Drakes need just a regular maintenance feed, often called grower feed. The calcium in layer feed is too much for their internal organs (kidneys, I believe).
When young ducks (girls) begin playing "hop on top" they are some weeks away from laying. At least, that is the conclusion I draw from my experience with our Runners. So at that point, I began setting free choice oyster shell next to their grower feed. Glad I did, it took another month or two for everyone to get laying. Once we were all laying, we switched to layer feed.
If we had drakes, I would have kept up with grower/maintenance feed and oyster for the girls. The rumor is they know they need it, and the drakes leave it alone.
I have also been advised that sufficient vitamin D helps proper calcium processing in ducks.