Windbreaks for cattle are critical
Published on Jan. 11, 2020
In climates where wind chill is an issue during colder months, windbreaks can reduce feed costs, illness and health costs, with less loss of body condition in cows – and better gains on young animals. Rows of trees or constructed wind barriers can give protection from wind and drifting snow.
Dr. Joseph Darrington, Extension specialist-livestock environment engineer at South Dakota State University says some shelterbelts of trees don’t have much foliage in winter/spring to stop the wind.
“If we plant a shelterbelt, use a couple rows of tall trees and a couple rows of smaller trees. If we want more protection, include some evergreens,” he says.
Main considerations when building windbreaks include height, orientation, length and porosity.
“Generally, the protected zone will extend out 10 to 15 times the height of the windbreak with a 50 percent reduction in wind speed. Length is the uninterrupted distance of trees or total length of a constructed windbreak. Ideally, the ratio of windbreak length and height is 10:1, which means a 10-foot-tall windbreak should be 100 feet long,” explains Darrington.
Orientation of the windbreak is ideally perpendicular to winter wind; windbreaks usually face the prevailing wind direction. If wind tends to come from several directions some people create a curved or cornered windbreak.
“In our region the predominant cold wind comes from the northwest, about 60 to 70 percent of the time. The best position would be to run a windbreak from southeast to northwest, to be perpendicular to prevailing wind. For a larger protected area, ranchers sometimes run another windbreak from northwest to southeast, creating an arrowhead shape pointing north to give the greatest protection,” says Darrington.
He continues, “When planting shelterbelts we situate them north-south and east-west. Constructed windbreaks or planted windbreaks have their corner in the northwest.”
Density/porosity is the ratio of solid space relative to total space. This impacts the effectiveness of a windbreak by controlling how much wind blows through it versus over it.
“The denser the windbreak the greater the initial reduction in wind speed, but wind speed increases faster on the downwind side, which decreases the protected area. Dense shelterbelts and solid fences create a larger negative pressure area just behind them, causing snow to build up in drifts. The target windbreak density is 60 to 80 percent,” he says.
Constructed windbreaks are generally made with boards, often placed vertically, leaving spaces between them rather than a solid barrier.
“The target porosity, according to several studies, is from 20 percent open and 80 percent solid, down to 65 or 70 percent solid. If we have more than 35 percent porosity, more than 65 percent solid, or closer to 50-50, we lose some benefits of the windbreak –more air velocity coming through rather than being pushed up and over,” explains Darrington.
“We don’t want a solid windbreak because it reduces wind speed right next to the windbreak. If there is snow the wind dumps more of it right behind the windbreak. A porous windbreak increases the size of the protected area, reduces the wind force/physical load on the windbreak and limits snow drift formation on the downwind side,” he says.
“Some producers make windbreaks using vertical metal roofing. A 30-inch piece of roofing is enough space for calves to nestle against it and be completely protected. We need a bigger gap, however, to create adequate porosity, so snow won’t collect right behind it,” says Darrington.
For cows, some people say these windbreaks are a little less effective because air speed coming through those larger cracks is high enough that if they are right next to it they may get cold, but if they are a distance of one or two heights away from it there is decreased velocity of air.
He continues, “Calves, however, can be very snug next to those 30-inch sections so we might get the benefits of both, solid windbreak plus some porosity.
“These are also faster to build, with fewer total pieces to put vertically. Spacing for 30-inch pieces of roofing metal would be 5 or 6-inch gaps between them or about 36 inches on center,” he explains.
“To build a windbreak six to 10 feet tall, we need posts deep enough to hold it, or build on skids for portability. When setting posts for a permanent windbreak, consider the wind load the structure must withstand and the density of the windbreak,” Darrington explains. “Wind pressure loads for a 10-foot high windbreak can exceed 20 pounds per foot if winds exceed 85 mph. This means for a solid windbreak, needing the strongest posts, with posts every 10 feet, the wind can exert over 2,000 pounds of force on each post.
When rotating pastures or strip grazing, moving the windbreaks when cattle are moved can be helpful.
“For portable windbreaks make the base heavy enough and wide enough they don’t tip over in the wind, or stake them down with two-foot lengths of rebar or T-posts,” Darrington says.
We can make these in sections so they can be moved with a tractor – pulled around or picked up with a loader.
“If we have to take them very far, we can lift them onto a flatbed to haul to the next pasture, strapping them down for hauling. Sections should be built so they can connect together and can be set up to create a corner which provides greater protection for multiple wind directions and reinforces each individual section,” he explains.
Heather Smith-Thomas is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to firstname.lastname@example.org.