Silage is grass which has been preserved or 'pickled'. It is made during the summer months when there is a lot of surplus grass i.e. not needed for grazing. Grass is cut 2 or 3 times a year, stored in a big heap and subsequently squashed down and covered with black plastic. Air is kept away from the grass to enable it to pickle. Grass crops for silage are fertilised to increase production and can look very much like a conventional arable crop. Silage is fed to cattle and sheep during winter because it is quite moist, easier to eat than hay and of higher nutritional value.