Composting large animal mortalities

Livestock producers face a growing problem: what to do with dead animals when the rendering truck doesn’t come around any more? Or, only comes at a high price. Burial is legal in some areas, so long as the ground water or nearby streams are polluted. Incineration is expensive and is restricted by air quality regulations. Composting dead animals is currently the most practical and economical method of on-farm disposal.

The ABCs Composting

Where to start? As always: with the basics. Composting is a pretty robust, natural process. Understanding the process and how all the ingredients work together to is necessary to make good compost.

The first thing to realize is that composting relies on naturally occurring microbes like bacteria and fungi. These microbes require both nutrients and moisture, plus they generate heat. If they have a well-rounded diet, some air, some water and some shelter, they thrive, turning their food into useful humus-like materials that doubles as organic fertilizer. (The most efficient ones are the thermophilic microbes, which grow the best when the temperature is up above 131 degrees F.) Take away one of those favorable conditions, however, and the composting process suffers.


Every living thing does the best when it eats a properly balanced diet, and compost microbes are no exception. The most important characteristic of a compost diet is the relative abundance of carbohydrate and protein - the carbon-to-nitrogen (C:N) ratio. If it’s around 30:1, this is optimal – but the microbes can handle anything from 15:1 all the way up to 50:1 and higher. Of course, tolerating a low or high C:N ratio is not the same as thriving, but if the C:N ratio’s slightly on either side of 30.00000:1, they’ll do fine.

Air and Water

In a compost pile, air and water are in competition for the empty pore space between the solid particles. The most efficient microbes, which are aerobic (requiring oxygen), need both of them, in the proper balance, so that they can eat the nutrients dissolved in the water but still breathe enough oxygen to digest the nutrients and harvest energy for essential functions. A moisture content of about 50-55% is just about right, although they can survive a bit above or below that. If the pore space dries out too much, the microbes will starve and the process will shut down. But if the pore space floods with water, a different set of microbes – the anaerobes – will out-compete the aerobes for food. Anaerobes are the ones that generate the distinctive smells of dead fish, rotten eggs and rotting carcasses.

Some Like It Hot

You’ll recall that the most efficient compost microbes are thermophilic and even generate their own heat. But they’re way too small to stay warm on their own. What they really need is insulation. A porous mixture of compost materials is a super insulator, and a 5-foot-deep layer of compost is thick enough to keep the microbes warm even in the most brutal Minnesota blizzard. In the Texas Panhandle, even 3-4’ is plenty of insulation for the core of a compost pile to stay in the thermophilic range.

Try This at Home!

Here are a few points to remember before composting a carcass:

  1. Left alone, large, intact carcasses will rot from the inside out.
  2. Rotting carcasses generate lots of odor and gas.
  3. Intact skin will contain the gas to a point.
  4. The larger the carcass, the more spectacular and explosive the eventual escape of gases.


To compost your carcass successfully, at a minimum you will need (a) a big pile of some carbon-rich material, (b) a big machine to move stuff around, (c) some kind of water source, (d) a long-stemmed thermometer and (e) something to open a carcass with.

(a) Carbon-rich material can include lots of different things. Sawdust is splendid stuff if you can get it. Wood shavings will work well, too, if they’re small enough – the larger they are, the less effective they are in a compost pile. Community brush-chipping sites often make a very nice material for composting. Rotten old hay bales, cotton gin trash, shredded sugar beets, peanut hulls and other browncolored crop residues will work nicely. Mixing two or three types of carbon-rich material together is even better. (The wider the variety of particle sizes in your pile, the better the balance among air movement, insulation, pile strength and surface area for the microbes to feed on.) You will need about 3-5 cubic yards of the material for every 1,000 pounds of carcass; less if the material yields its carbon easily, more if it does not. “Seeding” the pile of material with some manure or unfinished compost will kick-start the composting process even before the carcass shows up. (We think that’s a pretty good idea, in fact.).

(b) Some type of front-end loader or other machine that can move the carcass, assemble and turn the pile and load the finished compost into a spreader truck is necessary. You won’t need the loader every day, but you will need access to it to build the pile, to turn the pile every three to six months, and also to turn it in emergencies, such as if rainfall drenches the pile or it gets too hot for safety. The larger the loader, the faster you’ll be able to get the job done.

