Photo: Cheatgrass, southwest Montana. © 2020 Delena Norris-Tull
Picloram (Tordon 22K) Herbicide
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
In a controlled experiment on methods of removing spotted knapweed, Skurski, Maxwell, and Rew (2013) studied the impact over three years of four removal treatments in sagebrush-grassland communities in Southwestern Montana. Spotted knapweed (Centaurea stoebe) is “one of the most widespread and heavily managed non-native plant species in the Intermountain West.”
In natural areas in the West, spotted knapweed has been shown to displace native plants, reduce forage quality and quantity, and impact wildlife habitats. However, few field studies have been conducted on the community-level impacts of spotted knapweed. Skurski, Maxwell, and Rew (2013) expressed concern that the pressure to control noxious weeds may cause agencies to treat weeds before fully understanding the impacts that control measures may have on ecosystems, including impacts on the large and small mammals, songbirds, upland game birds, and raptors that rely on those ecosystems.
The four treatments Skurski, et al., 2013, compared were: no treatment, manual removal of knapweed, ground disturbance with hand tools, and treatment with the herbicide picloram (Trodon 22K), which has been commonly used to treat knapweed populations. Skurski, et al.,2013, included ground disturbance to find out whether manual removal was successful in reducing knapweed populations merely due to the ground disturbance.
Skurski, et al.,2013, found that herbicide treatment was “highly effective at reducing C. stoebe cover.” But there was no significant difference in C. stoebe cover between manual removal and herbicide treatment. Manual removal was just as effective as herbicide at reducing knapweed cover. They found that knapweed cover increased with no treatment (145%) and with ground disturbance (118%).
However, the study also uncovered some surprising negative and positive impacts of herbicide treatment.
“The change in total plant cover (not including C. stoebe) was not significantly different among treatments.” But they found that, after three years, the change in native forb cover differed among treatments. “Native forb cover had increased… in response to manual removal and no treatment… [But] native forb cover decreased… in response to herbicide treatment…
“Native grass cover increased… with no treatment, manual removal, and herbicide treatment,” with the greatest increase (41%) in native grasses occurring in the plots with herbicide treatment.
But “total non-native plant cover (other than C. stoebe) increased across all treatments,” with the greatest increase occurring in the plots with herbicide treatment. “Three years after treatment, total non-native cover had increased by an estimated 248%...with herbicide, 62%...with no treatment, 19.4%... with disturbance, and 16.7% with manual removal.” Also, both with herbicide and no treatment, “nearly all of the increase came from non-native grasses,” primarily cheatgrass. Herbicide treatment resulted in an estimated 200% greater increase in cheatgrass cover than in untreated plots. In the plots with disturbance and manual removal, the increase in non-native plants was primarily in forbs (Skurski, Maxwell, & Rew, 2013).
They also noted that “there were no response variables for which the effect of manual removal of C. stoebe was greater than the effect of no treatment, other than [in reducing] C. stoebe cover.”
They concluded that picloram, while effective at reducing spotted knapweed and having the positive effect of increasing native grass cover, also had the detrimental effect of increasing non-native grass cover, primarily cheatgrass. “The tradeoff of one non-native plant for another is an increasingly observed side effect of picloram.” The herbicide also reduced native forb cover. They also concluded that spotted knapweed is not having “significant negative impacts on the cover, richness, or biomass of native forbs or grasses” in the study plots. Spotted knapweed does not appear to be displacing native species in the sagebrush-grassland habitat, and does not necessarily lead to competitive exclusion of native species. Thus, they call into question the rapidity with which agencies are aggressively treating knapweed with herbicides, before thoroughly understanding the impacts of the non-native species on specific habitats, and the potential negative impacts of herbicides.
Skurski, et al.,2013, note the limitation that, in their study area, spotted knapweed has not caused as great an infestation as has been seen in some habitats and climates included in other studies. Thus, in more heavily infested areas, herbicide treatment may be warranted. But they conclude that in some habitats, it may be that “the benefits of herbicide treatment do not justify the costs.”
While these results highlight the complexity of assessing various non-native plant removal treatments, none of the treatments in the study by Skurski, et al., 2013, included planting with native seed. These results should cause agencies to reconsider the importance of supplementing manual removal with planting with native seed.
