I'm now wondering if I have to worry so much about Brewers yeast as mine are 5 months oldBrewer's yeast is a good source of niacin. Niacin is needed when ducklings are growing fast, especially large breeds and ducks that can't free range for insects, etc. Layer pellets are high in calcium and can be hard on a drake and can cause liver or kidney damage. Take a look at storey's guide to ducks for recommendations on home-mixed feed, but you can do a lot with cracked corn, cat kibble, potatoes, free ranging and misc supplements. How old is your drake? What symptoms is he showing?