What makes manure so stinky?

The three factors


Of all the smells in the world, from egg salad to wet dog, there is one that Bryan Woodbury cannot stand.

“When I walk through a store and I come across potpourri, I immediately get a headache and I hold my breath,” said Woodbury, a U.S. Department of Agriculture scientist who knows a thing or two about stinky scents.

Woodbury, an agricultural engineer, works for the USDA in Clay Center, Nebraska, and specializes in the smell of cattle manure.

Studying manure odour may seem odd or trivial, but Nebraska has more cattle on feed than any state in the U.S.

Some of those feedlots are near towns and close to people who detest the smell of manure. When citizens don’t like something politicians usually hear about it, which puts public pressure on feedlot owners and the livestock industry.

Woodbury and his USDA colleague John Gilley have studied ways to reduce the odours associated with cattle manure. As part of their research, they conducted a smell study to identify the chemical compounds responsible for the distinctive odours in cattle manure.

After taking hundreds of air samples, in close proximity to the cattle excrement, Woodbury and Gilley determined that three compounds cause two-thirds of all the odours in the manure.

Two of the chemicals are volatile fatty acids: isolaveric acid and butyric acid.

Woodbury said a volatile fatty acid is a chemical that wants to move from the liquid to gas phase.

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“Vinegar. Acetic acid. That would be a volatile fatty acid,” he said, adding butyric acid has an unforgettable smell. “I’m looking for a poetic way to put this… it’s what you would smell in vomit. Smelling that can cause a gag reflex.”

The third most common chemical in cattle manure odours is something called 4-methylphenol, an aromatic compound.

Brandon Gilroyed, a University of Guelph, Ont., assistant professor who specializes in anaerobic digestion and biofuel research, said dozens of other chemicals can be found in manure odour.

“I think it’s been shown there are over 150,” he said, adding the chemicals include recognizable compounds like ammonia, methane and hydrogen sulfide.

Gilroyed said microbes are to blame for most smells.

“All of those (chemicals) are produced by microbes… feeding on what… the cow or pig didn’t eat,” he said from his Ridgetown, Ont. office. “You get these anaerobic conditions and you get all these things which are basically fermentation products.”

Gilroyed said when it comes to manure, three factors determine the severity of the stink:

  • diet
  • animal’s digestion process
  • method of storage

Woodbury, who has studied how dried distillers grains affect cattle manure odours, agreed diet is critical. He said manure odour from grass-fed cattle is distinct from corn-fed cattle manure, which is different from the manure smell linked to dried distillers grains.

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When it comes to storage, an airless and wet environment produces the most putrid manure.

“The longer you have anaerobic conditions, the smellier that manure is going to be,” Gilroyed said. “If you don’t clean out your lagoons very often… when you do get around to spreading that, it’s going to be very, very smelly.”

As manure gets drier and the microbes become less active, it reduces the foulness of the manure.

“Dry feedlot surfaces vs. wet feedlot surfaces, the difference is night and day,” said Woodbury, who has built up a lifetime of immunity to manure odours.

“I grew up on a ranch (in Montana)… I don’t find it terribly offensive,” he said. “Unless it really gets wet and sits around a long time. Some of that stuff can be pretty strong, even by my standards.”

Gilroyed said what smells and what doesn’t is more complicated than the chemicals within the odour. There is also a psychological component because smell is closely associated with memory.

“Odours are perception. And those are hard to nail down, as to this one is offensive and that one isn’t offensive,” he said.

“Most of our smell re-sponses are learned. They’re tied to memory. So when you first come out of the womb, you don’t necessarily find something offensive… that an adult would find offensive.”

Sense of smell: dogs compared to humans

  • Humans have approximately five to six million olfactory receptors or smell receptors, which can detect odour molecules.
  • Dogs have 125 million to 300 million olfactory receptors, depending on the breed.
  • Several studies and anecdotes have documented the incredible acuteness of the sense of smell in dogs.
  • Scientists have estimated that dogs can detect odours at concentrations in parts per trillion. In the often-used Olympic-sized swimming pool comparison, some dogs can detect a teaspoon of sugar in the volume of water needed to fill two Olympic sized pools.
  • A Labrador Retriever trained in scent detection smelled the stool samples of people who possibly had colon cancer. The dog correctly diagnosed the presence of cancer cells 97 percent of 
the time.
  • In one case in the U.S., a drug sniffing dog detected a plastic container filled with marijuana, submerged within a tank of gasoline.

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