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Fighting Purple Threeawn With Fire

By   /  July 30, 2018  /  No Comments

Commonly known as purple threeawn, red threeawn, wiregrass or democrat grass, this plant can take over pastures and out-compete other, high quality forage. Agricultural Research Service scientists have discovered how to use fire to beat it back.

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This comes to us from Dennis O’Brien, Agricultural Research Service Information Staff, and the Agriculture Research Journal. Researchers Lance Vermeire and Dustin Strong are with the USDA-ARS Livestock and Range Research Laboratory, in Miles City, MT.

Carefully staged “prescribed fire”can reset a rangeland’s biological clock, awaken dormant plants, and breathe new life and diversity into an ecosystem. When fire rolls over a rangeland, it gives perennial sod-forming grasses, which are good sources of forage for livestock, a better chance to take hold. That’s why an Agricultural Research Service team in Miles City, Montana, began looking at fire as a way to control purple threeawn, a grass that is colonizing disturbed soils and overtaking rangelands used for grazing.

This is Aristida purpurea Nutt, commonly known as purple threeawn, red threeawn, wiregrass, or democrat grass. It can be found from Iowa and Minnesota west to British Columbia and south to California, Texas, and northern Mexico. It can spread in disturbed or overgrazed areas. Livestock eat it early in the spring, and fall and winter, but it is not a high quality forage.

The researchers—range ecologist Lance Vermeire, range technician Dustin Strong, and their colleagues at the ARS Range and Livestock Research Unit in Miles City ran experiments to find out which season is best for using fire to control purple threeawn, and to see if fire treatments not only reduced the abundance of purple threeawn, but also created more balanced ecosystems where desirable grasses can flourish. Their intent is to give land managers a way to keep rangelands viable so that cattle grazing on them can stay healthy and well fed.

“We don’t just want to discourage the bad weeds; we also want to promote the good grasses, such as western wheatgrass, needleandthread, and other native perennial grasses,” Vermeire says.

Effects of Fire

The researchers started with a controlled test to see how fire impacted the weedy grass. They grew purple threeawn and two desirable grasses—western wheatgrass and blue grama—in a greenhouse to study the effects of fire under controlled conditions. Some purple threeawn was grown in isolation, and some was intermixed with the two grasses. They applied fire to some plants and let others grow unimpeded, and they clipped both the burned and control plants to specific heights to simulate cattle grazing. Combusted materials, clipped material, and final biomass of all plants were measured to assess plant production levels. Results showed that fire killed 36% of the purple threeawn and reduced its biomass by 61%, indicating that fire treatments would likely have significant impacts for reducing purple threeawn on open rangelands.

The Right Timing

Another key question is when to use fire. Most prescribed fires are set in rangelands during spring or fall, but a plant’s response to fire varies with its stage of development and activity level. Purple threeawn is a warm-season species that grows during the summer, which should make it more susceptible to summer fires.

This research plot in Miles City, Montana, is made up of about 75 percent purple threeawn plants. ARS scientists are looking at ways to use fire to control the weed, which is taking over rangelands in the Northern Plains.

In a study at two field sites, plots were either burned during the summer or fall or were not burned at all, and each of those treatments had either no nitrogen or one of two levels of nitrogen added. Precipitation levels varied widely: Spring 2011 saw record rainfall, but spring 2012 was one of the driest on record.

The results showed that while fall fires reduced purple threeawn production, summer fires were much more effective, particularly after a wet spring. “Few rangeland treatments have shown such immediate effects with just a one-time application,” Vermeire says. In comparison to the control plots, the weed’s overall biomass was reduced 90% by the summer fire and 73% by the fall fire after the wet spring. Adding nitrogen to the soil had no effect on purple threeawn production at any of the sites nor on growth of the more desirable perennial grasses after the dry spring, but it doubled grass production after the wet spring.

Vermeire said the results could also be attributed to purple threeawn’s growth cycle. “Because purple threeawn grows and reproduces during the summer, setting it on fire in summer rather than fall is more likely to shut down that process,” he says.

The Right Heat Dosage and Duration

Thermocouples (the silver wires) and data loggers are used in prescribed burns in research plots planted with purple threeawn. The loggers record plant temperatures every second the fire burns.

When conducting prescribed burns, land managers can control certain factors such as the temperature, duration, and the amount of heat applied. The duration of a fire, for instance, can be prolonged by allowing plant litter to accumulate or by burning in light winds or when the plant material is moist. “Heat Dosage” is how hot and how long a fire burns.

For this part of the study, the researchers placed data loggers at the base of purple threeawn plants to record the temperatures during prescribed fires. The purpose was to assess the effects of temperature, heat duration, and heat dosage. The loggers recorded temperatures every second the fires burned. The results showed that heat dosage and duration are more important than maximum temperature. The scientists conclude that summer fires, with their higher dosage and duration levels, provided more benefit than fall fires.

What Can You Do With This?

Because it’s not as nutritious as other grasses, cattle may choose other forages first. That means that, in drought or cases of overgrazing, purple threeawn (and other threeawn species) have a competitive advantage and can take over a pasture or rangeland. If you find yourself in this situation, consider talking with your local land management agencies, or your Natural Resources Conservation Services or Conservation District Office about whether or not fire would be a tool you could use to get back on the right foot.

Wait – you don’t have threeawn? Good for you. But you might have something else that could respond to a fire treatment. Let me know what your problem forage is and we can check it out together.

This research is part of Pasture, Forage, and Rangeland Systems (#215), an Agricultural Research Service national program.

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About the author

Publisher, Editor and Author

Kathy worked with the Bureau of Land Management for 12 years before founding Livestock for Landscapes in 2004. Her twelve years at the agency allowed her to pursue her goal of helping communities find ways to live profitably AND sustainably in their environment. She has been researching and working with livestock as a land management tool for over a decade. When she's not helping farmers, ranchers and land managers on-site, she writes articles, and books, and edits videos to help others turn their livestock into landscape managers.

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