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Volatile Fatty Acids in a Redox-Controlled Cattle Manure Slurry¹

ABSTRACT

Fatty acids have been implicated as a part of the overall odor problem associated with the management of animal wastes. This study was conducted to determine the relationship between redox potential (E_h) and volatile fatty acid (VFA) transformations in a cattle manure slurry. The controlled redox system was sequentially lowered in 100-mV increments from +300 to –200 mV, and acid concentrations of the suspension were determined by gas chromatography. Acids associated with the added dry manure disappeared at + 300 mV E_h, and only acetic acid was detected at very low concentrations at E_h levels of +200, +100, and 0 mV. Acetic, propionic, and isobutyric acids started to accumulate at –100 mV and butyric acid began to accumulate at –200 mV. The rate of acid formation was considerably higher at –200 mV than at –100 mV, except for isobutyric concentrations, which began forming at –100 to –200 mV but remained consistently low. Acid accumulations during the 7-day period at –200 mV were 6.66, 0.86, and 0.05 mg/g manure for acetic, propionic, and butyric acids, respectively. Saturated manure samples incubated at constant temperatures ranging from 5 to 35°C reached an E_h of –100 mV after 2 hours at 5°C and after 1 hour at 15°C. A –200-mV E_h was reached after 0.5 hour at 25 and 35°C. Therefore, the conditions required for acid formation and, perhaps, release as odor components, could occur within hours after a feedlot has received moisture that initiates and accelerates microbial activity.

Key Words: oxidation-reduction • E_h • odors • VFA

¹ Contribution of USDA-SEA-AR, P. O. Box E, Fort Collins, CO 80522, in cooperation with Colorado State University Exp. Stn., Sci. J. Ser. no. 2561.

² Soil Scientist and Chemist, respectively, USDA.

Received for publication August 25, 1980.