Photo: Musk Thistle, southwest Montana. © 2020 Delena Norris-Tull
Picolinic Acid Family of Herbicides
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
This group of herbicides includes aminopyralid, clopyralid, picloram, and triclopyr, and others.
Aminopyralid (Milestone, Grazon) and Clopyralid (Curtail)
Aminopyralid was first registered by the US EPA for conditional use in 2005. It is used to treat broadleaf weeds, primarily on rangelands and pastures. Concerns about its use quickly emerged. It is generally considered to be non-toxic to mammals (including humans), birds, fish, and invertebrates. But long-lasting soil residues (residues lasting up to 533 days) have been shown to enter the food chain through manures, composts, mulches, and hays from forage, rangelands, or lawns on soils that were treated with aminopyralid. When the manure, compost, or mulch is used on crops, soil residues of aminopyralid and other members of this group of herbicides have caused deformities in various vegetable and flower crops. This effect can occur when aminocyclopyrachlor, clopyralid, picloram, and triclopyron are used on forage crops. With clopyralid, which has been widely used on lawns, contamination of garden composts from grass clippings started appearing in the US in 2000. Due to these concerns, use of aminopyralid was discontinued in the UK in 2008, but was reintroduced there in 2010.
DiTomaso, et al., 2017, report that “the use of aminopyralid to control Centaurea solstitialis in California can lead to the subsequent invasion and proliferation of even less palatable and equally noxious long-awned annual grasses,” such as barb goatgrass and medusahead.
Mikkelson & Lym, 2011, evaluated the presence of aminopyralid and picloram in soil residues in North Dakota. Initially, the intended use for aminopyralid was in invasive weeds in noncroplands, particularly Canada thistle. It was widely used in Conservation Reserve Program lands. For example, in 2008, it was applied to about 18,000 hectares in North Dakota CRP lands. Many CRP lands have since been placed back into agricultural use. Glyphosate resistant soybeans were subsequently planted on much of the CRP lands that had been placed back into agriculture. But soybeans are particularly susceptible to damage from aminopyralid. For purposes of EPA registration, aminopyralid half-life in five soils were evaluated to range from 32 days to 533 days. Fall-applied treatments, particularly in northern States, take longer to degrade than do spring-applied treatments, due to colder fall and winter temperatures. Picloram has a somewhat shorter half-life in soils.
Mikkelson & Lym, 2011, evaluated the effects of aminopyralid and picloram soil residues in a silty, clay soil near Fargo, ND, in alfalfa, corn, soybean, and sunflower crops planted 8 to 23 months after the last application of aminopyralid or picloram. Other herbicides were used to treat weeds within the various crops. Soil residues were measured 20-23 months after aminopyralid or picloram application. There was minimal crop damage documented in this study (see results below), thus a second study was conducted near Casselton, ND. In this study, crops were planted either 8 or 11 months after treatment with aminopyralid. The soil was a silty, clay loam. Soil was collected 8-11 months after aminopyralid was used. Fertilizer was applied and crops were planted immediately after soil samples were taken. Glyphosate was used to treat weeds in corn and soybeans. Clethodim and imazamox were used to treat weeds in sunflowers, and clethodim and bromoxynil were used in alfalfa.
Mikkelson & Lym, 2011, results of the first study: “Aminopyralid concentration in soil 20 or 23 [months after treatment] was approximately two to three times greater when fall-applied compared with that applied in spring…For instance, aminopyralid concentration when spring applied at 240 g ha-1 was 0.17 ug kg-1 compared with 0.48 ug kg-1 when fall-applied at the same rate. Picloram concentration in soil was 1.21 ug kg-1 when spring-applied at 560 g ha-1 and less than the detection limit… when fall-applied at the same rate. The lower picloram concentration from the fall-applied treatment was unexpected, and the reason is unknown because spring-applied picloram had an extra growing season to degrade.”
