A 1000-pound horse will defecate from 4 to 13 times each day and produce 35 to 50 pounds of wet manure (feces plus urine) daily, or approximately 9.1 - 12 tons per year. Typically a ton of horse manure will contain 11 pounds of N, 2 pounds of P, and 8 pounds of potassium (K). To obtain more accurate numbers, manure should be tested for nutrient content. Check Rutgers Cooperative Extension fact sheet, FS935, Feed and Forage Testing Labs for more information.

A horse kept in a stall will require about 10 to 20 pounds of bedding per day. This bedding should be replaced on a regular basis. Because of many types of bedding materials, wood byproduct (shavings, chips, sawdust), straw, or paper, the composition of the mix of manure and bedding will vary from farm to farm. In general, manure plus bedding will have a volume of between 2 and 3 cubic feet per horse per day.

Environmental Impacts and Benefits

Environmental Benefits: When managed properly, manure can be a valuable resource on a farm. Manure can be a source of nutrients for crop production and can improve soil quality. The organic matter present in manure can improve both tilth and water holding capacity of the soil. Manure can also be used as a fertilizer (N and P) for crops. However, most horse owners do not have enough land to use the amount of manure that is produced. Monitoring horse manure so that it does not cause environmental impacts is the goal of manure management.

Nutrients: When not managed properly, horse manure (feces and urine) can pollute the environment, mainly as ground or surface water pollution due to the nutrients nitrogen, phosphorus, and carbon (organic matter). These nutrients can reach waterways as surface runoff or leachate from the manure pile.

Nitrogen excreted from horses is usually present either as urea in urine, which is quickly converted to ammonia (NH₃), or it remains in association with organic matter in the feces. Ammonia (NH₃) can be volatilized into the atmosphere. If NH₃ from horse manure comes into contact with surface waters, it can cause nutrient enrichment and excessive algae growth. This process is referred to as eutrophication. Eutrophication is the process of nutrient enrichment in a lake or slow-moving stream occurring when excess nutrients from manure, fertilization, sewage, etc. are deposited. This can result in waters rich in mineral and organic nutrients that promote a proliferation of plant life, especially algae, which reduces the dissolved oxygen content and often causes the extinction of other organisms. (Adapted from Concise Columbia Electronic Encyclopedia, 1999 & American Heritage Dic. of the English Language, 4th ed.) In the case of nitrogen, the excessive algae and conversion of ammonia to nitrate (NO₃) causes a reduction in dissolved oxygen in the water, which can contribute to fish kills through oxygen depletion.

Nitrogen present in organic matter in the feces will be converted in the soil to ammonia and then nitrate, which can be taken up by plants. If plants do not take up nitrate it will easily move through the soil and can eventually leach into the groundwater where it can be a human health concern. Nitrate can also undergo the process of denitrification in the soil and be lost into the atmosphere as gaseous nitrogen (NO, N₂O, or N₂).

Phosphorus is also present in manure. When spread on the land it will not leach like nitrogen, unless the soil matrix where phosphorus binds becomes overly saturated with phosphorus. However, phosphorus will run off if applied at the wrong time of the year and/or when soil erosion occurs. This can lead to contamination of surface waters where it may cause eutrophication.

When manure is not properly incorporated into the soil, organic matter present in manure (contains carbon, nitrogen and phosphorus) can be a concern when it runs off into surface waters. Eutrophication and additional oxygen depletion may occur due to the decomposition of the organic matter.