an alternative to brewers yeast is doing your own fermented feed, it's a lot cheaper and they seem to do really well with that but still want their lettuce lol, we have ducklings in with our chicks.
we use non medicated starter/grower feed as they are still young and mix in a hand full of the treat block which is mostly grain and seeds , when they get a bit older going to have to decide to go with either a mixed flock feed or layer feed though. can't find the page they had linked to me on the natural chicken keeping thread sorry it's late and we had a hard day as friend got another small duckling and the older duckling and one of the chickens had to be put it jail for picking on it lol. and for treats they get fruit and veggies
here is a link to the information lindab220 posted on the other thread on fermented food, hope this helps with your questions
https://tikktok.wordpress.com/2014/04/13/fermented-feed-faq/