Composting livestock mortalities is a viable on-farm option. Here are the mechanics.
With rendering costs, biosecurity concerns and environmental regulations increasing, it's getting harder — and more expensive — to deal with carcasses on the farm. One alternative is composting — where naturally occurring bacteria (microbes) digest carcasses into a humus-like material called compost. The process works for manure, too.
Compost is a mixture of organic material that's been piled to undergo thermophilic (high heat, 120-160°F) decomposition. The heat generated by microbes known as thermophiles thrive on a diet of carbon (C) and nitrogen (N) in the right ratios, sterilizing pathogens and seeds.
Chris Augustin, North Dakota State University Extension specialist in Carrington, explains thermophilic bacteria break down the short organic chains of manure and carcasses, converting them into long, stable chains that are resistant to weathering and breakdown. The bacteria require oxygen, which is why bulking material is part of the compost equation.
He says the trick to composting is to maintain a C:N ratio between 20:1 and 40:1, 30:1 being ideal. Expect varying ratios of C:N as protein (low C source) and plant residues (high C source) intermix.
“Composting is more art than science,” Augustin says, as variables such as temperature, weather, carcass size and management make this process far from straightforward.
Step 1. Select the site
Site selection is important, Augustin says. Most large-animal compost piles are built in long, uncovered windrows, based on the carcass size and volume of material needed to cover them.
Piles should be constructed atop concrete or clay pads to avoid leaching or contaminating ground water. In fact, a permanent pad or base for composting piles makes it easier to work around in wet or dry weather.
Consider choosing a site with vegetation to reduce the environmental impact of leachate. Leachate is a flush of liquid, either from the decomposing carcass, or rain and snowmelt filtering through the pile. Harmful leachate can include chloride and ammonia nitrogen. A containment pond for leachate is ideal; Augustin recommends up to a 4% slope.
Follow good neighbor practices and avoid locating compost piles next to homes, main roads or picnic areas. Tom Glanville, Iowa State University (ISU) professor of ag and biosystems engineering, suggests locating compost sites 500 ft. away from homes and public roads, 200 ft. from public wells or visible bedrock outcrops, and 100 ft. from private wells or streams.
Step 2. Build the pile
Long, narrow windrows (base width 16-18 ft., about two loader widths) are easier to manage than broad-based piles. Though they will require more space, these windrows are easy to construct and maintain with typical farm equipment (Figure 1).
Pointed crowns and steep sides are recommended for wet climates to shed moisture, while a flat or concave top can be used in drier climates to accumulate moisture.
“It's important to have the right kind of envelope material under and around the carcasses,” Glanville says.
These materials can vary. Glanville field-tested corn silage, ground cornstalks and feedlot manure capped with ground hay. Base material should be about 2 ft. thick to absorb leachate. The heavier the carcass, the thicker the base should be.
Materials should allow air to pass through the pile, yet absorb leachate. Fine-textured material doesn't allow enough air to move through compost piles, which can leave large portions of carcasses intact. Conversely, overly coarse material can result in foul odors, flies and scavengers. Glanville had good success using a 2-in. grinder screen for envelope material.
“It can be too coarse, and it can be too tight. But there's a big wide friendly zone in between,” Glanville says.
For single or small carcasses, Glanville suggests a minimum pile width of 8-10 ft., and doesn't recommend placing more than two, full-sized carcasses back to back.
It's crucial to use enough material when composting and leave no part of the carcass exposed. A minimum 12-in. layer of co-compost (such as manure) should be placed around all sides of the carcass to discourage predators. This also helps heat the carcass and reduce odor.
Glanville recommends about 2 ft. of material over the top and sides of the carcass. He used 12 cu. yards of cover/base material for every 1,000 lbs. of carcass, equivalent to 1 ton of ground hay or straw, 1.4 tons of ground cornstalks or 3.2 tons of corn silage for large, uncovered and un-turned compost piles.
“It boils down to having about as thick of material underneath and over the top as the thickness of the animal itself,” he says.
Next Page: Managing the Pile
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Step 3. Manage the pile
Research shows a 1,000-lb. carcass can be reduced to skeletal remains in four months if the process began in warm seasons, or up to 8-12 months when started in cool seasons.
Glanville composted 54 tons of 1,000-lb. carcasses without ever turning piles when researching disease-related emergency disposal options. He notes that turning piles during warm weather will speed the process; doing the same in cold weather can chill the pile and slow things down. He cautions against turning too soon as it can increase odors and fly problems. Adding more cover material will prevent both.
“Turning incorporates more air into the system so you have aerobic rather than anaerobic conditions,” Augustin says. Anaerobic conditions can give off a hydrogen sulfide odor — a rotten egg smell. The incorporation of oxygen is needed for thermophiles and causes the pile to heat up.
Be sure to monitor temperature and moisture content of the compost pile throughout the process. “If it gets too dry, or wet, it won't compost properly,” Augustin says.
Try a “hand-squeeze” test, he says. Grab a handful of compost and squeeze — if a lot of water comes out, it's probably too wet. Augustin says the consistency is similar to that of a wet rag after being heavily wrung out. “You can feel it's moist, but without a lot of water coming out,” he says.
Using a long-stemmed thermometer (a 48-in. probe, about $150), take temperatures at the core. A properly built pile will reach temperatures of 130-150°F within 2-3 days of construction and remain there for at least two weeks. If the temperature remains below 130°F, the pile is either too wet or too small; the carbon source may be too coarse or fine for adequate oxygen supply.
Once carcasses are in a compost windrow, Glanville suggests recording when each pile was started to determine the amount of decomposition.
As the compost process begins, leave piles alone except to check temperature and moisture content. Wait 60-90 days before turning piles with large carcasses to avoid foul odors.
Augustin says the curing process is complete after 4-5 compost turns. Use your nose to determine if what's in the pile is compost or decomposing material. “Compost smells earthy,” he says. “Manure smells like manure.”
If it smells like compost, the final step is to let the pile's temperature acclimate with outside temperature.
Step 4. Compost byproducts
Bones and compost are the byproducts of a successful composting process. Glanville didn't have much luck getting rid of large bones and skulls during composting, noting, however, they were dry, clean and odorless. In drier climates, bones are intact but easily crushed under tractor tires.
Compost can be used for crop production; however, it should be managed as a phosphorus (P) fertilizer because some N is lost through ammonia volatilization. Whereas raw manure has 50% N available for the plant, compost will have 20-25% N, Augustin says. Be cautious about over-fertilizing with N or P, which can lead to eutrophication — excessively nutrient-rich runoff that can cause algae blooms in lakes and ponds.
To prevent that, calibrate manure spreaders to agronomic rates. On the plus side, compost's organic matter contains natural glues that reduce erosion.
Manure compost is great for gardens, but Glanville doesn't recommend carcass-compost be used to produce food for human consumption. Compost's greatest benefits are increased organic matter, lowered bulk density and promotion of root growth. The microbe-rich product also improves soil biology and water-holding capacity.