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Large carcass composting in cold semi-arid climates

Thomas Bass and Julia Dafoe Published on 12 November 2013

Proper management of animal mortalities on livestock operations has important implications in nutrient management, herd and flock health, as well as the health of farm families and public health.

For this reason it is imperative to be familiar with best management practices for dealing with dead animals. It is also important to understand that provinces may have laws related to proper disposal or processing of animal mortalities.

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Provincial ministries of agriculture/livestock and environmental agencies are great places to start looking for information on local laws.

The purpose of proper mortality disposal is to prevent the spread of infectious, contagious and communicable diseases – and to protect air, water and soil quality. For this reason, there are legal issues and requirements related to nutrient management.

During the permitting process and development of nutrient management plans for animal feeding operations, disposal of routine operational mortalities and catastrophic mortalities must be defined.

Mortality composting is often a preferable practice to other methods of large carcass disposal.

Composting large carcasses can save labour and land. This practice allows a dedicated area to be used and reused countless times for carcass management; it is done above ground, thereby reducing the number of labour-intensive burial pits created as well as minimizing the number of buried carcasses on the property.

Many dairies across the U.S. and Canada have demonstrated success in managing operational and catastrophic mortalities by composting.

However, there is still value in discussing this practice in order to assist more producers in adopting it and in sharing information to improve success where mortality composting is already in use.

In 2009, the Montana Agricultural Experiment Station at Havre and MSU Extension developed an applied research project to demonstrate the practice, with particular focus on issues encountered in our cold semi-arid climate.

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Additionally, only carbon materials available locally or on-site were used: they included wood shavings, hay and straw, manure and silage. Winter temperatures in northern Montana routinely drop below -17°C, while summer temperature can reach up to 38°C.

Annual precipitation is in a 30-centimeter to 35-centimeter range. The Havre research station maintains a small beef feedlot. Full-grown cattle and calves that died of natural causes were used in the project.

Accepted protocols, documented in scientific literature and Extension bulletins, were followed. The Cornell Waste Management Institute has an excellent publication on mortality composting (click here for the website); additionally four land-grant universities in the Rocky Mountains co-published a manual and set of tools to further guide the adoption of this practice (click here to read the article).

Those familiar with manure composting, or even garden composting, may be aware of the 30:1 carbon-to-nitrogen ratio recommended for successful composting.

This is not a requirement for mortality composting as the carbon materials grossly exceed this ratio, and for good reason. The abundance of carbon in this practice serves to insulate the pile, prevent leaching and filter odours.

The basic take-home points outlined in these publications and followed by the Montana State demonstration project include:

  • Site selection: The composting site should offer all-weather access for vehicles and basic power equipment. It should be on high ground and protected from any additional run-on (storm water other than direct precipitation).

The site should also be separated by at least 60 metres from water wells and surface water.

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It is advisable to have some visual screen from casual passersby, particularly public roads and neighbouring facilities. The site should also be separated from live animal/production areas.

  • Pad and optional structures: A natural or imported clay pad will help prevent any potential leaching to groundwater. In some states, an engineered pad may be required. (In the Montana case, we had adequate low permeable soils and we were well above groundwater.)

This practice can be done via windrows or in bins. We chose temporary bins made from old large hay bales. This allowed us to contain the pile’s footprint and offered the option to barricade the front of the bin to deter dogs and coyotes.

Windrows, adding additional piles up against the previous over time are preferable to some operators and sites. Some producers use permanent containment structures, such as bins or short walls; these may be made of heavy wood or concrete.

  • Building a pile: Successful composting of mortalities has been reported with carbon base thicknesses between 30 to 60 centimeters.

The base should be comprised of a material that is both absorbent and bulky, such as wood chips and shreds with sizable pieces being 10 to 15 centimeters in length.

This base material is important for achieving satisfactory porosity for aeration. Material that packs tightly or is excessively wet is not recommended. To save time, always have a couple bases ready to accept animal mortalities.

The carcass can be placed once a satisfactory base is established. Where wood chips are not available or are expensive, other materials may suffice; examples include coarse-chopped corn stalks or even straw.

  • Incorporating the carcass, covering: A large carcass or multiple small carcasses can be placed on the centre of the carbon base. Core carbon material can now be added around the mortality.