(c) Water availability is critical, especially in arid climates. Mixing water into the pile will be necessary every couple of months as the high interior temperatures dry out the pile. If you have a nearby well or water line already, a polyethylene tank can be rigged with a float valve so that it stays “topped off” with water and ready for use. If one of your stock tanks stays wet year ‘round, you can simply lay a suction line with a foot valve into the middle of the pond – hang it from an inflated inner tube if there’s a chance the water level will drop significantly during the year – and run the suction line into a small, gasoline-powered water pump. A highpressure nozzle, especially one with an adjustable spray pattern, will give extra flexibility and improve efficiency.

All other things being equal, if you have a choice between pristine drinking-quality water and some sort of non-potable water, use the non-potable water, as long as your pump will handle it.

(d) Some people say that a long-stemmed thermometer is optional. We don’t. Spend the money and get one. (More on that later.)

(e) Open the carcass? Yes. Leaving large, intact carcasses to their own devices generally results in a mess. There’s usually no obvious need to leave the carcass intact, unless you suspect a zoonotic disease risk (that is, a disease that can be transmitted from livestock to humans or other animals) associated with it – in which case you need to contact your local health authorities for disposal advice instead of putting the carcass in the compost pile.

But if the animal passed away in some routine way, opening up the carcass will (1) prevent carcass explosions and (2) increase contact between the carcass and the carbon-rich materials around it, speeding up the process. Carcasses may be opened with anything from a shotgun to large shears or a hunting knife. If you want to go a step further or if you need to limb the carcass for some reason, a big hacksaw will help.

Opening up the carcass is optional. Typically, we put the carcass in the pile long before it has a chance to ripen, and then the compost microbes perforate the carcass for us. When this happens, you will notice that the pile seems to have collapsed not too long after you assembled it - this is called “yielding”.

Building the Pile

The first thing to recognize when you’re learning to build a compost pile for carcasses is that these animals are more than 50% water, much of which is contained in the gut and the blood stream. When the carcass “yields,” a flush of liquid will drain from the carcass shortly thereafter.

This liquid should never percolate into the ground or run downhill into a creek or a stock tank or a wellhead. Two key principles for establishing a carcass pile are (a) good site selection and preparation and (b) dry, absorbent base material.

Site Selection

Many complaints can be avoided if you select a site that:

  1. Is away from the road. This goes without saying, perhaps, but messing with dead animals in an area that is highly visible to neighbors and passers-by is an excellent way to get complaints, even if nothing improper is going on.
  2. Is vegetated. Vegetated land interferes with environmental impact by soaking up and using the nutrients that are contained in liquids that may leach from the compost pile, converting them to plant matter and keeping them from draining into the water table or running off into surface water. Root systems help by maintaining soil porosity, which in turn reduces runoff.
  3. Features soils with low permeability. Soil permeability, a measure of how fast water can move through the soil under given conditions, is one of those soil conditions that is desirable in moderation but may cause problems in excess. Those of you with shallow ground water beneath the property may end up polluting your neighbor’s ground water if you build a compost pile on highly permeable, sandy soil. In the first place, these soils do not have a lot of structural strength, which means that heavy machinery will easily disturb them. Second, they drain very quickly, reducing the opportunity time for plant roots to capture the nutrients and other dissolved solids contained in pile “leachate” before it reaches the shallow water table. Clayey soils, caliche, crushed “bottom ash” (a cement-like residue from coal-fired power plants) or other less-permeable material can be used to pave the site or can be mixed into the soil and compacted in place to reduce seepage into the ground water.
  4. Has good air movement. All other things being equal, higher wind speeds cause odors to disperse more readily. When winds are light, odor intensity persists further downwind than when stronger winds mix odors into the upper air via turbulence. Choosing a sheltered site in a valley frequently decreases exposure to the higher wind speeds as compared to a site on a hilltop.
  5. Is away from public areas or the neighbor’s property. This is just common sense.

