Windbreaks, both constructed and planted, can improve conditions for livestock in windy and cold conditions.

“Windbreaks increase the effective temperature that an animal is exposed to during cold weather, keeping them comfortable, more efficient users of feed, and at a lower risk of cold stress which can lead to disease,” explained Joe Darrington, South Dakota State University (SDSU) Extension livestock environment associate.

What to consider with windbreaks

The main considerations of windbreak design are windbreak height, orientation, length, and density.

• Height: 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 to 15 times the height of the windbreak with a 50 percent reduction in wind-speed.

• Orientation: Orientation of the windbreak is ideally perpendicular to the cold winter wind. “Given that wind patterns fluctuate around the state, wind roses can be used to evaluate the frequency of wind direction in your area,” Darrington said.

• Length: Windbreak length is the uninterrupted distance between roads or paths through the trees. Ideally, Darrington explained 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.

• Density: Density 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 versus blowing over the windbreak,” Darrington said. He added 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. This causes snow to build up in large drifts. The target for livestock windbreak density is 60-80 percent. Figure 1 is a conservative example of the protected area calculation for a windbreak.

Constructing windbreaks

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,” Darrington said.

Wind pressure loads for a 10-foot-high windbreak can exceed 20 pounds per square foot if winds exceed 85 miles per hour.

“This means that for a solid windbreak (most extreme condition) with posts in the ground every 10-feet, the wind can exert more than 2,000 pounds of force on each post,” he explained.

Posts of diameter 8-inches or greater, with underground portion below the frost line (3 to 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 mobile systems connect together and can be set up to create a corner which provides greater protection for multiple wind directions and reinforces each individual section,” Darrington said.

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. See Figure 2 for equation and example.

Planting windbreaks

When planting windbreaks the principles described above still apply, but Darrington said 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,” he explained.

Coniferous trees maintain their leaves throughout the winter and improve the winter time density. Deciduous trees lose their leaves and provide less density in the winter time.

“A 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,” Darrington said.

Shelterbelt sizing

When sizing a shelterbelt, Darrington said it is important to determine how many head will be in the pasture at one time through the winter. Multiply by the amount of space per head depending on how much space you would like to give them.

“Remember, the less space, the greater the likelihood of muddy conditions in spring,” Darrington said.

Once the desired area is determined, you can calculate the planting length required using the equation found in Figure 3.

Roads, feed alleys and cattle alleys should never be less than 75 feet downwind of a shelterbelt, or 50 feet upwind, as snow accumulation and storage usually occurs within these areas and could cause unnecessary snow blowing/moving duties. — Lura Roti, SDSU iGrow Extension

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