What do cows eat in factory farms?

They are essentially "harvested" for their meat, eggs or milk. Though factory farms don't exactly exist (they're rather called Confined Animal Feeding Operations or CAFOs), animals are primarily raised so that they don't have to work for humans, but the humans are working for them, feeding them, attending to their health concerns, making sure their living areas are clean. Of course what really happens to the animals depends on what animals you're referring to in these "factory farms."