Photo: Kochia, southwest Montana. © 2020 Delena Norris-Tull
Controlling herbicide-resistant invasive plants in herbicide-resistant crops
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education,
University of Montana Western, October 2020.
As predicted (Green, 2007), the innovation of herbicide-resistant crops has exacerbated the problem of herbicide-resistant weeds. In order to grow herbicide-resistant crops, the farmer has to use the herbicide promoted by the company that developed the resistant crop seeds. Thus, many farmers have come to rely too heavily on a single herbicide, glyphosate being the most commonly developed for crop-resistance. The overreliance on a single herbicide leads to increased herbicide-resistance in weeds. The first instance “identified in a glyphosate-resistant crop system was horseweed. Glyphosate-resistant horseweed populations appeared just 3 years after the initial use of glyphosate-resistant soybeans” (Green, 2007).
Blatt, 2020, reported on problems associated with weeds that are resistant to glyphosate. These weeds are particularly common in monocultures when the same crop is grown year after year. The rise of herbicide-resistant weeds, or superweeds, pose a variety of threats: they reduce crop yields; farmers often respond to superweeds by using increased amounts of herbicide, or ever-more potent herbicides; superweeds have the potential to out-compete with native plant species; and they can reduce usable farmlands, resulting in increased habitat loss by increasing expansion of farms.
Research has shown that, unless they perceive rapid economic benefits, many farmers are reluctant to adopt the best management practices for preventing the development of herbicide-resistant weeds. Dong, et al., 2016, developed a tool to evaluate the extent to which farmers adopt the best management practices for preventing the development of herbicide-resistant weeds (results of that study are described in the section "Best Management Practices"). They stated that, in 2016, glyphosate was the “world’s most widely used herbicide. It is highly effective and provides control of a broad spectrum of weeds yet is also toxicologically and environmentally relatively safe.” [However, refer to the section “Glyphosate” for information on the 2019 financial settlements to farmers who have developed non-Hodgkin lymphoma after years of using glyphosate]. Dong, et al., 2016, reported that, “In 2013, herbicide-tolerant crops accounted for 93% of soybean planted acres, 85% of corn planted acres, and 82% of cotton planted acres in the U.S. (USDA National Agricultural Statistics Service, 2013), with the vast majority planted to [RoundUp Ready] varieties.”
By 2015, various weed species were exhibiting resistance to multiple herbicides, including both ALS-inhibiting herbicides and glyphosate. Meyer, et al., 2015, conducted a two-year field experiment to evaluate the effectiveness of the use of multiple herbicides in controlling Palmer amaranth and waterhemp. They cite the cause of the increase in herbicide-resistance as the result of the use of reactive, rather than proactive strategies by farmers, who often wait too long to tackle the problem. By 2015, Palmer amaranth, which has very high seed production and rapid growth, was estimated to cost soybean producers millions of dollars in the midsouth.
Meyer, et al., 2015, evaluated the effectiveness of 25 herbicide programs in controlling glyphosate-resistant Palmer amaranth and waterhemp in six midsouth States that grow herbicide-resistant soybeans: 5 programs used pre-emergent herbicides only, in various combinations of two herbicides; 10 used pre- and post- herbicides 3-4 weeks apart (EPOST); and 10 used the same pre-herbicides followed by post-herbicides applied 6 to 7 weeks later (LPOST). A variety of combinations of herbicides were applied as pre-emergents. And a variety of combinations of herbicides, including glyphosate, were applied as post-emergents. Various preplanting procedures were used that were common to each State, including tillage and burndown herbicides. The authors assessed the impact of the various herbicide treatments with a visual assessment of each plot, scoring the plots as 0% to 100% control, relative to the non-treated control, with 0% meaning no weed control and 100% meaning death of all weeds of that species.
“Herbicides included in these experiments are either currently available or are herbicides that were program concepts for use in the developmental herbicide-resistant soybean technologies….The goals were to compare current and future herbicide programs...and to identify those that provide the greatest control of Amaranthus spp. Therefore, no crop was planted.”
Results of Meyer, et al., 2015, on Palmer amaranth control:
“All PRE-only programs provided > 95% Palmer amaranth control at 3 to 4” weeks after application. For these PRE-only programs, weed control was reduced at 6-7 weeks, depending on the combination of herbicides used, with > 85% control for most combinations of herbicides, but only 80% control with dicamba + acetochlor.
All programs with post-emergent herbicides applied 3-4 weeks after pre-emergent herbicides (EPOST) “provided > 95% control of Palmer amaranth” at 3 to 4 weeks after the post-emergent herbicide applications. Weed densities in these plots were at <5%.
