In the same way that microorganisms degrade vegetative waste and turn it into a rich soil amendment, animal carcasses can be turned into an organic matter-rich material that can be spread on pastures and other agricultural land. To successfully compost animal mortality requires attention to the basics of any good compost pile: 1) proper carbon to nitrogen ratio (C:N), 2) moisture content, 3) available oxygen, and 4) temperature.

Carbon:Nitrogen ratio

Microorganisms that do the work of composting require nutrients in the form of carbon and nitrogen in a C:N ratio of roughly 30:1 or 30 parts carbon for each part nitrogen. Animal carcasses are high in nitrogen and the surrounding compost material should be high in carbon to create an acceptable C:N ratio. There are many suitable carbon sources for mortality composting. When deciding upon a carbon source to use, consider year-round availability, needed amounts, and cost. Producers are urged to seek out easily obtained materials for composting to make the process as easy and low-cost as possible. Materials such as wood chips from utility companies or municipalities or forage unsuited for feeding can be used. Used bedding after a livestock show at a local fairgrounds or horse arena can be an easily obtained, inexpensive carbon source. Other carbon sources used in mortality composting include old or moldy hay, crop residues such as straw and corn stover, silage, rice hulls, and ground corncobs, among others (Table 1). One commonly used material is sawdust. Sawdust has a high C:N ratio, small particle size, and the ability to absorb and retain moisture.

Particle size of the carbon source affects aeration of the pile with subsequent effects on pile temperature and decomposition rate. A general guideline for particle size given in some publications is roughly 1/8 to ½ inch (up to 1 cm) in diameter. Materials such as hay, straw, and corn stover will work better if coarsely ground prior to use. Alternatively, these materials can be mixed or layered with other material, such as manure or finished compost, in a 50:50 mixture and used. Mixtures of animal bedding and manure, such as that from horse stalls, work well as a carbon source.

Poultry or turkey litter has been used in mortality composting as a source of nutrients and microorganisms but it is very high in phosphorus. Check state guidelines before composting with litter. Because of environmental concerns, the Oklahoma Department of Agriculture, Food and Forestry requires mortality composting piles using poultry litter to be covered and runoff prevented. In Alabama, all compost piles regardless of carbon source must be covered to prevent runoff.

Moisture content

Microorganisms require proper moisture conditions to work and the optimum moisture content for a compost pile is approximately 50%. If the compost pile material is too dry, the bacteria have insufficient moisture and composting will be very slow. If the material is too wet, water fills the pore spaces in the compost pile resulting in the desired aerobic bacteria being replaced by anaerobic bacteria that do not require oxygen. Decomposition by anaerobic bacteria is very slow, generates odors, and does not produce sufficient heat to incativate pathogenic organisms in the compost pile. Too much water also increases the chance for liquid (called leachate) to run out of the pile potentially contaminating soil and water. Add water to the carbon material to obtain an adequate moisture level. While tap or well water can be used, pond or effluent water has the advantage of adding additional microorganisms and nitrogen to the pile.

Microorganisms require proper moisture conditions to work and the optimum moisture content for a compost pile is approximately 50%. If the compost pile material is too dry, the bacteria have insufficient moisture and composting will be very slow. If the material is too wet, water fills the pore spaces in the compost pile resulting in the desired aerobic bacteria being replaced by anaerobic bacteria that do not require oxygen. Decomposition by anaerobic bacteria is very slow, generates odors, and does not produce sufficient heat to incativate pathogenic organisms in the compost pile. Too much water also increases the chance for liquid (called leachate) to run out of the pile potentially contaminating soil and water. Add water to the carbon material to obtain an adequate moisture level. While tap or well water can be used, pond or effluent water has the advantage of adding additional microorganisms and nitrogen to the pile.

To test for moisture content, squeeze a handful of the compost material. If water drips out, it is too wet. If none sticks to your hand, it is too dry. The material should feel like a damp sponge. For a more accurate moisture level reading, use a portable moisture probe.

Available oxygen

n addition to proper carbon and moisture content, aerobic microorganisms require oxygen. The amount of oxygen available to microorganisms in a compost pile is largely dependent on the particle size of the carbon material. If the particle size is too small, there will be inadequate pore space to trap oxygen. If the material is too large, such as corn stover or unchopped straw, there can be too much air transfer and heat, odors, and moisture can escape the pile. Sawdust, mixtures of shavings and manure, or bedding and manure all have good sized particles providing adequate pore space.

Temperature

Microorganisms working in a compost pile include bacteria, fungi, and actinomycetes with bacteria by far outnumbering other organisms. In initial stages of composting, mesophilic bacteria (those that work best at temperatures of up to about 105°F) dominate. But as temperature increases, thermophilic bacteria that grow at temperatures up to 160°F (70°C) take over. Mortality compost piles work best in a temperature range of 130 to 150°F (54 – 65°C). A compost pile temperature above 131°F (55°C) for a minimum of 3 days reduces pathogens below detectable levels. Temperature in excess of 145°F (63°C) kills most weed seeds. A pile temperature that is too high, greater than 160°F (70°C), can affect bacterial survival, hampering the composting process.

Requirements for biosolids as given by the Environmental Protection Agency can provide some guidelines as to desired temperatures in a mortality compost pile. Requriements for Class A biosolids, allowing the completed compost to be used on public and private land, call for a temperature of 131°F (55°C) for a minimum of 3 days. Requirements for Class B biosolids are less stringent and require a temperature in excess of 104°F (40°C) for 5 consecutive days with a temperature of 131°F (55°C) or greater for at least 4 hours during that period. Class B biosolids can be applied to agricultural land. Producers wishing to provide compost to others or for use off farm may need to follow these guidelines. For further information see http://www.epa.gov/owm/mtb/biosolids/503pe/index.htm.

It is best to monitor temperature using a 36” or 48” (3 m) compost thermometer thrust into the pile’s core. Compost thermometers range in cost from $60 to over $100. Potential sources include Novatech International dial thermometer for Soil and Composite testing (http://www.novatech-usa.com/), REOTEMP Instrument Corporation, Heavy Duty Windrow Thermometers (http://www.reotemp.com/), and Omega Engineering Corp., Compost Thermometers (http://omega.com/). If a thermometer won’t be used, insert a long piece of metal rod, such as a piece of rebar, into the pile’s core, withdrawing it occasionally to feel if the pile is heating. At temperatures above 131°F (55°C), the tip of the rod can be held in one’s hand for only one or two seconds.

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