Dry, absorbent base material. For efficient composting, most of the compost pile will need to be maintained at an optimum moisture content of between 45 and 60% by mass (wet basis), but precipitation and/or a collapsing carcass may yield a flush of moisture that saturates the pile locally and then drains by gravity through the base of the pile. Capturing that “leachate” before it reaches the soil or pavement surface is the job of a dry, absorbent base layer. This layer should be thick enough and strong enough to support the full weight of the carcass without being significantly compacted; the heavier the carcass, the thicker the base layer should be. This layer will need to be a minimum of 18” thick. It should also be dry, absorbent and carbon-rich. Examples of excellent base materials include ground hay or straw, gin trash, finished compost, nut hulls or sawdust.

After laying down a base layer to capture leachate before it escapes, we then prepare to surround the carcass with the optimum composting mix. Step 2 should be adding a 12-inch layer of moistened, well mixed, co-composting materials. This is the stuff that should be at its optimum moisture content (45- 60%) and carbon:nitrogen ratio (25-45) with a well-graded distribution of pore sizes. With highly carbonaceous materials like wood shavings or sawdust, mixing in some manure or spare nitrogen fertilizer will accelerate composting. The better approach is to have this material mixed up, moistened and already composting in a pile off to the side somewhere close by – then bite into that pile with a loader, spread a bucketful atop the dry base layer, and away we go. The pile is now ready for the carcass.

The carcass should be surrounded on all sides with at least a foot of co-composting material. Having a hoof, a tail or an ear sticking out of the pile is a great signal to predators in its own right; however, minimizing the depth of cover material on all sides, whether flesh is showing through or not will also draw predators. (Once a predator has discovered the presence of an animal carcass, he will return again and again to the same site, no matter how well you cover the carcass after he discovers it. At that point, predator-control options are limited to traps, fences or a stakeout.) A 12-inch layer of co-composting material around all sides of the carcass will ensure thorough, uniform heating of the carcass, reduce odorous emissions and discourage predator access.

To reduce odors and predator access further, some composting specialists recommend covering the entire pile with dry co-composting materials similar to that which was used as the absorptive first layer. Although this approach has its merits, at some point it becomes a matter of balancing economics and the Law of Diminishing Returns. This approach still may not reduce odorous emissions and predator signals to zero, and although it will improve pile insulation and heat retention, it will reduce oxygen transfer and increase the overall requirement for compost feedstocks. Merely covering a poorly constructed pile with additional dry feedstock will obscure a problem rather than solving it.

Nature Can Disrupt Composting

Rain, snow and cold can all disrupt carcass-composting. Where rainfall is normally plentiful, piles can easily go anaerobic and generate the nastier odorants unless you design your piles and structures to shed rainfall rapidly. Conversely, folks in semi-arid regions will find it more likely that the pile will dry out and go dormant if the pile is left to the whims of nature.

Shedding excess rainfall is a matter of either putting a roof over the pile or building the pile with a steep, pointed crown and sloping sides. Construction plans for covered compost bins are beyond the scope of this article, but you can still get them from the MidWest Plan Service

(MWPS; see or the Natural Resource, Agriculture and Engineering Service (NRAES; see A pile with a triangular cross section will shed water more efficiently and will increase the rate of loss of heat and water vapor as compared to a pile with a rectangular, trapezoidal or semi-circular cross section. Capturing scarce rainfall or snow, by contrast, is usually accomplished by building the pile such that its top is flat to concave, a geometry that also retains more heat and dries out more slowly. Where rainfall is scarce, efficient composting will require a ready source of supplemental water as the piles dry out over time.

It is possible to compost carcasses successfully even in the northern latitudes. An active compost pile will melt snow accumulations provided that it is sufficiently large to insulate itself and retain heat in the core of the pile. The larger the pile, however, the greater the threat of spontaneous combustion as biological self-heating gives way to chemical oxidation, which does not require microbes. Allowing a hot pile to dry out increases the risk of fire by spontaneous combustion, but those fires can be prevented by careful management using a long-stemmed thermometer and a rule of thumb concerning compost moisture.


