Keeping cows for milk is common in many parts of the world, except for China. Dairy farms are usually quite small. In Europe, the average size is 10 hectares, with about 9 cows in a herd. The cows graze on grass in the summer. In colder climates, the cows are kept indoors in cowsheds the winter, and are fed on hay. The dairy farmer has to milk the cows twice a day, every day of the year.