Scientists in the field monitoring the effects of experimental herbicide treatments for controlling exotic annual grasses in the Boise River Wildlife Management Area.
Longevity of Herbicides Targeting Exotic Annual Grasses in Sagebrush-Steppe Soils
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By Forest and Rangeland Ecosystem Science Center
August 31, 2023
Pre-emergent herbicides applied to soil are a primary tool for reducing exotic annual grasses, such as cheatgrass, in sagebrush steppe rangelands. Effective herbicide application can allow existing perennials to grow and spread to outcompete cheatgrass, preventing reinvasion, but most perennials are slow-growing and require more years of relief from annual-grass competition than standard herbicides are thought to provide. We are measuring how long it takes for exotic annual grasses to reinvade areas sprayed with the herbicides imazapic, rimsulfuron and indaziflam, and evaluating how the risk of reinvasion varies across climate conditions and soil types.
Background
Pre-emergent herbicide treatments are a key part of the strategy used to control the invasion of exotic annual grasses such as cheatgrass and medusahead in the sagebrush steppe. Pre-emergent herbicides prevent germinated weed seedlings from establishing. Pre-emergent herbicides are used to limit the spread of exotic grasses and reduce competition with native perennials, making it easier for the native plants to grow. Currently, there are very few datasets that contain information on how long the effects of pre-emergent herbicides last in the sagebrush steppe landscape. The Fire, Invasion, and Ecology in Sagebrush Steppe (FIREss) lab is working to address this knowledge gap by studying the longevity of pre-emergent herbicides in the sagebrush steppe across a variety of plant communities, soil types, and climates by using five carefully designed field experiments.
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We are specifically looking at the longevity of the effects of the pre-emergent herbicides imazapic, rimsulfron, and indaziflam. In addition to reinvasion by exotic annual grasses, we are looking at secondary invasion by other, often more noxious invaders such as rush skeletonweed. Previous studies have shown that indaziflam has greater longevity compared to other types of pre-emergent herbicides, which could benefit desirable native perennials by giving them more time to grow without competition from invaders.
We selected a variety of unique sagebrush-steppe environments for our experiments with different climate and soil conditions, and interim findings from some trials have been published. Herbicide treatment experiments began in 2016 and include the longest-term trials of indaziflam to date. In 2019, we worked with the U.S. Fish and Wildlife Service on the Minidoka National Wildlife Refuge to create a well-replicated and controlled trial application of indaziflam across 1000 acres of recently burned soils. Fish and Wildlife Service staff spread tarps throughout the study area to block the aerial spray and create untreated control plots to compare with the broader treated area. The Fish and Wildlife Service has completed yearly monitoring of plant communities with support from the FIREss lab for data analysis and synthesis. The objective was to “break” the cheatgrass-fire cycle while simultaneously learning about effectiveness of the herbicide, with and without co-application of other herbicides and seeding of native species.
Objectives
1. Determine the longevity of the target and non-target effects of the pre-emergent herbicides imazapic, rimsulfron, indaziflam, and the post-emergent herbicide aminopyralid
2. Determine the impacts of climate and soil type on herbicide longevity
3. Better understand the relationship between pre-emergent herbicide use and secondary invasion
Sources/Usage: Public Domain. View Media Details
Published Results
This research is still ongoing, but interim results from some experiments have been published by our team. In 2016, we started to monitor the longevity of imazapic and rimsulfuron at three sites for one experiment. One site is located in the Idaho Batholith, one in the Snake River Plain, and one in the Northern Basin. The FIREss team collected data on plant cover before treatment, and then conducted prescribed burns at the sites to simulate a wildfire. After the fire, imazapic and rimsulfuron were sprayed. After monitoring the effects of the herbicides for five years, we found that both treatments generally decreased exotic annual grasses in the first year, but after that the results were varied. Plots with rimsulfuron were re-invaded after 1-2 years, whereas imazapic plots resisted invasion for four-five years after herbicide spraying. There was no increase in native perennial grass cover found, and rimsulfuron was found to be more damaging than imazapic to shallow-rooted perennial bunchgrasses. In some soil communities, the spraying resulted in an increase in moss and lichen cover, which is a key aspect of soil integrity. Both herbicides were also found to cause an increase in secondary invasion. Overall, the short term-effects of the herbicides used were consistent and predictable at reducing exotic annual grass cover, but the long-term effects were heavily dependent on which herbicide was used and the plant community present. The longevity of rimsulfuron and imazapic at these plots is still being measured by the FIREss team, we have recently completed our seventh year of monitoring.
