bacteria like E. coli or salmonella.
Fermentation has been used for thousands of years around the world to preserve the year's crop in the form of pickles, sauerkraut, beer/wine/spirits, cheese, yogurt, etc.
If you've ever made sourdough bread, it's the same process used to make fermented chicken feed, only that you don't cook it.