Winter 2008

Planning Considerations for Livestock and Poultry Mortality Disposal: Part 3 - Composting

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By Tom Glanville, Department of Agricultural & Biosystems Engineering, Iowa State University

In the summer and fall issues of the Odor and Nutrient Management Newsletter, the pros and cons of rendering, incineration, and on-farm burial of poultry and livestock were discussed.  In this issue, I’ll review key aspects of the relatively new on-farm disposal option of composting.

Spurred by declining availability of rendering service, and growing public concern over groundwater pollution potential of on-farm burial, use of composting for management of mortalities was pioneered in the large-scale broiler and turkey production operations in Maryland and Delaware during the late 1980s.  Composting turned out to be particularly well suited for poultry operations since the sawdust and wood shavings typically used as floor bedding in poultry barns proved to be an excellent cover material for carcass composting.

As word of the success of composting spread, it was applied to larger species, often in response to reduced access to timely rendering service. Swine mortality composting was pioneered by Charles Fulhage at the University of Missouri in the early to mid 1990s, and today is used by a significant and growing proportion of swine producers throughout the Midwest.  In the late 1990s composting was adapted for disposal of routine cattle mortalities in Colorado, Texas, Oklahoma and Kansas.

The composting methods used in poultry and livestock production are relatively simple. The process is started by laying down a thick base layer of absorptive crop residue such as ground cornstalks, ground straw or -- in the arid regions of Texas and western Kansas -- dry scraped feedlot manure.  A layer of carcasses is placed on top of the absorptive base, and then covered with 6 to 24 inches of the same material used in the base.  The larger the species, the thicker the layer of cover material necessary to absorb liquids, retain heat, and discourage insects and rodents.  If carcasses are large (greater than 500 pounds), the compost pile is usually limited to a single layer of carcasses.  For smaller species, it is common to stack several layers of carcasses, separated by a suitable thickness of cover material, until pile height reaches roughly 5 feet. 

The composting process is sensitive to moisture levels that are too high or too low.  In Iowa’s humid climate, excessive wetness is the most common problem.  Since carcasses themselves are 65 percent water, it doesn’t take a great deal of precipitation to cause excessive pile wetness, a condition that can slow decay and cause odor problems.  For this reason, it is recommended that composting in Iowa be done in roofed bins that protect the pile from precipitation during wet seasons.  If composting will not be done on a permanent basis, then covering piles with a tarp can provide temporary protection against excessive precipitation.

In cases of catastrophic loss caused by fire, flood, or disease, unsheltered windrows are often the most practical emergency composting option.  Piles are formed into long narrow “windrows” that increase the potential for natural ventilation and drying of the piles.  Since construction of a roof, or even use of tarps, may not be a practical option during emergencies, extra thick base and cover layers are used to temporarily absorb precipitation until it can evaporate from the pile.

In field research conducted for the Iowa DNR and USDA by Iowa State University, emergency composting in unsheltered windrows proved effective in heat treating and decomposing 1,000 pound cattle carcasses during all seasons of the year. With cover and base layers of sufficient thickness, odor emissions were negligible and inoffensive (usually characterized as smelling like straw or cornstalks), and the total amount of N added to the soil beneath the windrows was only 10 to 25 percent of the total amount of N in the carcasses and which would have been added to the soil by burial. Furthermore, the highest N concentrations were in the top two feet of soil where crop uptake is possible, rather than 5 to 6 feet below ground as is typical for burial. 

Producers who use composting say the things they like about it include the flexibility to handle carcasses of all sizes. Producers also say that it puts them in control of timely disposal during hot weather, and can be done with typical farm equipment such as a skid loader, tractor loader and solid manure spreader. 

Negative aspects of composting include the need for relatively large quantities of cover material (7 to 8 cubic yards of ground straw or cornstalks per 1,000 pounds of carcasses in bin composting, and as much as 12 cubic yards per 1,000 pounds for emergency windrow composting of large species). Use of finished compost as a cover material can help reduce cover material quantities somewhat. 

Another important consideration for composting is that the process takes longer than other disposal options. Decomposition time depends on both carcass size and seasonal temperatures. During warm weather, birds and small stock weighing 20 pounds or less can usually be decomposed in 2 to 4 weeks.  Larger sheep and swine weighing up to 200 pounds can take 2 to 3 months during warm weather, and 1,000 pound cattle can take 4 to 5 months.  If composting is begun during cold weather, decompositions times can be 2 to 3 times those for warm weather.  The finished compost usually contains bones (particularly for large species), and the end product must be disposed of, usually by applying it to cropland much like solid manure. Unlike manure, however, the N and P content of mortality compost is usually quite low unless dry manure is used in the composting process. 

As for all mortality disposal options, cost is a key consideration. Composting costs vary widely depending on whether special structures are erected, if cover material is available on the farm or must be purchased, and also on the size of the operation. In a comprehensive 2001 survey of 300 Iowa swine producers using the four major disposal options (rendering, incineration, burial, and composting) the average reported total cost (sum of all capital and operating costs) for composting was less than for incineration and burial, but greater than for rendering.

For more detailed information on routine and emergency composting procedures, check out the following websites maintained by the Agricultural & Biosystems Engineering Department at Iowa State University.


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