The milk is the product of cows. Feed them regularly to increase the amount of milk you can sell. Milk could be sold from the begining of animal industry in Farming Simulator 2011 to Farming Simulator 15. The milk gets sold automatically in Farming Simulator 2013 and farming Simulator 2011, by a Rolo truck in the property of Supreme Milk that checks your milk tank, collects and money automatically comes to you. There are various Mods that have the theme of a milk trailer (This letting you collect the milk for yourself and then selling it at the dairy.)