Horse manure makes a suitable and inexpensive fertilizer for plants. Horse manure can give new plants a jump start while providing essential nutrients for continual growth. It's also slightly higher in nutritional value than cow or steer manure.
Horse manure contains grass and grain fibers, minerals, shed cells, fats, water, and sand or grit, depending on the type of soil the hay or grass was growing in. About 3/4 of the total weight of manure is water. It may also contain undigested grain and weed seeds, which is why it should be composted before fertilizing your garden because these could still sprout.