The stacks are sealed on the sides and ends with plastic tarps. The tarps are pulled into tight contact with the stack sides by the suction of fans that pull the air out the bottom. The warm air coming in from above has no place to go but down through the hay. Gas-fired heaters warm the air to a range of 140 to 150F. Fans move a huge volume of air at about three inches static pressure. But systems with unheated air from below only work with low-density bales that are usually too soft for mechanical handling and shipping.
Heat is turned off two hours before the fans stop.
That cools the hay and finish-drys it as the last heat is removed. A stack retriever takes over next to move the green leafy bales to the warehouse storage barn where 50,000 bales can be stacked to await shipping. Some of the hay makes it all the way to the customer without ever being restacked by hand. If you make a couple of moves, that can cut earnings drastically. The automatic bale wagon and stack retriever avoid that cost.