What factors influence the odors in manure?

Beyond the manure itself, there are measureable and descriptive ways to describe the odors of manure as well as how and why people perceive these odors certain ways

It’s a beautiful spring day as you are driving along a back country road. The sun is out, your windows are rolled down and the birds are chirping their songs when all of a sudden an offensive odor hits you right in the nostrils. Someone obviously hit a skunk not long before you came along. What is it about this smell that makes it so offensive and does this have any relation to the odor of livestock manure

There are several factors and characteristics that play into odors, specifically manure, that allow us to describe and measure them. In the article Odor Measurement for Animal Agriculture, Eileen Fabian-Wheeler (Penn State), Michael Hile (Penn State) and David Schmidt (University of Minnesota) give direction on the five characterizations of odors: concentration, intensity, persistence, hedonic tone and character descriptor.



Concentration can be measured from the odor detection threshold (ODT), which is the volume of non-odorous air divided by the volume of odorous air. Additionally, the odor recognition threshold also comes into play in that it measures the volume of non-odorous air that is necessary to dilute an odorous sample of air.


Intensity is the strength of an odor sample and can be measured via a five-step scale involving different dilutions of n-butanol (type of alcohol).


Persistence is defined as how easily a full-strength odor is diluted below the detection threshold (ex. the more persistent an odor, the more air is needed to dilute the odorous air and vice versa).

Hedonic Tone

Hedonic tone is the pleasantness/unpleasantness of an odor that is measured on a scale with negative numbers (unpleasant) to positive numbers (pleasant).

Character Descriptor

Character descriptor is the type of odor that is detected (floral, fruity, vegetable, earthy, offensive, fishy, chemical and medicinal).  


Concentration and intensity are considered objective parameters and are used for scientific and regulatory measurements while persistence, hedonic tone and character descriptors are more commonly considered subjective as these last three can differ from person to person.

Perception (the ability to discern and comprehend) is another factor of manure odor that should be considered. In a field study done by Robert Mikesell, Kenneth Kephart and Charles Abdalla (Overview of Social Issues Related to the Swine Industry), the impact of odors from a swine operation on neighbors were assessed. Factors examined included:

  • Distance of the neighbor from the swine operation
  • Direction of the wind
  • Whether or not the neighbor knew the producer
  • The visual attractiveness of the farm and,
  • The neighbors’ self-recorded health rating

The further the neighbor lived from the swine operation, the less odor observed and less odor was also observed when the neighbors were not downwind from the operation. Additionally, neighbors reported lower odor if they knew and had a good relationship with the producer. Interestingly, the more attractive and clean looking the farm, less odor was recorded by the neighbor and those neighbors that had higher self-recorded health ratings (those that considered themselves generally healthy), associated less odor with the swine operation.

Michigan State University Extension recognizes that a better understanding of the factors that classify an odor, specifically manure odor, allows farmers to better manage livestock manure for optimal neighbor relations and will result in fewer odor complaints. So the next time you are driving along and smell that oh so familiar skunk, perhaps you will be able to distinguish the characteristics that go into that odor.

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