Composting Horse Manure

By: Brent W. Auvermann, Lanny A. McDonald, Robert Devin and John M. Sweeten

Uncontrolled stockpiles of horse manure can be an unsightly, smelly, and fly-infested mess. Stockpiles also can cause runoff pollution in nearby streams and ponds.

For horse enthusiasts, veterinarians, and operators of boarding stables, handling and disposing of horse manure can be tricky. Operators of equine facilities and large-animal veterinary clinics often address that challenge by paying a third party to take the manure off their property.

However, there is an excellent way to stimulate demand for a product that would otherwise be a liability. Composting manure can eliminate a messy problem and may provide a modest additional income.

The horse enthusiast needs to understand the basic scientific principles and other factors that contribute to successful compost management.

 

What is composting?

Composting is the controlled breakdown or degradation of organic material into a product known as humus. The composting process must be:

1. Aerobic. In the presence of air, or specifically oxygen, organic material degrades without creating problems associated with odor. Biological degradation that takes place without oxygen, or anaerobic digestion, is often called “fermentation.” It usually is associated with intense and disagreeable odors
2. Biologically mediated. A biologically mediated process like composting takes advantage of naturally occurring bacteria and fungi to digest the organic material. Because those microbes generate heat on their own, the addition of supplemental heat normally is not required, even in colder climates. By contrast, a purely chemical oxidation of organic material is generally known as “combustion.” (Needless to say, we don’t want that to occur!)
3. Thermophilic. High-quality compost for agricultural or horticultural use is produced at temperatures between 130 degrees F and 160 degrees F.At those elevated temperatures, most microbial pathogens such as coliforms and Salmonella are destroyed, and all but the hardiest weed seeds are inactivated. (Notable exceptions include Bacillus anthracis, the spore-forming bacterium that we know as “anthrax;” if anthrax is suspected in any of the animals nearby, composting the associated manure is not recommended.) Composting processes that take place at ambient temperatures between 50 degrees F and 90 degrees F are known as mesophilic composting.
4. Controlled. Wherever manure is stockpiled, some degree of aerobic and/or anaerobic digestion occurs. However, the end result is variable and unpredictable. Composting is a controlled process. Organic material is managed in a systematic way and within a designated time frame to yield a consistent, predictable end product.

 

In complete composting, organic matter, such as carbohydrates, sugars, proteins, fats, and cellulosic compounds, reacts with oxygen and water to produce carbon dioxide, water vapor, and humus. Incomplete composting produces intermediate products, such as fatty acids, that have an offensive odor and that may be toxic to plants. As a result, composting systems should be properly designed and managed to ensure the complete breakdown of organic matter.

Compost maturity

When manure and other organic matter are completely oxidized or degraded, they are termed “mature” compost. Compost maturity is crucial to quality assurance and quality control when marketing a compost product for agricultural or horticultural use.

There are several well-established ways to determine compost maturity. The easiest way is to monitor the internal temperature of the compost bulk using a long-stemmed thermometer. Temperatures in fresh compost piles rise quickly—up to 160 degrees F and greater—and then decline slowly until the compost temperature again approaches the air temperature. If the composting material reheats spontaneously after turning, aerating, or rewetting, it is probably not mature. Instead, the aerobic microbes probably ran out of oxygen, nitrogen, or water before all of the organic material degraded.

A more direct method of determining compost maturity is based on the rate of microbial respiration or gas exchange. Respirometry-based maturity tests create standardized moisture and aeration conditions within the compost and then measure oxygen consumption or carbon dioxide production during a specified period of time. If oxygen consumption during that period is below a certain threshold, the compost is considered mature.

One well-known commercial test for compost maturity is the SolvitaTM test kit from Woods End Laboratories. The SolvitaTM test is a simple, 4-hour maturity assay in which the color of a test strip changes in response to changes in oxygen and carbon dioxide saturation in a closed container. A maturity index between 1 (immature) and 8 (mature) is then determined by comparison with a set of standard colors.

The Texas Department of Transportation requires maturity assays using the SolvitaTM test or its equivalent on all compost used in vegetation along public roadways. A minimum value of 7 is required to meet the TXDOT maturity specification.