Sources/Usage: Public Domain. View Media Details
Additionally, in 2018 in the Boise foothills, we began to monitor the longevity of the effects of the herbicides imazapic and indaziflam in replicated 1-acre plots. We compared imazapic, indaziflam, control groups, and combinations of both herbicides. We found that exotic annual grasses, in particular medusahead, were reduced for up to three years when indaziflam was used. Comparatively, imazapic was only found to reduce exotic annual grasses when sprayed twice. It was found that indaziflam effects were greater when there was more precipitation immediately after spraying, and also when indaziflam was coapplied with imazapic. However, reapplying indaziflam did not lead to additional reductions in exotic annual grasses. Aminopyralid was also used in this experiment, but it only was able to prevent the secondary invasion of the highly noxious forb rush skeletonweed for one year. We recently completed our fifth year of monitoring at this site.
Next Steps
New funding for phase two of this project is allowing us to continue monitoring previously established plots until 2025, enabling us to conduct some of the longest standing trials for imazapic, rimsulfuron, and indaziflam. In addition to continuously monitoring these plots we will produce a document synthesizing the results of our long-term trials with best-practice guidance on the safe and effective application of the herbicides.
If you are unable to access or download a product, email fresc_outreach@usgs.gov.
Vegetation monitoring in Idaho
Scientists in the field monitoring the effects of experimental herbicide treatments for controlling exotic annual grasses in the Boise River Wildlife Management Area.
Experimental herbicide treatment plots in Idaho
Experimental site in the Snake River Plain in southwest Idaho three years post-herbicide treatment. The plot towards the lower left of the image was not treated with herbicide. The plot towards the upper right was treated with the pre-emergent herbicide imazapic and is free of cheatgrass. This is one of the longest-lasting imazapic treatments recorded.
Experimental site in the Snake River Plain in southwest Idaho three years post-herbicide treatment. The plot towards the lower left of the image was not treated with herbicide. The plot towards the upper right was treated with the pre-emergent herbicide imazapic and is free of cheatgrass. This is one of the longest-lasting imazapic treatments recorded.
Pre-emergent herbicides applied to soil are a primary tool for reducing exotic annual grasses, such as cheatgrass, in sagebrush steppe rangelands. Effective herbicide application can allow existing perennials to grow and spread to outcompete cheatgrass, preventing reinvasion, but most perennials are slow-growing and require more years of relief from annual-grass competition than standard herbicides are thought to provide. We are measuring how long it takes for exotic annual grasses to reinvade areas sprayed with the herbicides imazapic, rimsulfuron and indaziflam, and evaluating how the risk of reinvasion varies across climate conditions and soil types.
Background
Pre-emergent herbicide treatments are a key part of the strategy used to control the invasion of exotic annual grasses such as cheatgrass and medusahead in the sagebrush steppe. Pre-emergent herbicides prevent germinated weed seedlings from establishing. Pre-emergent herbicides are used to limit the spread of exotic grasses and reduce competition with native perennials, making it easier for the native plants to grow. Currently, there are very few datasets that contain information on how long the effects of pre-emergent herbicides last in the sagebrush steppe landscape. The Fire, Invasion, and Ecology in Sagebrush Steppe (FIREss) lab is working to address this knowledge gap by studying the longevity of pre-emergent herbicides in the sagebrush steppe across a variety of plant communities, soil types, and climates by using five carefully designed field experiments.
Sources/Usage: Public Domain. View Media Details
We are specifically looking at the longevity of the effects of the pre-emergent herbicides imazapic, rimsulfron, and indaziflam. In addition to reinvasion by exotic annual grasses, we are looking at secondary invasion by other, often more noxious invaders such as rush skeletonweed. Previous studies have shown that indaziflam has greater longevity compared to other types of pre-emergent herbicides, which could benefit desirable native perennials by giving them more time to grow without competition from invaders.
