As we mentioned, the ideal water content of your horse waste compost piles should be 50%-60%, or the increment of oxygen and temperature will be difficult for your compost. You can dehydrate your fresh horse manure before the process of composting. Also, leaves and straw can be mixed with your compost to reduce the water content.
During composting, water in your compost will evaporate because of the increasing heat. Hence, you need to dampen your horse manure if it is dry. Alternatively, you can cover your compost with a layer of plastic tarp to prevent a great loss of water.