All programs with post-emergent herbicides applied 6-7 weeks after pre-emergent herbicides (LPOST), had no significant differences in either weed or crop counts, than did the programs with only pre-emergent herbicides. All LPOST programs that used dicamba or 2,4-D as post-herbicides had >90% control of Palmer amaranth at 3-4 weeks after the POST-herbicide applications.
“The programs that provided the greatest control at both rating timings consisted of PRE [followed by] POST applications using three or more sites of action.”
“Overall, control was greater for programs with LPOST herbicides than it was for programs with EPOST herbicides at 3 to 4 [weeks after] LPOST application, and an orthogonal contrast shows significant difference between programs with EPOST herbicides and programs with LPOST herbicides... This indicates that by the 3 to 4 [weeks after] LPOST rating, herbicide programs with a residual herbicide included in the EPOST application were no longer providing residual control, leading to new emergence. However, a different situation occurred with programs with LPOST herbicide applications. Data collected immediately before the LPOST application… show Palmer amaranth plants were present in the plots at the time of application… and were not fully controlled at 3 to 4 [weeks after] LPOST application… Having plants not fully controlled by LPOST herbicides is less desirable than applying EPOST herbicides and risking emergence 3 to 4 [weeks] later.”
“Overall, new technologies performed better for controlling Palmer amaranth than did glyphosate- or glufosinate-resistant systems.”
Results of Meyer, et al., 2015, on waterhemp control:
All PRE-only programs “provided > 95% control of waterhemp at 3 to 4 [weeks] after application.” For these PRE-only programs, waterhemp control at 6-7 weeks “was > 90% control for dicamba + acetochlor only.” This may be related to the rainfall requirements of acetochlor, but other environmental factors also may have influenced efficacy.
“All programs with EPOST herbicides provided 96% control of waterhemp at 3 to 4 [weeks after] EPOST application… and reduced waterhemp density to < 1% of the density in the nontreated control.”
For programs with LPOST herbicides, “By 3 to 4 [weeks after] LPOST treatment, flumioxazin + pyroxasulfone applied PRE [followed by] a S-metolachlor + glyphosate application, which contains no effective sites of action with POST activity, provided only 74% control... The programs that provided the greatest control at both rating timings typically consisted of PRE [followed by] POST applications using three or more sites of action…, with the exception of dicamba + acetochlor applied PRE [followed by] S-metolachlor + glyphosate + dicamba applied LPOST (two sites of action).”
“An orthogonal contrast between programs containing either a dicamba or 2,4-D treatment showed no difference between those technologies, and future technologies performed better than current technologies….
"Even though programs with EPOST herbicides provided less control than did programs with LPOST herbicides at 3 to 4 [weeks after] LPOST application, waiting until 6 to 7 [weeks after] PRE treatment to make a POST application gave weeds a longer opportunity to emerge and compete with the crop.”
Implications of Meyer, et al., 2015: “The herbicide programs evaluated in these experiments that contained new soybean technologies and PRE [followed by] EPOST herbicides should effectively manage glyphosate-resistant waterhemp and Palmer amaranth. Current technologies that include a POST herbicide treatment with an effective site of action (e.g., glufosinate) in combination with a residual product will also control glyphosate-resistant Amaranthus spp. To delay herbicide-resistance, applying residual herbicides PRE and POST to minimize selection pressure on herbicides with only POST activity is recommended... Furthermore, applications of POST herbicides should occur no later than 3 to 4 [weeks after] PRE treatment to ensure that herbicides are applied at labeled weed sizes and that residual herbicides applied POST are effectively used. However, it is possible residual herbicides may not be effective if no rainfall occurs after application.” If little rain falls or canopy-closure does not occur within a few weeks, “another application of a POST herbicide may be necessary for season-long control.”
In the 1980s, it was found that two mutations in corn provided a greater degree of ALS herbicide-resistance than did a single mutation. This led to the development of new biological technologies to produce crops seeds for corn, soybeans, and other crops that are resistant to the entire array of ALS herbicides and to glyphosate. The new mechanism would use a gene from Bacillus licheniformis. Recombinant DNA can be used to increase the glyphosate acetylation activity of this gene. Since 2007, additional genetic engineering techniques have been tested and found to have some success, but more research is needed and several techniques have not yet been approved (for information on the status of this research, refer to Daniell, et al., 1998; Duke, S.O., 2015; Lombardo, et al., 2016).
