The Causes of Livestock Odors By: Rick Koelsch
Departments of Biological Systems Engineering and Animal Science, UNL
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Odors associated with livestock manure represent a growing challenge for livestock producers. Livestock production trends such as increased animal density and expanded reliance upon manure storage have added to the odor nuisance. In addition, implementation of manure nutrient management practices and maintenance of surface crop residues for soil conservation can result in additional odor challenges. An understanding of the cause of livestock odors is critical to identifying the solution.
Common Odors
Odors originating form livestock manure are a result of a broad range of odor producing compounds. Researchers have identified a total of 168 compounds associated with livestock manure. Thirty of these compounds have very low odor detection thresholds (less than 1 part per billion). Commonly reported compounds associated with livestock waste includes sulfur containing compounds, ammonia, volatile organic acids, phenols, alcohol's and others (see Table 1). The substantial range of odorous compounds from manure adds to the complexity of odor control solutions.
Table 1. Characteristics of common odorous compounds from livestock manure.
Ammonia / Sharp, pungent irritating odor / Both aerobic and anaerobic conditions / Lighter than air, disperses quickly, high pH and warm temperatures increase release
Hydrogen sulfide and other sulfur compounds / Powerful, rotten egg odor / Anaerobic conditions / Toxic, very low detection threshold, heavier than air, disperses slowly
Volatile organic acids / Key source of livestock odor / Anaerobic conditions / Results from partial completion of anaerobic processes
Phenolics / Highly odorous / Exist in raw manure and worse with anaerobic conditions / Lower detection threshold than hydrogen sulfide
Contributing Biological Processes
If manure can be maintained in an aerobic state (free oxygen present), chemical reactions will stabilize the organic compounds and minimize odors. Heavily bedded manure spread daily has only modest odor because aerobic conditions are prevalent. However, anaerobic process (absence of oxygen) add to the odor nuisance. Concentrations of odorous compound in liquid manure can increase by factors two to ten-fold for storage periods of only 24 hours (see Table 2).
Table 2. Concentration of odorous results form anaerobic conditions during storage of pig manure (From Storage of Piggery Slurry, A.G. Williams and M.R. Evans, 1981).
Fresh Slurry / Compound in: ------
Anaerobically Stored Slurry After 24 hrs. / % Increase
Total Sulfides / 1.6 ppm / 23.6 p.m. / 1350%
Phenol / 5.6 / 13.5 / 140
p-cresol / 24.9 / 31.4 / 26
Indole / 2.1 / 5.3 / 160
Proprionic acid / 310 / 571 / 84
Acetic acid / 1233 / 1923 / 56
Two broad groups of bacteria are involved in anaerobic decompositions. Acid-forming bacteria react with manure solids forming acids and many other compounds, many having strong odors. Next, methane-forming bacteria convert acids to odorless methane and carbon dioxide. Hydrogen sulfide also results from completed anaerobic processes which results in some odors.
Greater sensitivity of methane-forming bacteria to lower temperatures, low pH, and overloading of organic solids often allows acid-forming processes to outpace methane forming processes in manure storages and most lagoons. This imbalance causes the high concentration of volatile, odorous compounds in stored manures. This is a common cause of odors from anaerobic lagoons in the spring. If conditions allow the anaerobic process to proceed to completion such as in a properly sized anaerobic lagoon or anaerobic digester, the manure solids are stabilized and few odors result during land application. However, odor nuisance form sulfur compounds produced by the lagoon or digester can still be a nuisance.
The ammonia related odors originating from manure result from a different set of chemical processes. Urea, the primary source of nitrogen in urine, is quickly converted to ammonium ion state under most conditions. The ammonium ion is converted to ammonia and lost to the atmosphere at a slightly slower pace. Elevated temperatures and pH values contribute to more rapid losses of ammonia.
Most odors perceived by people as associated with manure are products of anaerobic decompositions (i.e. Sulfur compounds, volatile organic acids, phenols). Ammonia concentration is a poor indicator of the level of odor nuisance experienced by neighbors.