Introduction
Preliminary research and observations made by farmers suggest
that shelterbelts placed around livestock production facilities may
effectively reduce movement of odors emitted by manure to
neighboring properties. Essentially, trees can be 'put to work' to
reduce the movement of livestock production odors off-site.
Although the idea of placing treed windbreaks and shelterbelts
around agricultural buildings and farm fields is not new, additional
benefits from farm shelterbelts continue to be learned and tested.
Shelterbelts alone will not prevent odor problems associated with
intensive livestock production but may provide farmers with one more
tool to help reduce negative visual perceptions and detection of
smell by neighbors and surrounding communities.
Figure 1. A single row of maturing
conifers can significantly change the appearance of livestock
production facilities and help filter out odor particles.
An odor-emitting source can include a livestock production barn,
manure storage or a farm field where manure is being spread.
Shelterbelts have the ability to reduce odor concentrations
significantly at or very near the source, which greatly improves the
effectiveness of separation distances.
There are five ways that treed windbreaks and shelterbelts can
reduce the effects of livestock odor and improve visual perception
of production buildings:
- Dilution and dispersion of gas concentrations of odor by a
mixing effect created by shelterbelts
- Deposition of odorous dusts and other aerosols (like snow
fencing) to the windward and leeward sides of windbreaks
- Collection and storage (sinks) within tree wood of the
chemical constituents of odor pollution
- . Physical interception of odor particles (dust and
aerosols)
- Aesthetic appearance
- Trees create a visual barrier to livestock barns
- Trees can make cropped fields and pastures more pleasing
to look at
- Trees represent an 'environmental statement' to
neighbors that the producer is making every effort to
resolve odor problems in as many ways as possible
Dilution and dispersion
Without wind management, odors emitted from livestock facilities
and manure storage areas tend to travel along the ground as a plume
with air movement, especially during atmospheric inversions with
little or no dilution of odor occurring.
Figure 2. Without
shelterbelts and without wind management, the odor plumes are picked
up by passing air masses and travel near the ground with little or
no dilution or filtration.
Shelterbelts create an obstacle for moving air masses. When
designed properly, shelterbelts force turbulent fresh air up and
over the tree row and will also moderate and evenly distribute a
more gentle airflow through the trees. Less air movement past barns
means less pickup and movement of odor off site.
It is believed that shelterbelts have the ability to lift some of
the odor plume into the lower atmosphere where winds aloft mix and
dilute the odor. The greatest dilution of odor occurs above and
downwind from the quiet zone created by the action of wind passing
over the shelterbelt. Beyond the quiet zone, more fresh air and less
odorous air returns to the ground, thereby reducing movement of
livestock odors off site.
Approximately 50 percent of the wind should be deflected up and
over the shelterbelt and 50 percent should pass through the canopy
of the trees. Two to five rows of spruce conifers spaced
appropriately can provide an ideal 50 percent density (or 50 percent
porosity) through the tree canopy. Shelterbelts are less effective
for odor reduction when wind is absent but visual appearance remains
in place.
Figure 3. Shelterbelts
located upwind and downwind of livestock facilities will reduce and
manipulate air flow around the facility to reduce the spread of
odors. Overhead winds can lift particles and gases into the lower
atmosphere to help dilute and disperse odors. Also, more clean air
diverts up and over the source of odor.
Shelterbelts create a 'quiet zone' of air that measures a
distance of 8 to 10 times the height of the tree row downwind of the
shelterbelt, and an additional moderation of wind speeds 10 to 25
times tree height, beyond the shelterbelt. Back-pressure created by
the blocking effect of the tree row also creates a small quiet zone
upwind of the tree line that is equal to 2 to 3 times the height of
the trees.
Livestock barns and manure storage areas are best located in the
quiet zone 75 to 100 feet downwind of shelterbelts. In addition,
shelterbelts located downwind of the odor source are also important
for filtering, absorption and trapping odors. Therefore, placing
shelterbelts around the entire perimeter of livestock production
areas is ideal. Shelterbelts should also be at least 75 feet from
access roads and driveways to prevent snowdrifts from blocking farm
vehicles during winter.
Deposition of odorous dusts
Shelterbelts create a physical barrier to wind and air movement.
The trees absorb wind energy and reduce its speed near the ground.
