Quite a common sight on dairy farms is a long, large heap, several metres high, covered in old car tyres. These are stacks of silage, which is surplus pasture harvested in spring and stored green in large stacks. The stacks are covered in polythene sheeting to keep out rain, and weighed down with tyres to hold the polythene in place. The stored grass ferments but retains its feed value, and is fed out in winter or during summer droughts, when available pasture is short. On the right is a silage bin, with two high parallel concrete walls, which provides a more efficient means of silage storage. Silage may also be stored in large, polythene-wrapped bales.