Why do horses chew wood?

Some horses when given access to lush spring grass will continue to want dry hay or will start to chew wood. This is because lush grass is a poor source of structural carbohydrates (fibre). Horses need fibre in their diet for proper digestive function. If horses are not given enough fibre they will seek other sources of fibre. If fibre is not provided as hay, horses may chew wooden fence rails or wooden boards in stables.