As a result, fewer dust particles and less odorous gases will be
picked up by the air coming from livestock facilities. Also in
calmer air, dusts and gases already caught up in the air will be
more likely to settle back to the ground on the downwind side of the
shelterbelt. This deposition effect is commonly seen with snow
fencing where snow settles downwind of the fencing or trees due to
reduced wind speed.
Figure 4. Reduced wind speed in the quiet
zone that is created downwind from a shelterbelt allows odorous
dusts and particles to settle to the ground, similar to what happens
with settling and drifting of snow.
Figure 5. Shelterbelts located downwind of livestock production
barns allow settling of odorous wind-borne dust particles.
Shelterbelts should be located 75 to 100 feet away from barns.
To be most effective for deposition of odorous dust, shelterbelts
need to be located upwind and downwind of odorous livestock
facilities. Upwind shelterbelts reduce the quantity of dust and odor
that is picked up by wind, and shelterbelts located downwind of the
facilities are essential to reduce wind speeds to allow settling of
odorous dusts that have become airborne.
For cropland, the same may hold true for reduction of odor
movement where manure is being spread onto farm fields. Shelterbelts
established around the full perimeter of farm fields should reduce
movement of odor and can accommodate winds that are approaching the
farm from any direction.
Figure 6. Mature shelterbelts around cropped fields may help lift
and disperse odors during application of manure as nutrient soil
amendment, in addition to sheltering crops from damaging wind.
Studies of mass transport have shown that shelterbelts can remove
35 to 55 percent of dusts being carried in moving air which would
provide a substantial reduction of offensive odors carried off-farm.
The amount of dust that is picked up or allowed to settle will
depend on wind speed, direction of the wind, density of shelterbelt
trees, height of shelterbelt trees and number of shelterbelts.
Collection and storage of pollutant odors within
trees (sink)
Scientific evidence of plant intake of livestock odors in field
situations is limited, however there have been many studies done on
the ability of plants to absorb air-polluting odors and chemicals.
Research in livestock odor control shows very strong evidence that
trees and shrubs of shelterbelts will help reduce odors. Trees and
shrubs clean the air of micro-particles of all sizes by
interception. Interception of air pollutants may be 20 times higher
in treed or forested areas than non-forested cropped or barren
lands. Conifers show a better ability to absorb air pollutants than
deciduous trees.
Figure 7. In air pollution research,
odorous gases and particles can be absorbed into the foliage of
conifers and deciduous trees during the growing season. Pollutants
diffuse inside leaves and needles through tiny openings called
stomata or adsorb into waxy coatings that naturally cover leaf
surfaces.
Odorous gases, chemicals and dust particles can become fixed to
plant surfaces and can enter into the plant tissue in three ways: 1)
gaseous diffusion through open stomata, 2) on wet leaves, soluble
air pollutants can enter through stomata in a dissolved liquid form,
3) pollutants can absorb directly into plant tissues.
Shelterbelt trees and shrubs absorb air pollutants when they are
healthy and not under drought stress. Trees and shrubs absorb more
air pollutants when leaf surfaces are wet. Higher humidity can
increase uptake of air pollutants into trees, which is commonly
measured within tree canopies.
Micro-organisms cover plant surfaces and there is evidence that
these micro-organisms associated with shelterbelt trees also
contribute to absorbing odorous chemicals. Forests are often
referred to as pollutant air filters. This may also apply to
shelterbelts trees.
Physical interception of odor particles
Trees are highly effective at physically intercepting dusts,
gases and microbial particles that are carried in the wind.
Shelterbelts are commonly used to catch spray drift of agricultural
chemicals, reduce and catch pollen drift from agricultural crops,
intercept and drop blowing snow and as barriers to trap blowing sand
and soil caused by wind erosion.
As leaf surface roughness increases, the capture ability of
particles and odor increases. Leaves with complex shapes (large
circumference to area ratios) collect particles most efficiently.
Therefore, conifers may be more effective at intercepting livestock
odors than deciduous tree and shrub species. Conifers also have
leaves (needles) year around.
Figure 8. Like the air filters of home furnaces, shelterbelt trees,
especially conifers, physically catch wind-borne odorous particles.
Conifers have foliage year-round.