Compost maturity has important implications for plant growth. Mature compost does not compete with plants for nutrients, especially nitrogen, that may already be in limited supply. If unfinished compost is mixed into the soil, the aerobic microbes may compete with plants for essential nutrients, stunting growth or killing the plants. In addition, immature compost is likely to be relatively high in fatty acids that are toxic to plants, or “phytotoxic.”

Optimal conditions for composting

A manager of a small-scale composting system must respond to a broad range of conditions such as variable manure composition and uncontrollable weather. Certain conditions, however, can be controlled to improve various elements of the composting process. These include moisture, carbon-to-nitrogen ratio, and oxygen saturation.

Moisture – Sufficient water provides soluble nutrients for the microbes necessary for composting. If the moisture content is too high, however, it reduces the supply of oxygen, and offensive odors are likely to develop. In most cases, a moisture content between 35 percent and 55 percent by weight results in efficient, thermophilic composting. Because the elevated temperatures accelerate evaporation, water must often be added to ensure that the composting process does not shut down prematurely. This is nearly always true in semiarid climates like the Texas Panhandle, the Trans-Pecos region, or the Southern Plains.

Carbon-to-Nitrogen (C:N) Ratio – Along with oxygen, the most important nutrients for microbes are carbon and nitrogen. Just as human metabolism benefits from a proper balance of protein and carbohydrate in the diet, microbes in the composting process function best when available nitrogen and available carbon are properly balanced with each other and with moisture and oxygen.

Generally, the average C:N ratio that optimizes the composting system is 25:1 to 30:1 by weight. Higher C:N ratios cause nitrogen-limited conditions and nitrogen-poor finished compost. Compost that has a low C:N ratio will not stabilize nitrogen completely and can result in excessive release of gaseous ammonia to the atmosphere. Most livestock and poultry manure has a C:N ratio on the order of 15:1 or 10:1. Therefore, other material with high carbon content, such as crop residues, wood chips or sawdust, should be added to manure. A mixture of horse manure and wood shavings is well suited to complete composting.

Oxygen Saturation – Because aerobic microbes require oxygen dissolved in water to complete their work, composting efficiency depends on maintaining free oxygen in the pore spaces around the compost particles. Oxygen saturation measures the availability of free oxygen and is defined as the volume fraction of oxygen in the pore gas. Standard air has approximately 21 percent oxygen by volume; because aerobic organisms in a compost pile are constantly consuming oxygen, the oxygen concentration in the pore space will generally be lower than 21 percent, but aerobic conditions may still be in place.

If the oxygen saturation drops too low, around five percent, the oxygen- dependent microbes will begin to shut down, and anaerobic microbes will assume responsibility for further digestion. That will be accompanied by a noticeable temperature drop. Oxygen saturation for aerobic conditions can be maintained by passive or active (forced-air) aeration, depending on the permeability of the combined feed-stocks. Horse manure combined with ample wood shavings or straw is porous and permeable to gas because of varied particle size. Mixtures of manure and sawdust, a finer material, require a slightly higher degree of management to maintain adequate permeability in the compost bulk. If managed properly, however, sawdust will compost faster than coarser bedding materials because of its much higher surface area.

Making compost from horse manure

There are several ways to design an on-farm composting system, and no single one is appropriate for all sizes and types of equine facilities. However, any system should consist of the following simple components:

1. a staging area for raw manure;
2. a set of four to six bins or free-standing piles large enough to maintain elevated internal temperatures;
3. a mechanism for turning the piles or moving the compost from bin to bin, such as hand labor for small operations or a small front-end loader for larger areas; and
4. a water faucet or a pump/water tank combination, and a spray nozzle.

The number and size of composting bins are determined by the amount of manure generated by the facility and the desired turning frequency. Establish a desired turning frequency of 2 to 3 weeks. Count the number of wheelbarrow loads of manure generated over that period of time. Estimate the capacity of each wheelbarrow load and multiply that by the number of loads to obtain the necessary bin volume. Then, to ensure adequate capacity for an increase in stocking rates, add another 50 percent of the volume. 

