Yes! In fact, it makes great compost, according to the Maryland Cooperative Extension Office.

Horses produce copious amounts of manure, which, when combined with straw or sawdust bedding, will make mountainous piles of waste each month, and if left alone, it will eventually break down and become compost. But with a few simple steps you can have usable, rich compost in just a few weeks. To learn how to do this on your farm, read Composting Horse Manure, produced by the Maryland Horse Outreach Workgroup.