Next section on Herbicide Resistance:
Links to additional information on herbicides and other pesticides:
Controlling herbicide-resistant invasive plants in herbicide-resistant crops
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education,
University of Montana Western, October 2020.
As predicted (Green, 2007), the innovation of herbicide-resistant crops has exacerbated the problem of herbicide-resistant weeds. In order to grow herbicide-resistant crops, the farmer has to use the herbicide promoted by the company that developed the resistant crop seeds. Thus, many farmers have come to rely too heavily on a single herbicide, glyphosate being the most commonly developed for crop-resistance. The overreliance on a single herbicide leads to increased herbicide-resistance in weeds. The first instance “identified in a glyphosate-resistant crop system was horseweed. Glyphosate-resistant horseweed populations appeared just 3 years after the initial use of glyphosate-resistant soybeans” (Green, 2007).
Blatt, 2020, reported on problems associated with weeds that are resistant to glyphosate. These weeds are particularly common in monocultures when the same crop is grown year after year. The rise of herbicide-resistant weeds, or superweeds, pose a variety of threats: they reduce crop yields; farmers often respond to superweeds by using increased amounts of herbicide, or ever-more potent herbicides; superweeds have the potential to out-compete with native plant species; and they can reduce usable farmlands, resulting in increased habitat loss by increasing expansion of farms.
Research has shown that, unless they perceive rapid economic benefits, many farmers are reluctant to adopt the best management practices for preventing the development of herbicide-resistant weeds. Dong, et al., 2016, developed a tool to evaluate the extent to which farmers adopt the best management practices for preventing the development of herbicide-resistant weeds (results of that study are described in the section "Best Management Practices"). They stated that, in 2016, glyphosate was the “world’s most widely used herbicide. It is highly effective and provides control of a broad spectrum of weeds yet is also toxicologically and environmentally relatively safe.” [However, refer to the section “Glyphosate” for information on the 2019 financial settlements to farmers who have developed non-Hodgkin lymphoma after years of using glyphosate]. Dong, et al., 2016, reported that, “In 2013, herbicide-tolerant crops accounted for 93% of soybean planted acres, 85% of corn planted acres, and 82% of cotton planted acres in the U.S. (USDA National Agricultural Statistics Service, 2013), with the vast majority planted to [RoundUp Ready] varieties.”
By 2015, various weed species were exhibiting resistance to multiple herbicides, including both ALS-inhibiting herbicides and glyphosate. Meyer, et al., 2015, conducted a two-year field experiment to evaluate the effectiveness of the use of multiple herbicides in controlling Palmer amaranth and waterhemp. They cite the cause of the increase in herbicide-resistance as the result of the use of reactive, rather than proactive strategies by farmers, who often wait too long to tackle the problem. By 2015, Palmer amaranth, which has very high seed production and rapid growth, was estimated to cost soybean producers millions of dollars in the midsouth.
Meyer, et al., 2015, evaluated the effectiveness of 25 herbicide programs in controlling glyphosate-resistant Palmer amaranth and waterhemp in six midsouth States that grow herbicide-resistant soybeans: 5 programs used pre-emergent herbicides only, in various combinations of two herbicides; 10 used pre- and post- herbicides 3-4 weeks apart (EPOST); and 10 used the same pre-herbicides followed by post-herbicides applied 6 to 7 weeks later (LPOST). A variety of combinations of herbicides were applied as pre-emergents. And a variety of combinations of herbicides, including glyphosate, were applied as post-emergents. Various preplanting procedures were used that were common to each State, including tillage and burndown herbicides. The authors assessed the impact of the various herbicide treatments with a visual assessment of each plot, scoring the plots as 0% to 100% control, relative to the non-treated control, with 0% meaning no weed control and 100% meaning death of all weeds of that species.
“Herbicides included in these experiments are either currently available or are herbicides that were program concepts for use in the developmental herbicide-resistant soybean technologies….The goals were to compare current and future herbicide programs...and to identify those that provide the greatest control of Amaranthus spp. Therefore, no crop was planted.”
Results of Meyer, et al., 2015, on Palmer amaranth control:
“All PRE-only programs provided > 95% Palmer amaranth control at 3 to 4” weeks after application. For these PRE-only programs, weed control was reduced at 6-7 weeks, depending on the combination of herbicides used, with > 85% control for most combinations of herbicides, but only 80% control with dicamba + acetochlor.
All programs with post-emergent herbicides applied 3-4 weeks after pre-emergent herbicides (EPOST) “provided > 95% control of Palmer amaranth” at 3 to 4 weeks after the post-emergent herbicide applications. Weed densities in these plots were at <5%.