Jim Pike, District Conservationist, NRCS, Laramie County Field Office, told me, “Here’s the technique I use [to control] cheatgrass [in crops]. I have the landowner plow it under to bury the seed, and then plant millet over it. They can hay the millet, leaving 4-5 inches of stubble. By year two, they can plant their crop over the millet. They may have to use RoundUp once or twice but I discourage its use.”
In my interview with Dr. Scott Nissen, weed scientist at Colorado State University, he said: “For the past four years, since George Beck retired, I’m now doing work on cheatgrass management. George’s research associate began this research. In 2010, we were using the herbicide Alion [Indaziflam; which is formulated with 2,4-D, dicamba, mecoprop, and penoxsulam], which was developed for California vineyards and orchards. George noticed that downy brome was listed on the label as one of the grass weeds this herbicide would kill. So he did research to show that this herbicide also kills brome on rangeland and pasture land in Colorado. It provides long-term control, and has the potential to eradicate downy brome. It can eliminate the seed bank for this species in four years, possibly with only one application. The research on this is funded by the Bayer Company.”
Sheley, et al., 2011, reported that “dicamba and picloram are highly mobile in the soil… High application rates, high rainfall following application, or direct application… to water bodies can pose a significant threat to water resources.” But little research has been conducted on the impacts of herbicides on water and soil resources and wildlife. And few studies have compared impacts of herbicides on native versus invasive plant species. “Herbicides such as 2,4-D, clopyralid, or picloram…[used to control broadleaf weeds] can greatly decrease native forb density and cover… There is evidence suggesting that herbicide effects on native forbs are long-lasting and can drive a local decline in species richness.” They concluded that, “The effect of herbicide on desirable vegetation remains difficult to predict.”
References:
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Picloram (Tordon 22K) Herbicide
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
In a controlled experiment on methods of removing spotted knapweed, Skurski, Maxwell, and Rew (2013) studied the impact over three years of four removal treatments in sagebrush-grassland communities in Southwestern Montana. Spotted knapweed (Centaurea stoebe) is “one of the most widespread and heavily managed non-native plant species in the Intermountain West.”
In natural areas in the West, spotted knapweed has been shown to displace native plants, reduce forage quality and quantity, and impact wildlife habitats. However, few field studies have been conducted on the community-level impacts of spotted knapweed. Skurski, Maxwell, and Rew (2013) expressed concern that the pressure to control noxious weeds may cause agencies to treat weeds before fully understanding the impacts that control measures may have on ecosystems, including impacts on the large and small mammals, songbirds, upland game birds, and raptors that rely on those ecosystems.
The four treatments Skurski, et al., 2013, compared were: no treatment, manual removal of knapweed, ground disturbance with hand tools, and treatment with the herbicide picloram (Trodon 22K), which has been commonly used to treat knapweed populations. Skurski, et al.,2013, included ground disturbance to find out whether manual removal was successful in reducing knapweed populations merely due to the ground disturbance.
Skurski, et al.,2013, found that herbicide treatment was “highly effective at reducing C. stoebe cover.” But there was no significant difference in C. stoebe cover between manual removal and herbicide treatment. Manual removal was just as effective as herbicide at reducing knapweed cover. They found that knapweed cover increased with no treatment (145%) and with ground disturbance (118%).
However, the study also uncovered some surprising negative and positive impacts of herbicide treatment.
“The change in total plant cover (not including C. stoebe) was not significantly different among treatments.” But they found that, after three years, the change in native forb cover differed among treatments. “Native forb cover had increased… in response to manual removal and no treatment… [But] native forb cover decreased… in response to herbicide treatment…
“Native grass cover increased… with no treatment, manual removal, and herbicide treatment,” with the greatest increase (41%) in native grasses occurring in the plots with herbicide treatment.
