There are many types of hay and straw, and each has a different effect on animals. Some animals sleep on hay or straw to stay warm, while others use it for nesting material. Still others eat the hay or straw.
The most common type of hay is alfalfa, which is high in protein and calcium. It's often used as animal feed, but can also be used for bedding. Straw is usually made from wheat or oats and is less nutritious than hay. It's often used as bedding because it's absorbent and keeps animals dry.