All programs with post-emergent herbicides applied 6-7 weeks after pre-emergent herbicides (LPOST), had no significant differences in either weed or crop counts, than did the programs with only pre-emergent herbicides. All LPOST programs that used dicamba or 2,4-D as post-herbicides had >90% control of Palmer amaranth at 3-4 weeks after the POST-herbicide applications.
“The programs that provided the greatest control at both rating timings consisted of PRE [followed by] POST applications using three or more sites of action.”
“Overall, control was greater for programs with LPOST herbicides than it was for programs with EPOST herbicides at 3 to 4 [weeks after] LPOST application, and an orthogonal contrast shows significant difference between programs with EPOST herbicides and programs with LPOST herbicides... This indicates that by the 3 to 4 [weeks after] LPOST rating, herbicide programs with a residual herbicide included in the EPOST application were no longer providing residual control, leading to new emergence. However, a different situation occurred with programs with LPOST herbicide applications. Data collected immediately before the LPOST application… show Palmer amaranth plants were present in the plots at the time of application… and were not fully controlled at 3 to 4 [weeks after] LPOST application… Having plants not fully controlled by LPOST herbicides is less desirable than applying EPOST herbicides and risking emergence 3 to 4 [weeks] later.”
“Overall, new technologies performed better for controlling Palmer amaranth than did glyphosate- or glufosinate-resistant systems.”
Results of Meyer, et al., 2015, on waterhemp control:
All PRE-only programs “provided > 95% control of waterhemp at 3 to 4 [weeks] after application.” For these PRE-only programs, waterhemp control at 6-7 weeks “was > 90% control for dicamba + acetochlor only.” This may be related to the rainfall requirements of acetochlor, but other environmental factors also may have influenced efficacy.
“All programs with EPOST herbicides provided 96% control of waterhemp at 3 to 4 [weeks after] EPOST application… and reduced waterhemp density to < 1% of the density in the nontreated control.”
For programs with LPOST herbicides, “By 3 to 4 [weeks after] LPOST treatment, flumioxazin + pyroxasulfone applied PRE [followed by] a S-metolachlor + glyphosate application, which contains no effective sites of action with POST activity, provided only 74% control... The programs that provided the greatest control at both rating timings typically consisted of PRE [followed by] POST applications using three or more sites of action…, with the exception of dicamba + acetochlor applied PRE [followed by] S-metolachlor + glyphosate + dicamba applied LPOST (two sites of action).”
“An orthogonal contrast between programs containing either a dicamba or 2,4-D treatment showed no difference between those technologies, and future technologies performed better than current technologies….
"Even though programs with EPOST herbicides provided less control than did programs with LPOST herbicides at 3 to 4 [weeks after] LPOST application, waiting until 6 to 7 [weeks after] PRE treatment to make a POST application gave weeds a longer opportunity to emerge and compete with the crop.”
Implications of Meyer, et al., 2015: “The herbicide programs evaluated in these experiments that contained new soybean technologies and PRE [followed by] EPOST herbicides should effectively manage glyphosate-resistant waterhemp and Palmer amaranth. Current technologies that include a POST herbicide treatment with an effective site of action (e.g., glufosinate) in combination with a residual product will also control glyphosate-resistant Amaranthus spp. To delay herbicide-resistance, applying residual herbicides PRE and POST to minimize selection pressure on herbicides with only POST activity is recommended... Furthermore, applications of POST herbicides should occur no later than 3 to 4 [weeks after] PRE treatment to ensure that herbicides are applied at labeled weed sizes and that residual herbicides applied POST are effectively used. However, it is possible residual herbicides may not be effective if no rainfall occurs after application.” If little rain falls or canopy-closure does not occur within a few weeks, “another application of a POST herbicide may be necessary for season-long control.”
In the 1980s, it was found that two mutations in corn provided a greater degree of ALS herbicide-resistance than did a single mutation. This led to the development of new biological technologies to produce crops seeds for corn, soybeans, and other crops that are resistant to the entire array of ALS herbicides and to glyphosate. The new mechanism would use a gene from Bacillus licheniformis. Recombinant DNA can be used to increase the glyphosate acetylation activity of this gene. Since 2007, additional genetic engineering techniques have been tested and found to have some success, but more research is needed and several techniques have not yet been approved (for information on the status of this research, refer to Daniell, et al., 1998; Duke, S.O., 2015; Lombardo, et al., 2016).
Next section on Herbicide Resistance:
Links to additional information on herbicides and other pesticides: