In field experiments by microbiologist Patricia Millner at the ARS Environmental Microbial Safety Laboratory in Beltsville, Md., 99.99 percent of the pathogens Escherichia coli and Salmonella were eliminated from manure after composting. But the scientists stressed that composting for the right amount of time, and at the proper temperature, is the key to controlling pathogens in compost.

Composting is one of several methods farmers can use to treat animal manure, sewage sludge and other organic residuals that contain pathogens or parasites of public health concern. The temperature of an aerated compost pile — one that's turned frequently to allow air to penetrate — must be at least 131 degrees Fahrenheit for three consecutive days to reduce pathogens to safe levels.

For un-aerated compost piles — those that are turned only five times — the temperature must reach 131 degrees Fahrenheit for two weeks.

In many states, untreated manure can be applied to farm fields. However, this can introduce pathogens and parasites into soils, and even into runoff or irrigation water. As organic vegetables and fruits gain popularity, the demand for animal manure is expected to increase.

Millner is conducting research on what she calls hybrid composting systems. Not only do these systems reduce numbers of pathogens like E. coli and Salmonella, they also reduce excess available phosphorus and keep the ratio of nitrogen to phosphorus within a range acceptable for use in areas that have nutrient-management plans. This approach means that composting can address nutrient, pathogen and odor concerns all at the same time.

ARS is the U.S. Department of Agriculture's chief scientific research agency.