The Tractor is an item that can be bought through the shop. It plows up cow poop and collects coins for you while the app is closed. The next time the app is opened, a message appears stating how many coins your tractor collected.
Initially the tractor must be emptied after two hours, but upgrades can be purchased to extend the collection time to 4, 5 and then 6 hours. The initial purchase and first upgrade cost coins, but the 5 and 6 hour upgrades can only be purchased with diamonds.