I don't mix the grains in - ducks are smart, they eat what they need. I offer it free choice because I have a layer feed, and 2 of my 8 ducks are male. The idea is they will supplement with the grain themselves to even out the higher calcium levels in the layer feed. I am actually using the old wheat from my food supply, and buying new wheat for my family.
Malnutrition in Cayuga Ducks? - Page 2
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You really don't need to do anything fancy. You can cut it even with something as simple as rolled oats like you find in a grocery store. I don't recommend buying it in the grocery store simply because that can be very expensive. If you have a farm/feed store nearby, they usually sell oats in bulk (like really, really large amounts) for hardly anything per pound.