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Do Trees Effectively Block Hog Odors?

Drive around the countryside and it is common to see small groves or rows of trees that provide an aesthetic aura to the farm site. But do they really help reduce odors and gases emanating from hog operations?

Drive around the countryside and it is common to see small groves or rows of trees that provide an aesthetic aura to the farm site. But do they really help reduce odors and gases emanating from hog operations?

That depends, says Dick Nicolai, Extension agricultural engineer at South Dakota State University in Brookings. New research highlights the value of vegetative shelterbelts adjacent to hog barns in providing effective barriers to reduce concentrations of hydrogen sulfide (H2S).

But farther downwind from the barn, the absence of shelterbelts or the presence of unstable air masses also appears to play an equal role in effectively reducing those gas levels, he says.

Studying Shelterbelts

For years, shelterbelts have been used to enhance odor dispersion, but their true impact on odor concentrations farther downwind from a source farm has never been documented, Nicolai points out.

In a research project completed last year, Nicolai measured hydrogen sulfide emissions from a hog barn with and without shelterbelts up to a half-mile downwind.

The effect on H2S containment without shelterbelts next to a hog barn was compared with a single row of 15-ft.-tall ash and honey locust trees, a second row of 15-20-ft.-tall evergreen trees, and a third row of 25-30-ft.-tall ash trees. Each row of trees was spaded in at the site, and data was recorded for approximately four months. Nicolai says only H2S concentrations were evaluated, at heights of 3 ft. and 18 ft. above the ground, and located downwind of the barn site. Odor was not monitored because it can only be measured by the human nose.

Dispersion Factors

Planting the first row of trees near the barn resulted in 84% porosity, meaning that plenty of air succeeded in passing through the single-row shelterbelt. Adding a second row of trees lowered porosity to 51%, and adding a third row reduced porosity to 38%, he says.

“Decreasing the porosity of the shelterbelt slows horizontal wind speed and causes increased turbulence, which enhances dispersion,” Nicolai says. “When the porosity was lowered by adding more rows of trees, the hydrogen sulfide concentration decreased immediately beyond the shelterbelt, but at distances farther away, there was less difference in H2S concentration relative to porosity.”

Stability of the atmosphere also played a role in odor and gas dispersion. Stability is based on the cloud cover, the amount of solar radiation, wind speed, air temperature and any other factors that would affect airflow, he says. Stable air means greater traveling distance of plumes, less mixing and greater odor concentrations. In contrast, unstable air, such as during a bright, sunny day with a lot of vertical movement of the air from the ground up, results in less movement of odors downwind, he explains. Most neighbor complaints of odor occur during times of atmospheric stability, which generally take place during the evening.

Data from the weather station indicated that the air turbulence for the farm site during the study was mostly neutral, indicating that odor and gas plumes had a reasonable chance of being maintained as they moved downwind from the building.


Figure 1 shows that the effect of the shelterbelt resulted in reduced hydrogen sulfide concentrations as the distance from the shelterbelt increased. The greatest impact occurred at the 182-ft. mark from the barn, with no windbreak or a single row of trees, and measured at a height of 18 ft. by the monitors.

Three rows of trees had an 81% reduction in average H2S concentrations compared to no trees.

However, Nicolai reports, the three rows of trees compared to no shelterbelt at the 812-ft. distance from the building (Figure 1) lowered gas levels 50%. At 1,683 ft. and 2,657 ft. from the source, there was only a 16% reduction in gas levels, regardless of the number of rows of trees in the shelterbelt as compared to no shelterbelt.

“Therefore, as the distance increased from the shelterbelt, the difference in H2S concentrations from no shelterbelt to three rows of trees becomes less,” he explains.

Figure 1 also shows average gas concentration at 2,657 ft. from the barn was higher than at 1,683 ft. from the barn, which could be attributed to a cattle farmstead located to the southeast of the farthest sampling location, Nicolai notes.

Figure 2 shows very little difference in any of the shelterbelt groups or no shelterbelt when H2S was measured at four distances at the 3-ft. height.

“The greater the distance from the shelterbelt, the less effect the shelterbelt has on H2S reduction. Overall, it was found that shelterbelts can enhance dispersion immediately beyond the shelterbelt, but may have limited impact over a quarter of a mile,” Nicolai observes.
Stability of the air mass also played a major role in the dispersion of the plume over a short distance, likely due to turbulency, he adds.


For maximum effectiveness of the shelterbelt in reducing barn emissions, Nicolai suggests placing the shelterbelt near a recipient’s farm. “In other words, plant these shelterbelts near a neighbor, between their farm and the odor source on your farm,” he says.

Producers should also consider planting a shelterbelt around a manure storage structure or a hog building as an additional barrier to hog odors escaping the farm, he says.

The recommendations only apply to reductions in H2S levels, but can be translated into effectively reducing odor levels, Nicolai assures.

Shelterbelts also add an aesthetic appearance to a hog farm and can help in lowering dust levels, he says.

The information garnered from this research project will be used to create a model incorporated into the South Dakota Odor Footprint Tool and other models for pork producers. It will help them determine how effective their shelterbelts are in reducing odor volume, by distance, and assist producers in siting shelterbelts when new pork units are constructed, Nicolai states.

Funding for the project was provided by the South Dakota Pork Producers Council, Nebraska Corn Board and several county conservation districts in South Dakota.