Neither alfalfa nor sunflowers sustained any injuries from either aminopyralid or picloram. Corn plants had no visual injuries, but corn density was slightly lower in the aminopyralid and picloram soils, compared to the control soil. However, corn grew taller and had greater yields in the treated soils. The increased yields may be due to better weed control in the crops grown on treated soils than in the control fields. Soybean height was not affected by soil residues. But in the soil in which aminopyralid was fall-applied at 120 to 240 g ha-1, significant visible injury to soybean plants appeared by 60 days after crop emergence. Soybean height was not affected. Soybean density, compared to untreated plots, was greater in the treated plots. “Soybean yield was reduced 28, 29, and 41% by residues from spring-applied aminopyralid at 120 g ha-1 and from spring or fall-applied aminopyralid at 240 g ha-1, respectively, compared with the nontreated control. Soybean yield was similar to the nontreated plots when picloram at 560 g ha-1 was applied 20 or 23 [months] before planting.”
Mikkelson & Lym, 2011, results of the second study: “Aminopyralid concentrations in soil [11 months after treatment] ranged from 0.8 to 2.3 ug kg-1 when spring-applied and ranged from 4.8 to 14 ug kg-1 [8 months after treatment] when fall-applied... Aminopyralid concentration approximately doubled as application rate doubled and was approximately sixfold greater when fall-applied compared with when spring-applied. Picloram concentration in soil when applied at 560 g ha-1 was 15.5 and 48.8 ug kg-1 when spring- and fall-applied, respectively. Aminopyralid and picloram concentrations were much less when herbicides were spring-applied than when they were fall-applied, so the summer months appear very important for metabolism and dissipation of these herbicides.”
Alfalfa showed significant injuries in the aminopyralid treated soil, with injuries much greater in the plots with aminopyralid applied in the fall, and at higher rate. In fall-applied aminopyralid and picloram plots, alfalfa height was reduced 30% to 100% compared to control plots. Alfalfa stand density was reduced on fall aminopyralid and picloram treated plots, and in spring aminopyralid treated plots. Alfalfa yield suffered greatly (75-100%) within the fall-applied aminopyralid and picloram plots.
“Corn was very tolerant to aminopyralid and picloram soil residues,” and only sustained minor injuries that fully recovered.
“Soybean planted 8 or 11 [months after treatment] was severely injured and height was reduced by almost all aminopyralid and picloram treatments.” Both soybean height and density were greatly affected. “Fall-applied aminopyralid at 60 g ha-1 or greater completely eliminated soybean yield when planted the following growing season... Spring-applied aminopyralid at 120 and 240 g ha-1 reduced soybean yield by an average of 47%. Soybean was killed when planted into soils with aminopyralid concentrations of 4.8 fig kg-1 or greater and was severely injured by aminopyralid concentrations of 1.2 ug kg-1.” Soybean yield was increased with spring-applied aminopyralid at 60 g ha-1, likely due to the continuing weed control effects of the treatment.
“Sunflower height was reduced from spring-applied aminopyralid at 240 g ha-1 and by all fall-applied aminopyralid and picloram treatments.” Sunflower density was reduced by fall-applied treatments of aminopyralid at the higher rates of application. “Fall-applied picloram at 560 g ha-1 caused sunflower injury, height reduction, and yield reduction similar to fall-applied aminopyralid at 60 g ha-1. Aminopyralid was more phytotoxic to sunflower than picloram…Picloram rate was ninefold greater than the aminopyralid rate was, but sunflower response to the two herbicides was similar….Sunflower yield was reduced an average of 99% when aminopyralid or picloram was fall-applied the previous growing season.”
Mikkelson & Lym, 2011, conclusions: “Corn should be a safe option for eastern North Dakota soils that were treated with aminopyralid within the previous 8 to 11 months.” Alfalfa and sunflower should not be planted within a year after aminopyralid application in these soils. But those crops may be safe 20 to 23 months after application, in soils with >5% organic matter. Soybean would require the longest time interval after aminopyralid application, before soybean planting. Aminopyralid appears to have a longer soil residual than previously reported, at least in northern climates.
References:
Further information on some of the Picolinic Acid Herbicides:
To return to previous sections on most widely used modern herbicides:
Next Sections on Herbicides and other Pesticides:
Picolinic Acid Family of Herbicides
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
This group of herbicides includes aminopyralid, clopyralid, picloram, and triclopyr, and others.
