Editor’s note: The following was written by Joe Darrington, South Dakota State University veterinarian and ag engineer, for the university’s iGrow website.
Windbreaks, both constructed and planted, can improve conditions for livestock in windy and cold conditions.
Increasing the effective temperature that an animal is exposed to during cold weather by cutting down the wind chill keeps them comfortable, more efficient users of feed, and at a lower risk of cold stress which can lead to disease.
The main considerations of windbreak design are height, orientation, length and density:
- Windbreak height is the highest point on the structure or tallest row of trees. Generally, the protected zone of the windbreak will extend out 10-15 times the height of the windbreak with a 50 percent reduction in wind speed.
- Orientation of the windbreak is ideally perpendicular to the cold winter wind. Given that wind patterns fluctuate in different regions, do some research to evaluate the frequency of wind direction in your area.
- Windbreak length is the uninterrupted distance between roads or paths through the trees. Ideally the ratio of windbreak length and tree/windbreak height is 10:1, which means that to develop a full protected zone a 10-foot-tall windbreak should be 100 feet long.
- The last major design consideration is density, which is the ratio or fraction of solid space in relation to total space. Density impacts the effectiveness of a windbreak by controlling how much wind blows through the windbreak vs. blowing over the windbreak.
One impact of density is that the denser the windbreak, the greater the initial reduction in wind speed, but the wind speed increases faster on the downwind side of the windbreak which decreases the protected area. Additionally, very dense shelterbelts and solid fences create a larger negative pressure area just behind the windbreak which causes snow to build up in large drifts.
The target for livestock windbreak density is 60-80 percent.
Windbreaks can be built to be mobile or permanent. The biggest considerations to take into account are the wind load that the structure needs to withstand and the density of the windbreak.
Wind pressure loads for a 10-foot-high windbreak can exceed 20 lbs./square foot if winds exceed 85 mph. This means that for a solid windbreak (most extreme condition) with posts in the ground every 10 feet, that the wind can exert over 2,000 lbs. of force on each post. Posts of diameter 8 inches or greater with the underground portion below the frost line (3-5 feet depending on location) should be adequate in permanent systems.
In mobile systems, the base needs to be broad enough and heavy enough not to tip over or move. An important note is that they connect together and can be set up to create a corner which provides greater protection for multiple wind directions and reinforces each individual section.
The density of the windbreak is important to control to increase the size of the protected area, reduce the physical load on the windbreak, and limit snow drift formation on the downwind side.
To target 80 percent density, measure the width of the solid material you are using for the windbreak panels and divide by 0.80, this will give you the center-to-center spacing that you need to reach 80 percent density.
For example, using 1x8s for the windbreak, we find a spacing of 9.0625 inches. Feel free to round up or down to the nearest quarter inch — if rounding up the density is 78 percent, if down the density is 80.5 percent. This means that between each board, there will be a 1.75-2 inch space.
When planting windbreaks, the principles described above still apply, but we have less control over the growth characteristics of the plants regarding density.
Density is controlled by the types of shrubs and trees planted, their spacing and how many rows are used. Coniferous trees maintain their leaves throughout the winter and improve the wintertime density. Deciduous trees lose their leaves and provide less density in the winter time.
Another benefit of living shelterbelts is that they can provide significant snow storage capacity, especially if they are wide. This can also be a detriment, however, if there is inadequate drainage out of the shelterbelt in the spring thaw.
When sizing a shelterbelt, determine how many head will be in the pasture at one time through the winter and multiply by the amount of space per head, depending on how much space you would like to give them (the lower the space, the higher the likelihood of muddy conditions in spring — each situation will be different).