Shelterbelt design and planting
Selecting the species of trees and shrubs to plant will vary at
each livestock facility and farm field site. Species selection will
be based on the characteristics of each site including: soil type,
natural drainage, common wind conditions, annual precipitation,
natural range of each tree and shrub species and genetic
preservation. If needed, advice of local experts can be sought such
as experienced farmers and landowners, conservation authorities,
tree horticulturists and foresters.
It is usually best to select several species of trees and shrubs
for use in shelterbelts to prevent loss or destruction of the entire
windbreak if attacking insect pests or tree diseases occur. Having
diversity also offers a better chance for tree survival during
alternating seasons of drought and wet soil conditions.
Figure 9. Pleasing to the eye and to the
rural landscape, shelterbelts comprised of diverse native tree and
shrub species may help reduce negative visual perceptions associated
with modern, intensive livestock production facilities.
Windbreaks may consist of one to three rows of alternating
conifer and deciduous species while shelterbelts may be wider with
more tree rows. Shrubs are generally planted in the outside rows,
followed by conifers with deciduous hardwoods towards the middle or
along the downwind side where they can grow more efficiently,
sheltered from wind that is provided by conifers.
Ideally once established, the tree barrier should have a density
of about 50 percent for best results for wind management. Conifers
such as spruce will provide uniform branch coverage from the ground
level up. Tree rows should be spaced wide enough apart to allow
access by a small tractor for mechanized management of vegetation.
Weed management is important during the first five years of tree
establishment using herbicides, or plastic or organic mulch. Weed
management is important until the young shelterbelt trees have
overtopped most weed competition and are free to grow.
Renovating old windbreaks
Maturing shelterbelts eventually begin to lose their ability to
manage wind effectively as lower branches begin to die off and a
percentage of trees are taken by diseases, insect pests,
environmental stresses and natural mortality. Excessive holes in the
tree row will enable wind to pass through without moderation. Wind
management is eventually reduced or lost and farmers may need to
renovate the shelterbelt. Renovation means 'to make new again' or
'to restore to good condition'.
Some farmers choose to remove the old trees first and clear the
way to plant new young trees. Others may prefer to establish new
trees adjacent to the old trees and wait for the new windbreak to
develop for several years before removing the old tree row. The
advantage of overlapping the new and old windbreak provides
continuous management of wind. Farmers may also decide that the tree
row would be more effective if it is relocated, that is,
re-established to a new site.
Transplanting larger trees using a hydraulic tree spade can give
a jump-start to growth of new shelterbelts, although transplanting
larger trees can be more expensive than starting with young tree
seedlings. Large transplanted conifers can provide an instant visual
barrier and quicker odor management advantages. Tree sizes can be
inter-mixed. More trees can always be added over several years to
spread out costs and work. Imagination, experimentation and artistic
talents among landowners can also be applied to enhance each
tree-planting project.
Figure 10. With a hydraulic tree spade, larger sized trees as
transplants can provide a jump-start to having functional
shelterbelts.
Other benefits
In addition to odor management, treed shelterbelts also act to
reduce the seasonal cost of heating and cooling of farm buildings
without disrupting ventilation in livestock barns.
Shelterbelts may also reduce the spread of specific infectious
disease of livestock by blocking, intercepting or diverting
wind-borne infectious organisms away from buildings.
Shelterbelts placed around farm fields reduce damage to forage
and crops (preserve crop yield potential) caused by damaging
turbulent winds while allowing normal air circulation to continue.
Shelterbelts reduce soil erosion by wind. Around pastures, mature
shelterbelts will relieve livestock of stress during hot summer days
and cold windy winter conditions. Avoid planting trees and shrubs
around livestock that are known to be poisonous.
Figure 11. Road salt applied in winter can mist onto and kill
shelterbelt trees. Colorado blue spruce and Austrian pine have some
tolerance to the effects of road salt and can be used near highways.
Figure 12. Where barns are surrounded by solid forest plantation, it
is important not to block ventilation fans with excessive tree
growth. Thinning the plantation and pruning off lower branches can
improve air circulation. Fifty to 100 feet is a good separation
distance between trees and barn.
References
J. Tyndall and J. Colletti. Literature Review on Air Quality and
Shelterbelts: Odor Mitigation and Livestock Production. 2000. Forest
Department, Iowa State University.
A. Knight and R. Walter. A Guide to Plant Poisoning of Animals in
North America. Copyright 2001. Teton NewMedia. Jackson WY.