Aminopyralid (Milestone, Grazon) and Clopyralid (Curtail)
Aminopyralid was first registered by the US EPA for conditional use in 2005. It is used to treat broadleaf weeds, primarily on rangelands and pastures. Concerns about its use quickly emerged. It is generally considered to be non-toxic to mammals (including humans), birds, fish, and invertebrates. But long-lasting soil residues (residues lasting up to 533 days) have been shown to enter the food chain through manures, composts, mulches, and hays from forage, rangelands, or lawns on soils that were treated with aminopyralid. When the manure, compost, or mulch is used on crops, soil residues of aminopyralid and other members of this group of herbicides have caused deformities in various vegetable and flower crops. This effect can occur when aminocyclopyrachlor, clopyralid, picloram, and triclopyron are used on forage crops. With clopyralid, which has been widely used on lawns, contamination of garden composts from grass clippings started appearing in the US in 2000. Due to these concerns, use of aminopyralid was discontinued in the UK in 2008, but was reintroduced there in 2010.
DiTomaso, et al., 2017, report that “the use of aminopyralid to control Centaurea solstitialis in California can lead to the subsequent invasion and proliferation of even less palatable and equally noxious long-awned annual grasses,” such as barb goatgrass and medusahead.
Mikkelson & Lym, 2011, evaluated the presence of aminopyralid and picloram in soil residues in North Dakota. Initially, the intended use for aminopyralid was in invasive weeds in noncroplands, particularly Canada thistle. It was widely used in Conservation Reserve Program lands. For example, in 2008, it was applied to about 18,000 hectares in North Dakota CRP lands. Many CRP lands have since been placed back into agricultural use. Glyphosate resistant soybeans were subsequently planted on much of the CRP lands that had been placed back into agriculture. But soybeans are particularly susceptible to damage from aminopyralid. For purposes of EPA registration, aminopyralid half-life in five soils were evaluated to range from 32 days to 533 days. Fall-applied treatments, particularly in northern States, take longer to degrade than do spring-applied treatments, due to colder fall and winter temperatures. Picloram has a somewhat shorter half-life in soils.
Mikkelson & Lym, 2011, evaluated the effects of aminopyralid and picloram soil residues in a silty, clay soil near Fargo, ND, in alfalfa, corn, soybean, and sunflower crops planted 8 to 23 months after the last application of aminopyralid or picloram. Other herbicides were used to treat weeds within the various crops. Soil residues were measured 20-23 months after aminopyralid or picloram application. There was minimal crop damage documented in this study (see results below), thus a second study was conducted near Casselton, ND. In this study, crops were planted either 8 or 11 months after treatment with aminopyralid. The soil was a silty, clay loam. Soil was collected 8-11 months after aminopyralid was used. Fertilizer was applied and crops were planted immediately after soil samples were taken. Glyphosate was used to treat weeds in corn and soybeans. Clethodim and imazamox were used to treat weeds in sunflowers, and clethodim and bromoxynil were used in alfalfa.
Mikkelson & Lym, 2011, results of the first study: “Aminopyralid concentration in soil 20 or 23 [months after treatment] was approximately two to three times greater when fall-applied compared with that applied in spring…For instance, aminopyralid concentration when spring applied at 240 g ha-1 was 0.17 ug kg-1 compared with 0.48 ug kg-1 when fall-applied at the same rate. Picloram concentration in soil was 1.21 ug kg-1 when spring-applied at 560 g ha-1 and less than the detection limit… when fall-applied at the same rate. The lower picloram concentration from the fall-applied treatment was unexpected, and the reason is unknown because spring-applied picloram had an extra growing season to degrade.”
Neither alfalfa nor sunflowers sustained any injuries from either aminopyralid or picloram. Corn plants had no visual injuries, but corn density was slightly lower in the aminopyralid and picloram soils, compared to the control soil. However, corn grew taller and had greater yields in the treated soils. The increased yields may be due to better weed control in the crops grown on treated soils than in the control fields. Soybean height was not affected by soil residues. But in the soil in which aminopyralid was fall-applied at 120 to 240 g ha-1, significant visible injury to soybean plants appeared by 60 days after crop emergence. Soybean height was not affected. Soybean density, compared to untreated plots, was greater in the treated plots. “Soybean yield was reduced 28, 29, and 41% by residues from spring-applied aminopyralid at 120 g ha-1 and from spring or fall-applied aminopyralid at 240 g ha-1, respectively, compared with the nontreated control. Soybean yield was similar to the nontreated plots when picloram at 560 g ha-1 was applied 20 or 23 [months] before planting.”
