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Background:
"Whence Come Hog Manure Odors?"
(March 2000 AR magazine story)

Antimicrobial Compounds May Reduce Swine Waste Odor

Agricultural Research Service scientists have found that certain microbes, called gram- positive anaerobic bacteria, often are responsible for offensive odors in stored swine manure. Now the researchers are seeking a way to help better bacteria prevail over the worst stinkers.

Gram-positive anaerobic bacteria are classified as such because of their ability to retain particular stains or dyes. Examples of these bacteria include clostridia, lactobacilli and streptococci.

At the National Center for Agricultural Utilization Research, Peoria, Ill., microbiologist Michael A. Cotta and his colleagues used classical microbiology and modern molecular techniques to identify the predominant bacteria in slurries from hog manure pits. The scientists work in the center's Fermentation Biotechnology Research Unit.

Slurries rich in bacteria associated with methane and foul-smelling compounds could become less offensive if hog producers treated the pits with some as- yet-to-be identified effective antimicrobial compounds, targeted against these bacteria.

To the scientists’ surprise, the commercial antimicrobial compound, monensin, which inhibits growth of gram-positive bacteria in the rumen of cows and other domestic animals to improve milk production or weight gain, just didn’t quite do the trick in laboratory research to reduce hog manure odors. But the results did provide encouragement that the concept might be sound.

For their experiments, the scientists used samples of slurry of manure from a swine barn pit, enriched with more hog feces. They found that, in closed bottles, monensin quickly reduced methane and carbon dioxide production and continued to do so for 28 days. And the total amount of volatile fatty acids in the bottled samples decreased, at least slightly.

However, one of the foulest-smelling kind of fatty acids, butyrate, increased 3-fold. So the search continues for an antimicrobial compound that will inhibit the populations of butyrate-producing microbes.

ARS is the USDA’s chief scientific agency.