This is an opportunity to use a variety of materials found on-site or regionally. The material added directly around the sides and top of the carcass does not need to be as porous as the base. If the carbon source has odour associated with it, the core around the carcasses is the ideal place for its use.

Manure, silage and other active materials with a low C:N ratio may be ideal for this layer. Finally, the cap may also be a finer material than the base and should be low in odour.

Core and cap materials such as silage or moist sawdust in the 50 to 60 percent moisture range are ideal. The addition of the cap should bring the final margin around the carcass to a range of 45 to 60 centimeters.

  • Winter tip: Surrounding the carcasses in warm or active compost will give them a quicker start, especially for winter or early spring mortalities.

In Montana, producers have been successful with attaining necessary temperatures by placing non-frozen carcasses in the pile and building the core with silage, warm compost or manure solids.

The pile should always be capped with a “clean” material such as sawdust or chopped straw. Likewise, getting carcasses started in compost before they freeze in the field helps the pile attain and maintain desirable temperatures.

  • Moisture, oxygen and temperature: The base materials discussed can be quite dry; however, core materials that promote composting are ideally 50 to 60 percent moisture.

Organic matter in this range can be squeezed in a fist, will feel moist but will produce little to no free water. Large carcass composting does not require immediate and frequent turning, a conventional method to oxygenate the pile.

Oxygen flow is passive, relying on the pore spaces in the carbon material. Based on the simple steps of constructing such a pile, core temperatures should naturally reach upwards of 60°C within a couple days.

This is a reflection of microbial activity in the pile. A long-stem (45 to 90 centimeters) thermometer inserted into the pile after construction can measure the internal temperature of a compost pile.

Temperatures in the 60°C to 70°C range, held for 48 to 72 hours, are necessary to sterilize weed seeds and destroy pathogens. If a pile fails to heat up, it may be too wet or the carbon material may be packed too tightly.

  • Management: Now that the mortality is properly enveloped or incorporated, the process of composting takes four to 12 months depending on mortality size and mixture.

During this phase it is a good practice to monitor the piles and intervene at the appropriate times (i.e.: when additional cover is needed or pile is emitting odour).

With windrows, some operators will leave a marker where the last mortality is located to avoid accidentally disturbing the active site. The process of composting mortality is passive.

This phase of the process should not be disturbed for three to six months depending on animal size. During this time, microbial activity from bacteria and fungi are performing their function by reducing the carcass to a homogenous organic material.

Most of the easily decomposed tissue is virtually “gone” within six weeks. Fungi need the extra time to continue working on the remains.

The pile can be disturbed for mixing, watering and stockpiling for curing after four to six months in the passive phase. This is the only time one actually needs to turn and mix mortality compost.

Any residual large bone fragments can be removed at this point and the pile left to cure for four to 12 weeks.

During curing, the pile will continually cool to near ambient temperatures. This compost may be selectively land-applied or reused as core material for future mortalities.

In the Montana demonstration, mortalities were incorporated into piles throughout the late winter and early spring. This included 545-kilogram cows and several smaller stock.

Though ambient temperatures reached into the single digits, core temperatures remained above 60°C for up to four weeks. Upon partial excavation at about three months, only some connective tissue and a few large bones remained. At six months, only a few bone fragments were identifiable.

Though known coyotes and farm dogs were in the area, the piles were not disturbed by scavenger activity.

Odour was minimal to non-existent. A field day was hosted at the site, where participants could view the partially excavated pile at about three months. There were neither perceptible odours nor flies while the pile was open.

The Montana site is now used as a dedicated mortality composting facility. It can be managed with a skid-steer or small tractor, whereas digging burial pits requires larger equipment, more operator hours and may pose site-specific threats to groundwater.

As rendering options become less available and land-fill disposal requires transportation and tipping fees, composting usually proves to be an economical choice as well.

Many producers have already realized these benefits and mortality composting may be a new best choice for many others.

In summary, composting is both science and art. A producer starts by following the basic recommendations but develops a sense of what works best for their site, materials, climate and management style. Some level of trial and error is to be expected.  PD

Julia Dafoe is from the Montana Agricultural Experiment Station.

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Thomas Bass

Livestock Environment Associate Specialist
Montana State University Extension

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