My runners don't eat grass either, at least until they can eat those unbloomed babyrose-bulbs I try to save from them (with very little success so far). Just as Tivona said, they can be teached to eat greens, even to select specific weeds from the grass: I gave them chopped iceberg for weeks daily as a snack, later mixed in some cut leaves of ragweed, Ambrosia artemisiifolia (I'm allergic from it's pollen so it is a great help if they eat it out from the garden when the weed is young).
Ducks search for young specimens of ragweed now, they like it - they learned that it must be something special because I gave it as a treat, only a pinch daily per beak, acting like I'm giving away the crown diamonds or something. Same play with moss. But my runners do not graze and eat greens that much, they search for bugs and ants and snails - the flock walks around several times, talking to each other while hunting. Food (2x pellet, 2x wheat, 1x corn - 5dl for five duck daily) is offered only at lunch, with a little more plus corn at dinner if the night is cold. For breakfast they run around and hunt, insects are slow when the sun barely rose, and best time to catch snails is in the morning dew.