Many automatic wheat farms using the new villager mechanics require that farmer inventories be full with seeds so that:

• farmers do not pick up crops that they harvest
• farmers never have any food in their inventory which would otherwise cause them not to harvest crops

However, it seems villagers cannot have more than 4 stacks of the same item. One apparent way around this is to give the villagers 4 stacks of wheat seeds and 4 stacks of beetroot seeds. Unfortunately, farmers do not seem to feel inclined to harvest in this configuration and will only harvest with empty inventories or with only wheat seeds in their inventory