I do not not know your available natural recources for them or your type of earth.
Where I live the ground has so much nutrients in them; and they can find a lot of hard-shelled sluggs and other critters; I don't even feed them extra grit/shells for calcium. I do feed them mealworms in the winter because the ground in too frozen/lack of animal-protein.
What I have learned; waterfowl food does not work for runners. Runners are an exception on all other ducks. Because they are island ducks surounded with salt water; they adapted to way more animal-protein then all other waterfowl. Chicken-feed is often higher in protein (atleast when it added to it. that depends on your country; they often sell enriched chicken feed in less-wet climates). Chicken feed is not easy digested by ducks. I give them chick-feed; the grinded up version, for chickens. It's not enriched here; due to our climate. IF your runners due to your climate can't get all of their animals to eat out of your terrain; give them enriched chick feed; and keep giving the mealworms (or maggots, baby-mice, etc.) Waterfowl-feed is made for birds that mainly eat plants in the water. If you look up information about food for waterbirds (ducks); you get information for all other ducks. Information that their protein intake is lower then chickens.. But runners are the exception.