Reference article
Seed (Duck Life 4 Classic)
Food dispensed in the world hubs raises a duck’s energy. Purchased feed costs 15 coins and gives three energy levels in Classic.
Buying and feeding
Use the feed dispenser while no previous seed remains. Let the intended duck eat it, or drag the food to direct feeding. It does not raise the five racing skills.
Original Flash
Purchased seeds also raise energy by three levels in the original Flash game.
Value
Fifty feeds would take a zero-energy Classic duck to level 150 for 750 coins, subject to tutorial food and any existing energy. No need to buy food past the cap.
Across the games
Compare games and editions on the 
Seed page.