When an animal is hungry, it will display a feed icon above its head.

Tap the icon and you will see what items the animal can produce when fed. Select the item you wish to be produced and drag and drop it on the animal, as demonstrated in the game’s tutorial:

The type of required feed will be briefly demonstrated along with the time it will take for the animal to produce the item and the County Fair ribbons awarded. Make sure to have enough of the right type of feed for each animal!