Starter feed for ducklings
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This is what Storey's Guide recommends because it should contain the niacin needed if I recall correctly, but I'm not 100% on that.
Generally, I recommend having some on hand regardless, as individual ducks may have a need for supplementation. I'd rather spend the $8-10 to have it on hand, than have a duck exhibiting signs of deficiency and have to wait for a store to open or have a purchase delivered before I'm able to start treating.
On the bottom of the bag of food there should be a white tag with the nutritional information on it. It should tell you how much of each vitamin and mineral that feed contains. I used flock raiser out of the bag with nothing added and I have three amazing Muscovies.
The flock raiser should be okay by itself. Niacin deficiency is pretty easy to spot and is easily corrected if caught early. The first sign is, generally speaking, the duckling lays down a lot. In some cases the legs will start to bow. If you see any signs add additional niacin. The good thing is that you know there is niacin in the feed so you won't need to worry about the more extreme symptoms, like neurological problems.
You may find that some of the duckling do just fine but one is showing signs. This is normal. Some ducks are just more sensitive to deficiencies than others.