It would be interesting to see a list of "starters" for fermented feed ... with people's opinions about each
I've read about:
Nothing, just let the air provide the good stuff
"Mother" purchased at a specialty store
Unpasturized Apple Cider Vinegar (with the mother)
Yogurt, the real stuff with live cultures
Kefir
...
I'm sure I'm missing a bunch ...
There's a detailed description of "add nothing" fermentation on the Natural Chicken Keeping blog: Fermented Feed
It begins with "What the heck is fermented feed and why on earth would you want to give it to chickens?" And then tells you what, and why.