Compost piles need attentive supervision, or else they are liable to create havoc. The most important diagnostic tool in the composter’s tool chest is a long-stemmed thermometer. An alternative is an electronic thermometer than can be buried in the compost pile and read without disturbing it. Long-stemmed thermometers provide the flexibility of using the same device to measure temperatures in several different locations to evaluate temperature uniformity in the pile. Select a probe length that will reach at least all the way to the center of your pile, and be sure to read the instructions carefully for techniques to insert the probe without bending or breaking the stem.

Now that you have temperature data, though, what do you do with it? If the temperature remains below 131 degrees Fahrenheit (131F), or 55 degrees Celsius (55C), more than three days after pile assembly, you’ve built a poor compost pile that may not compost properly. Temperatures above 131F indicate that the core of the pile has reached conditions favoring our thermophilic aerobes, and odor nuisance is much less likely. A properly built pile should reach thermophilic conditions within 2 or 3 days and remain above that threshold for at least two weeks. But pay attention! If the temperature gets much above 150F (66C), the risk of spontaneous combustion increases dramatically, and the pile will need to be turned and moistened some.

To sum up, then, temperatures in the pile core need to remain between 131F and about 150F for a couple of weeks to ensure that the pile is composting properly. If the temperature does not reach 131F, it probably means that the pile is either too wet or too small, and/or its carbon source may be either too large-grained to insulate well or too small-grained to permit oxygen to enter the pile and keep the thermophilic microbes happy.

Moisture is the other major influence on composting efficiency that can be measured rather easily. The base layer should be very dry, but aside from that, the rest of the co-composting material covering the carcass should be between 45 and 60% moisture by weight. Checking compost moisture is a pretty simple matter. Grab a handful of the mixed material and squeeze it –and look for droplets coming out of your fist. If they do, that sample of material is too wet. If not, then drop the material and look at the palm your hand or your glove, and if it does not have an obvious sheen of water on it, the sample was too dry. Material that is too wet may also smell rancid, sour or otherwise offensive; material that is too dry may not smell at all.

Once the pile is cooking right along, leave it alone for a few months except to check the temperatures and moisture content. The larger the animal, the longer it will take for the soft tissue (muscles, viscera, ligaments, cartilage, fat) to be converted to humus. A 1,400-lb dairy cow or a 2,000- lb horse may need 6 months of active composting to clean off the bones; a 500-lb calf may need only 3- 4 months. If the temperature gets too high, though, and the compost is drying out, turn the pile thoroughly and add water to get the moisture back up to near 60%.

If any soft tissues are exposed in the process, you will need to add more of the carbon-rich feedstock to meet the 12” cover requirement again. Phase I, the active composting phase, ends when the soft tissues are degraded and the bones are clean, and it’s time to turn the pile and add water again. Even though the carcass itself is history, the compost should heat up again and continue where it left off during Phase II.

This secondary phase is the time during which the microbes finish off most of the available carbon and nitrogen and convert it into microbial biomass. At this point, except for the presence of a skull, a pelvis, a couple of femurs and some other large bones, the compost pile is just like any other compost pile. Let the Phase II pile sit for a couple more months until the temperature drops to within 10 degrees or so of the air temperature. At this point, it is ready to be turned one more time and allowed to “cure,” which means that the remaining, undegraded organic compounds – for example, foul-smelling plant-killers like acetic acid – may still need some time to degrade all the way to carbon dioxide and water vapor. This curing phase isn’t really composting any more, but it does help ensure that the compost can be safely applied to pastures, gardens or cropland.

We don’t necessarily recommend that you try to sell the finished product to someone else. Its one thing to market manure-based compost, but it is another thing entirely to market a material derived from dead animals. If your neighbors are interested in taking some of your compost, make sure you are clear with them about what the “ingredients” were. In general, it’s probably wiser to use this compost on your own property.

Wrapping it Up

If you want to get along with your neighbors and protect the air, water and soil resources around you from environmental insult, it’s going to take some thought, planning and effort. Composting large carcasses successfully is not difficult, but neither is it automatic.

Summarized by staff from an article by:
Brent Auvermann and K. Heflin, Texas A&M University, TX, USA.
Originally published in: Proceedings of the 7th Western Dairy Management Conference, Mar 2005