Horses eat the hay and sleep/defecate/urinate on the straw. Wheat and rye are used for most straw, hay can be a variey of plants varying from grasses to alfalfa. A hayride is usually a straw ride, not a hay ride. A roll in the hay is really a roll in the straw. Hay doesn't go on the ground. I used to deliver hay and straw to racehorse farms right before I became an electrician. What a miserable job that was. Snakes like straw. Carrying bales up in old failing barns, or in oven hot metal shipping containers. It was so dusty, I'd spit and it would be all black. 8 months of mostly misery lol.