Livestock barns with shelterbelts around them smell better because they look better, says Iowa University researcher
Intensive livestock operations are tremendously efficient at converting grain into meat.
But all those animals gorging themselves under one roof generate a lot of odoriferous byproducts.
Shelterbelts, known in academic circles as vegetative environmental buffers (VEB), can help such operations stay on friendly terms with neighbours downwind by trapping and dispersing odours.
What’s more, beautifying the production site with greenery somehow makes it smell better, said John Tyndall, a professor at Iowa State University.
“Studies have shown that as farms are viewed as being more attractive, perceptions of odour and other impacts from that facility tend to be reduced,” said Tyndall, who gave a presentation on shelterbelt designs for odour control at the recent joint U.S.-Canada Great Plains Windbreak conference.
Apart from keeping massive barns “out of sight, out of mind” and providing “visual screening,” shelterbelts provide more than just subjective benefits at a cost of around one to three cents per pig, he added.
Odour from intensive livestock operations happens because anaerobically decomposing manure gives off volatile organic compounds composed of 400 different chemicals.
Ammonia is lighter than air, and quickly dissipates into the atmosphere. Hydrogen sulphide is heavier than air, moves very slowly, and rarely makes its way far from the barn.
What really ruins barbecues and pool parties downstream, however, are the particulates. Volatile organic compounds cling to dust particles as they drift away from the facility on gentle currents of air.
“So, if you can control the movement of particulates, to a large degree you are going to be managing the movement of odours,” said Tyndall.
Odours can be controlled to some degree by adjusting feed rations. However, preventing particulate movements with shelterbelts or VEBs is the most effective, “last line of defence” tactic.
Temperature inversions during the hot summer months can trap odour “plumes” near the ground level and prevent them from being diluted. Landscape features such as hills and ravines can also funnel noxious odours in different ways.
Mechanical turbulence from VEBs mixes and dilutes the plume, slows its release, and the branches and leaves trap particulates and their stinky cargo. Particulate buildup over time leads to even more captured dust, because “dust grabs dust.”
One row of trees can make a difference, and three to five more rows is generally better, but at some point, the law of diminishing returns takes hold.
All that gunk clinging to the leaves and branches isn’t great for tree health, he said, but periodic rains help to keep them clean.
How effective are shelterbelts for odour mitigation?
There’s no definitive answer for that, said Tyndall. Topography, prevailing wind direction and strength, ambient weather, distance from the “sensory receptor,” as well as a multitude of other factors mean that effectiveness is largely site specific.
Also, the fact that trees grow very slowly and research funding is typically short term, means that collecting hard data is “incredibly difficult,” he added.
“However, most ag engineers are comfortable with a five to 50 per cent reduction in odour concentration moving downwind because of the VEB,” said Tyndall, adding that incremental improvements in frequency, intensity, and offensiveness of odours are well documented.
Obviously, people like to see trees on the landscape, and VEBs around intensive livestock operations help to soften the visual impact of industrial agriculture. Focus group surveys of pork consumers have also found that there is a strong appreciation for farmers who make an effort to be a good steward and a good neighbour by managing odours.
“You can’t see feed additives or a diet change,” he said. “And some of the things you can see, like chemical scrubbers outside of buildings, are more intimidating than anything.”
While mechanical solutions depreciate and add costs over time, trees — if well maintained and healthy — only get taller and more effective throughout their lifespan, he added.
Design is critical, however, because poor placement of shelterbelts can create back pressure and impact tunnel ventilation and airflows in hot weather, as well as snow accumulation in winter.