Mikkelson & Lym, 2011, results of the second study: “Aminopyralid concentrations in soil [11 months after treatment] ranged from 0.8 to 2.3 ug kg-1 when spring-applied and ranged from 4.8 to 14 ug kg-1 [8 months after treatment] when fall-applied... Aminopyralid concentration approximately doubled as application rate doubled and was approximately sixfold greater when fall-applied compared with when spring-applied. Picloram concentration in soil when applied at 560 g ha-1 was 15.5 and 48.8 ug kg-1 when spring- and fall-applied, respectively. Aminopyralid and picloram concentrations were much less when herbicides were spring-applied than when they were fall-applied, so the summer months appear very important for metabolism and dissipation of these herbicides.”
Alfalfa showed significant injuries in the aminopyralid treated soil, with injuries much greater in the plots with aminopyralid applied in the fall, and at higher rate. In fall-applied aminopyralid and picloram plots, alfalfa height was reduced 30% to 100% compared to control plots. Alfalfa stand density was reduced on fall aminopyralid and picloram treated plots, and in spring aminopyralid treated plots. Alfalfa yield suffered greatly (75-100%) within the fall-applied aminopyralid and picloram plots.
“Corn was very tolerant to aminopyralid and picloram soil residues,” and only sustained minor injuries that fully recovered.
“Soybean planted 8 or 11 [months after treatment] was severely injured and height was reduced by almost all aminopyralid and picloram treatments.” Both soybean height and density were greatly affected. “Fall-applied aminopyralid at 60 g ha-1 or greater completely eliminated soybean yield when planted the following growing season... Spring-applied aminopyralid at 120 and 240 g ha-1 reduced soybean yield by an average of 47%. Soybean was killed when planted into soils with aminopyralid concentrations of 4.8 fig kg-1 or greater and was severely injured by aminopyralid concentrations of 1.2 ug kg-1.” Soybean yield was increased with spring-applied aminopyralid at 60 g ha-1, likely due to the continuing weed control effects of the treatment.
“Sunflower height was reduced from spring-applied aminopyralid at 240 g ha-1 and by all fall-applied aminopyralid and picloram treatments.” Sunflower density was reduced by fall-applied treatments of aminopyralid at the higher rates of application. “Fall-applied picloram at 560 g ha-1 caused sunflower injury, height reduction, and yield reduction similar to fall-applied aminopyralid at 60 g ha-1. Aminopyralid was more phytotoxic to sunflower than picloram…Picloram rate was ninefold greater than the aminopyralid rate was, but sunflower response to the two herbicides was similar….Sunflower yield was reduced an average of 99% when aminopyralid or picloram was fall-applied the previous growing season.”
Mikkelson & Lym, 2011, conclusions: “Corn should be a safe option for eastern North Dakota soils that were treated with aminopyralid within the previous 8 to 11 months.” Alfalfa and sunflower should not be planted within a year after aminopyralid application in these soils. But those crops may be safe 20 to 23 months after application, in soils with >5% organic matter. Soybean would require the longest time interval after aminopyralid application, before soybean planting. Aminopyralid appears to have a longer soil residual than previously reported, at least in northern climates.
References:
- DiTomaso, J.M., Monaco, T.A., James, J.J., Firn, J. (2017). Invasive plant species and novel rangeland systems. In D. Briske (Ed.), Rangeland Systems: Processes, Management & Challenges: 429-465. Springer Series on Environmental Management. Cham, Switzerland: Springer.
- Mikkelson, J.R., & Lym, R.G. (July-Sept., 2011). Aminopyralid soil residues affect crop rotation in North Dakota soils. Weed Technology, 25(3): 422-429.
Further information on some of the Picolinic Acid Herbicides:
To return to previous sections on most widely used modern herbicides:
Next Sections on Herbicides and other Pesticides: