To add to what Chuck said, The law requires that the slop to be disposed of by either 1) selling/giving away to farmers in wet form or, 2) dried and sold as feed. If the operation is too big of an operation, then a drier house is a requirement. Jack Daniel's and Brown-Forman, Shively both produce more slop than they could possibly get rid of in wet form so they have a drier house, whereas Woodford Reserve produces in less quantity and is able to simply sell it to local farmers. The price the farmers pay for the slop is really very low - about a dollar a gallon - but the distillery does not need to make much money on the sale. The farmers are doing the distillers a favor by taking the slop, keeping the distiller from having to invest in a drier house.
On a historic note, the old distillers regulations stated that a government agent could not destroy a still if the distiller was found to have broken the law if 1) the still was of a capacity of more than 3 barrels a day or 2) if it could be proved that 20 head of livestock was dependent upon the slop from the distillery.
"Our people live almost exclusively on whiskey" - E H Taylor, Jr. 25 April 1873