If a crop is fully grown, passive mobs will munch off its top layer

When a passive mob / animal passes by a fully grown crop, it has a chance to go over and eat the top layer off the crop, IF the type of the crop is their breeding food; for example, pigs eating carrot crops, or cows eating wheat crops. After eating from the crop (using an animation similar to how sheep munch grass to restore their wool), the crop is set back to its second-to-last growth stage. This could also work with foxes eating from berry bushes.

This feature serves a number of purposes:

• New players get a hint of which foods can breed which animals.

• It improves the overall atmosphere of the game, especially in making villages more lively.

• It will give crops and mobs more of a purpose in the game, outside of waiting to be used by the player, similar to how illagers actively attack villagers.

• It would even allow for passive mob farms, making a new way to attract animals without active player interaction. 

This action should NOT make the mob enter love mode. There are a number of reasons for this: the mob population could control its own numbers and get out of control quickly, while the player is away; in addition, one crop could support infinitely many mobs in breeding, as compared to the usual breeding method that uses one food item per animal.

-And here are some branching suggestions by other people: Eating a crop would heal the mob a little if it's hurt, and that it could boost the growth of baby mobs a bit. Farmer villagers could sometimes chase animals away from their crops.