But “total non-native plant cover (other than C. stoebe) increased across all treatments,” with the greatest increase occurring in the plots with herbicide treatment. “Three years after treatment, total non-native cover had increased by an estimated 248%...with herbicide, 62%...with no treatment, 19.4%... with disturbance, and 16.7% with manual removal.” Also, both with herbicide and no treatment, “nearly all of the increase came from non-native grasses,” primarily cheatgrass. Herbicide treatment resulted in an estimated 200% greater increase in cheatgrass cover than in untreated plots. In the plots with disturbance and manual removal, the increase in non-native plants was primarily in forbs (Skurski, Maxwell, & Rew, 2013).
They also noted that “there were no response variables for which the effect of manual removal of C. stoebe was greater than the effect of no treatment, other than [in reducing] C. stoebe cover.”
They concluded that picloram, while effective at reducing spotted knapweed and having the positive effect of increasing native grass cover, also had the detrimental effect of increasing non-native grass cover, primarily cheatgrass. “The tradeoff of one non-native plant for another is an increasingly observed side effect of picloram.” The herbicide also reduced native forb cover. They also concluded that spotted knapweed is not having “significant negative impacts on the cover, richness, or biomass of native forbs or grasses” in the study plots. Spotted knapweed does not appear to be displacing native species in the sagebrush-grassland habitat, and does not necessarily lead to competitive exclusion of native species. Thus, they call into question the rapidity with which agencies are aggressively treating knapweed with herbicides, before thoroughly understanding the impacts of the non-native species on specific habitats, and the potential negative impacts of herbicides.
Skurski, et al.,2013, note the limitation that, in their study area, spotted knapweed has not caused as great an infestation as has been seen in some habitats and climates included in other studies. Thus, in more heavily infested areas, herbicide treatment may be warranted. But they conclude that in some habitats, it may be that “the benefits of herbicide treatment do not justify the costs.”
While these results highlight the complexity of assessing various non-native plant removal treatments, none of the treatments in the study by Skurski, et al., 2013, included planting with native seed. These results should cause agencies to reconsider the importance of supplementing manual removal with planting with native seed.
Jim Pike, District Conservationist, NRCS, Laramie County Field Office, told me, “Here’s the technique I use [to control] cheatgrass [in crops]. I have the landowner plow it under to bury the seed, and then plant millet over it. They can hay the millet, leaving 4-5 inches of stubble. By year two, they can plant their crop over the millet. They may have to use RoundUp once or twice but I discourage its use.”
In my interview with Dr. Scott Nissen, weed scientist at Colorado State University, he said: “For the past four years, since George Beck retired, I’m now doing work on cheatgrass management. George’s research associate began this research. In 2010, we were using the herbicide Alion [Indaziflam; which is formulated with 2,4-D, dicamba, mecoprop, and penoxsulam], which was developed for California vineyards and orchards. George noticed that downy brome was listed on the label as one of the grass weeds this herbicide would kill. So he did research to show that this herbicide also kills brome on rangeland and pasture land in Colorado. It provides long-term control, and has the potential to eradicate downy brome. It can eliminate the seed bank for this species in four years, possibly with only one application. The research on this is funded by the Bayer Company.”
Sheley, et al., 2011, reported that “dicamba and picloram are highly mobile in the soil… High application rates, high rainfall following application, or direct application… to water bodies can pose a significant threat to water resources.” But little research has been conducted on the impacts of herbicides on water and soil resources and wildlife. And few studies have compared impacts of herbicides on native versus invasive plant species. “Herbicides such as 2,4-D, clopyralid, or picloram…[used to control broadleaf weeds] can greatly decrease native forb density and cover… There is evidence suggesting that herbicide effects on native forbs are long-lasting and can drive a local decline in species richness.” They concluded that, “The effect of herbicide on desirable vegetation remains difficult to predict.”
References:
- Sheley, R.L., James, J.J., Rinella, M. J., Blumenthal, D., & DiTomaso, J.M. (2011). Invasive plant management on anticipated conservation benefits: A scientific assessment. In D.D. Briske (Ed.) Conservation benefits of rangeland practices: Assessment, recommendation, and knowledge gaps. (pp. 293-336). USDA Natural Resources Conservation Service.
- Skurski, T.C., Maxwell, B.D., & Rew, L.J. (2013). Ecological tradeoffs in non-native plant management. Biological Conservation, 159, 292-302.
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