For example, if the wheelbarrow holds 3 cubic feet of manure when heaping full, 16 wheelbarrows of manure are generated per day, and compost is turned every 2 weeks, the first bin should have a volume capacity as follows. Capacity = (3 cubic feet/load × 16 loads/day × 14 days) × 1.50 = 1,008 cubic feet

The floor area of the first bin is computed by dividing the volume capacity by the design depth in feet. Compost piles should be at least 4 feet deep, so the floor area of the first bin is calculated as 1,008 ÷ 4 = 252 square feet, or measures approximately 16 feet × 16 feet. If the front-end loader has sufficient reach, the floor area can be reduced by increasing the operating depth to 6 or 7 feet. For a depth of 6 feet, the floor area would be 168 square feet, or 12 feet × 14 feet. The volume of material in each bin will decrease over time as materials degrade, so subsequent bins may be slightly smaller if necessary. By the time the compost is mature, its volume may have decreased by as much as one half.

The moisture content of the manure and bedding is normally 50 to 60 percent in the raw state, so additional moisture probably will be unnecessary until the compost is moved to the second or third bin. Have an ample water supply and pressure to moisten the compost as it is turned. Moisture content can drop as low as 25 percent within 4 weeks. To increase the moisture content of compost from 25 percent to 55 percent, add about 20 to 30 gallons of water per 100 cubic feet of compost. For a system in which four bins (1,000 cubic feet each) require additional moisture, approximately 1,200 gallons of water are needed every time the bins are turned. However, the actual amount of water needed will vary substantially depending on the kind of bedding used, the size of the particles in the bedding and other site-specific factors.

Don’t try to add all the water at once. Instead, use a spray nozzle to deliver the water to the compost as each loader bucket is turned into the bin. It is easy to verify the proper moisture content. Grab a handful of compost from the middle of a bin that has been mixed well, and squeeze the compost tightly in your fist. You should not be able to squeeze any free water droplets out of the compost, but it should leave your hand slightly wet. If you inadvertently get the compost too wet, don’t panic; just keep an eye on the compost temperatures in that bin and turn the compost if temperatures do not rise in a couple of days. If rancid odors emanate from any of the bins, the moisture content is probably too high. Turning the compost will help drive off some moisture and oxygen and alleviate the problem.

Measuring compost temperatures

Checking compost temperature is the easiest and quickest way to keep track of a composting system. A simple, long-stemmed thermometer (or two) and some good record-keeping skills are all that are needed.

Carefully insert the thermometer halfway down into the compost bulk and allow the needle or digital display to stabilize. This may take up to 2 minutes for dial thermometers. Record the date, time, bin or pile number, location within the bin (e.g., center, north-west corner, etc.) and temperature. Temperatures should be highest near the center, but take temperatures at several locations to average out any odd readings. Sometimes a thermometer will be inserted directly into a cold or wet spot that is not visible from the outside and that is not characteristic of the bin as a whole.

Measure temperatures at least daily for the first week after the compost has been turned. Then, if temperatures in active bins are in the thermophilic range between 130 degrees and 160 degrees F, you won’t need to measure temperatures as frequently; weekly may be adequate. Temperatures immediately after turning and wetting obviously will be near air temperature, but they should rebound markedly within 48 hours. Keep the temperature measurements in a handy file to help document to prospective buyers that weed seeds and pathogens should not be a problem in your compost.

Monitoring compost maturity

Because compost maturity is extremely important for horticultural and agricultural users, it makes sense to begin a systematic process of maturity testing. Respirometry-based test kits cost between $15 and $20 each, so use them wisely! Do not use maturity tests for piles or bins that respond quickly to additions of moisture or to aeration (easily deter-mined from temperature data). However, tests should be used in material that is within four weeks of sale to give you time to make adjustments. Again, temperatures will help in diagnosing problems, but they will not indicate maturity.

Laboratory analysis of compost

Agricultural users and commercial nurseries are keenly interested in compost levels of nitrogen, phosphorus, and potassium and, in some cases, micronutrients such as iron or zinc. For use in potting mixes, salinity also is important because excessive salinity can interfere with seed germination. Typically, the nutrient content of compost in horticultural use usually does not limit growth in any manner. It is beneficial to have a laboratory analysis of compost from time to time.