We selected a variety of unique sagebrush-steppe environments for our experiments with different climate and soil conditions, and interim findings from some trials have been published. Herbicide treatment experiments began in 2016 and include the longest-term trials of indaziflam to date. In 2019, we worked with the U.S. Fish and Wildlife Service on the Minidoka National Wildlife Refuge to create a well-replicated and controlled trial application of indaziflam across 1000 acres of recently burned soils. Fish and Wildlife Service staff spread tarps throughout the study area to block the aerial spray and create untreated control plots to compare with the broader treated area. The Fish and Wildlife Service has completed yearly monitoring of plant communities with support from the FIREss lab for data analysis and synthesis. The objective was to “break” the cheatgrass-fire cycle while simultaneously learning about effectiveness of the herbicide, with and without co-application of other herbicides and seeding of native species.
Objectives
1. Determine the longevity of the target and non-target effects of the pre-emergent herbicides imazapic, rimsulfron, indaziflam, and the post-emergent herbicide aminopyralid
2. Determine the impacts of climate and soil type on herbicide longevity
3. Better understand the relationship between pre-emergent herbicide use and secondary invasion
Sources/Usage: Public Domain. View Media Details
Published Results
This research is still ongoing, but interim results from some experiments have been published by our team. In 2016, we started to monitor the longevity of imazapic and rimsulfuron at three sites for one experiment. One site is located in the Idaho Batholith, one in the Snake River Plain, and one in the Northern Basin. The FIREss team collected data on plant cover before treatment, and then conducted prescribed burns at the sites to simulate a wildfire. After the fire, imazapic and rimsulfuron were sprayed. After monitoring the effects of the herbicides for five years, we found that both treatments generally decreased exotic annual grasses in the first year, but after that the results were varied. Plots with rimsulfuron were re-invaded after 1-2 years, whereas imazapic plots resisted invasion for four-five years after herbicide spraying. There was no increase in native perennial grass cover found, and rimsulfuron was found to be more damaging than imazapic to shallow-rooted perennial bunchgrasses. In some soil communities, the spraying resulted in an increase in moss and lichen cover, which is a key aspect of soil integrity. Both herbicides were also found to cause an increase in secondary invasion. Overall, the short term-effects of the herbicides used were consistent and predictable at reducing exotic annual grass cover, but the long-term effects were heavily dependent on which herbicide was used and the plant community present. The longevity of rimsulfuron and imazapic at these plots is still being measured by the FIREss team, we have recently completed our seventh year of monitoring.
Sources/Usage: Public Domain. View Media Details
Additionally, in 2018 in the Boise foothills, we began to monitor the longevity of the effects of the herbicides imazapic and indaziflam in replicated 1-acre plots. We compared imazapic, indaziflam, control groups, and combinations of both herbicides. We found that exotic annual grasses, in particular medusahead, were reduced for up to three years when indaziflam was used. Comparatively, imazapic was only found to reduce exotic annual grasses when sprayed twice. It was found that indaziflam effects were greater when there was more precipitation immediately after spraying, and also when indaziflam was coapplied with imazapic. However, reapplying indaziflam did not lead to additional reductions in exotic annual grasses. Aminopyralid was also used in this experiment, but it only was able to prevent the secondary invasion of the highly noxious forb rush skeletonweed for one year. We recently completed our fifth year of monitoring at this site.
Next Steps
New funding for phase two of this project is allowing us to continue monitoring previously established plots until 2025, enabling us to conduct some of the longest standing trials for imazapic, rimsulfuron, and indaziflam. In addition to continuously monitoring these plots we will produce a document synthesizing the results of our long-term trials with best-practice guidance on the safe and effective application of the herbicides.
If you are unable to access or download a product, email fresc_outreach@usgs.gov.
Vegetation monitoring in Idaho
Scientists in the field monitoring the effects of experimental herbicide treatments for controlling exotic annual grasses in the Boise River Wildlife Management Area.
Scientists in the field monitoring the effects of experimental herbicide treatments for controlling exotic annual grasses in the Boise River Wildlife Management Area.
Experimental herbicide treatment plots in Idaho
Experimental site in the Snake River Plain in southwest Idaho three years post-herbicide treatment. The plot towards the lower left of the image was not treated with herbicide. The plot towards the upper right was treated with the pre-emergent herbicide imazapic and is free of cheatgrass. This is one of the longest-lasting imazapic treatments recorded.
Experimental site in the Snake River Plain in southwest Idaho three years post-herbicide treatment. The plot towards the lower left of the image was not treated with herbicide. The plot towards the upper right was treated with the pre-emergent herbicide imazapic and is free of cheatgrass. This is one of the longest-lasting imazapic treatments recorded.