After a while farmer villagers will only have wheat seeds in their inventory becaus they drop more often than wheat (or other food) and won't be thrown to other villagers becaus it's no food.

So if you let your farmer, farm long enough a wheat field he will only have wheat seeds in his inventory and won't be able to provide food for the village.

Maybe they should use the composter to throw the seeds away but i haven't seen them doing this and there is no bone meal in it so i don't think they use the composter.