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Researchers Determine Fatty Acids, Compound Responsible for Manure Odor

In a recent study conducted by U.S. Department of Agriculture (USDA), Agricultural Research Service (ARS) scientists indicated that just three compounds in beef manure were responsible for generating over two-thirds of detectable odors.

These findings by ARS agricultural engineers Bryan Woodbury and John Gilley could help with developing techniques for controlling objectionable odors from manure used to amend crop fields. ARS is the USDA’s chief intramural scientific research agency, and this work supports the USDA’s priority of responding to climate change.

Woodbury and Gilley conducted a comprehensive study to identify compounds responsible for beef manure odor and to evaluate how land application practices, diet, soil moisture and application procedures affect odor emissions. The team used manure collected from feedlot pens where cattle consumed diets containing 0%, 10% or 30% wet distillers grain solubles. The scientists also evaluated two application methods — no-till surface manure application or disk tillage that incorporated manure into the soil — and collected air samples before and after water was added to the soil to assess the effect of moisture levels on emissions.

Beef cattle manure was applied at levels that provided 135 lb. of nitrogen per acre, which met the one-year nitrogen requirement for corn. After collecting and analyzing the air samples, the researchers determined that two volatile fatty acids — isovaleric acid and butyric acid — and the aromatic compound 4-methylphenol were responsible for over two-thirds of detectable beef manure odors. Most of these odors were released within 24 hours after manure was applied to the soil.

Incorporating the manure into the soil and irrigating afterward reduced most of the odor compounds that were measured. But the manure needed to be incorporated almost immediately after being applied to obtain the most effective odor mitigation.

The importance of tilling manure into soil was highlighted by emission measurements the researchers obtained for 4-methylphenol. The greatest emissions of this compound occurred from dry soils on no-till plots and were sometimes as much as 10 times greater than similar emissions from tilled soils.

Woodbury works for the ARS Nutrition and Environmental Management Research Unit in Clay Center, NE, and Gilley works for the ARS Agroecosystem Management Research Unit in Lincoln, NE. They published their research in the Journal of Environmental Quality in 2013.

Read more about the study in the October issue of Agricultural Research magazine.

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