The tall, cylinder-shaped farm structures known as silos are used to store silage, which is animal feed. Silage is moist feed made from green crops that ferment when stored in an airtight place. This fermentation process preserves the feed, which is used along with or instead of hay (dried grasses) to feed livestock like horses, cattle, and sheep during the winter when they cannot feed in green pastures. Silage gives farm animals needed nutrients. Before farmers started to raise food crops to feed their livestock (during the eighteenth century), they had to kill most of their animals when winter approached, because grass in pasturelands stopped growing and the creatures faced starvation. But herds of livestock could be kept year-round once farmers began to grow crops for winter feed. Root crops like turnips, as well as leafy crops, were sometimes used. Today, corn is the crop most often used for silage.