Do’s and don’ts for the small-scale composter

• DO make provisions for adding supplemental water when needed. Capturing rainwater from roofed buildings or putting a float valve on a bleeder line from outdoor plumbing may be all that is needed to keep a nearby tank full. In dry climates, shaping the tops of the piles to capture rainfall may be helpful.
• DO move enough dirt to ensure the compost area drains well. Ponded water, especially around manure and compost, will cause odor and fly problems. A small box blade will help keep the area smooth and well drained.
• DO monitor compost temperatures every few days. Temperature alone will not tell the full story, but it can be an indicator of success or of imminent problems.
• DO instruct employees to keep garbage, plastics, carcasses and animal-health products (syringes, vials etc.) out of the compost piles.
• DO keep the composting area clean and well maintained. A good image is vital to marketing success.
• DO use the finished product in your own landscapes, planters and gardens. If you use it and like it, your clients will be more inclined to try it, too.
• DO have laboratory analysis performed on compost samples from time to time. Knowing your product will reassure your clients and will help you identify ways to improve your system. A routine analysis will include nitrogen, phosphorus, potassium, sulfur, and total salinity. Organic matter analysis adds significantly to the cost, but it will help you determine if manure-harvesting methods are picking up too much mineral soil, which reduces compost quality. If interested in using compost as a bedding material, periodic analysis for pathogens is good insurance.
• DO collect manure from the corrals and pens carefully. Try to keep mineral soil out of the manure, and keep track of how many wheelbarrow loads are delivered to your system every day.
• DO try to ensure good drainage from any outdoor horse pens from which manure is collected. Muddy conditions give you soil-laden manure, reducing the organic matter content per unit of compost.
• DON’T try to start a composting operation during extremely cold weather unless enough warm manure (above 50 degrees F) is available to immediately make a pile at least 4 feet deep. Small piles lose heat too rapidly to sustain temperatures suitable for the microbes. Turning piles during extremely cold weather may result in a slower rebound.
• DON’T use the compost piles to degrade or store carcasses if you plan on marketing the material to the public. Composting carcasses is more difficult than composting manure, carries a significant marketing risk, and is better suited to those operations that will be using the compost on their own property. If you want to use the composting system for carcass disposal, first make sure your system works well without carcasses; then add them and refine your technique as needed. In a good system, composting works well for animals of all sizes, from chickens to full-grown dairy cows. Professional guidance is important if you go down that road.
• DON’T give the compost away. A nominal fee stimulates interest, and the revenue will help you refine your techniques and equipment when opportunities arise.
• DON’T neglect damaged bin structures. Small damage quickly turns into heavy and expensive damage that can interrupt your compost system and make a quick recovery difficult.
• DON’T let employees use the compost piles as an alternative to the trash can. Baling wire, syringes, soda cans, twine, and other inert materials can be fatal to a marketing effort. Quality, consistency and appearance are the cornerstones of marketing success.
• DON’T assume that bins are the only workable configuration for a composting system. Bins lend an air of organization to the system, but the major difference between a bin and a pile is just having something sturdy to push against. If you can devise a static pile system that is manageable, and if appearance is not the most important consideration, don’t bother with the extra effort or expense.

Additional information

Beck, M. The Secret Life of Compost: A How-to and Why Guide to Composting—Lawn, Garden, Feedlot or Farm. Acres USA, 1997.

Epstein, E. The Science of Composting. Technomic Publishing Co., Inc. 1997.

Sweeten, J. M. “Composting Manure and Sludge.” L-2289, Texas Agricultural Extension Service.

TXDOT. “Furnishing and Placing Compost.” Special Specification Item 1009, Texas Department of Transportation.

Acknowledgments

Thanks to Gregg Veneklasen, D.V.M., for making the facilities at Timber Creek Veterinary Hospital in Randall County, Texas, available for this project.

The following individuals assisted with compost management, data collection and data analysis: Kevin Heflin, TAEX-Amarillo; Megan Campbell Williams, TAES-Amarilllo; and Bob Burkham, TAEX-Canyon. Thanks also are due to Dr. John M. Sweeten and Dr. Bob Robinson for providing resources for this demonstration through the Tierra Blanca Creek project.

Thanks to the reviewers for their many helpful comments and suggestions.

This demonstration and implementation project was funded in part by a Section 319 water quality grant from the United States Environmental Protection Agency and the Texas State Soil and Water Conservation Board.

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