In the past few weeks, two decisions have highlighted the precarious nature of the list of chemicals tillage farmers have to protect crops from weeds and pests. Chlorpyrifos (better known as Dursban, Clinch etc) was taken off the market with almost immediate effect (31 March 2016) due to a health and safety risk to the sprayer operator. Glyphosate (better known as Roundup, Gallup etc) had its re-registration decision referred to a political forum even though a comprehensive body of scientific evidence supports its continued use as a herbicide.
While growers and advisers can only play with the hand they are dealt, our aim should be to utilise pesticides as efficiently as possible. This is as simple as following the label but also involves keeping up to date with new research and avoiding practices that may affect future performance. One such area is herbicide resistance.
What is herbicide resistance?
Herbicide resistance describes a weed that can survive (and produce seeds) after being treated with a rate of a herbicide that would normally kill it. For example, herbicide A normally controls weed B, but due to continued use over time, weed B adapts and can now de-toxify herbicide A and survive a robust rate.
Herbicide resistance is not poor control of weeds as a result of difficult spraying conditions or a poor choice of herbicide being applied etc. In practice, I find most complaints on poor weed control can be attributed to weather factors and/or errors in application technique. However, for a small number of weeds – namely chickweed, marigold and poppy – I find that herbicide resistance is increasingly to blame for control failures.
Some confusion can also arise from the weed charts written on herbicide labels. Manufacturers use the term ‘resistant’ on their product labels to guide the user as to which weeds are not controlled by a herbicide. For example, DuPont deems cleavers to be resistant to Ally Max and does not claim control on the label. It is naturally resistant rather than having developed resistance.
How does resistance occur?
This is a simple question but without a simple answer. However, this knowledge is crucial to help prevent herbicide resistance becoming unmanageable. Scientists are very good at confirming the presence of resistance and explaining the genetic and biochemical reasons as to why a weed can survive a herbicide spray. However, there is no consensus as to how this resistance occurs.
While being careful not to attempt to over-simplify the issue, resistance generally occurs in one of two ways.
1. A very small number of mutant weeds (single gene alterations) exists in the population, either before or after a herbicide is introduced. When the herbicide is applied over a period of time the mutant weeds can survive and reproduce, thus increasing their proportion in the population.
2. Weeds (multiple gene alterations) slowly develop partial resistance over many generations. This is less dramatic in the field and harder to confirm in experiments.
In either case, there is no convincing evidence to suggest that herbicides cause such mutations but rather select out the resistant weeds over time. Despite this, resistance is not inevitable and it is probable that resistant genes simply do not exist in certain weed species. Irrespective of how resistance occurs, the more you control weeds with herbicides, the more you select and the more likely you will find resistant weeds in your fields.
How can I spot resistance?
In my experience, growers or advisers will not spot herbicide-resistant weeds at low levels in a field. It’s only when the weed becomes noticeable that any action is taken. One of the best times to spot a low-level problem is during the harvest – from the combine cab. If you see some weeds that were poorly controlled, or are occurring in patches (not coinciding with a sprayer run), it is time to be suspicious and talk to your adviser.
Best practice guidelines to spot herbicide resistance are as follows:
A gradual decline in control level over several years.Healthy plants beside dead plants of the same species.Poor weed control leading to discrete weed patches.Poor control of one susceptible species when other susceptible species are well controlled.How common is herbicide
resistance worldwide?
There is no absolute answer to that question because it is not a static phenomenon. However, data on the subject is being continuously updated and Dr Ian Heap, a weed researcher in the US, edits an excellent website (www.weedscience.org) on the subject. According to this source, there are currently 249 weeds (144 broad-leaved and 105 grass weeds) that have developed resistance to 160 different herbicides worldwide.
Most of the scientifically proven resistant weed cases have occurred in the US, Canada and Australia, primarily in wheat, maize and cotton. Spain, France and Italy have the most reported cases of herbicide resistance in the EU, but these are mainly in horticultural crops.
ALS inhibitor herbicides
In Ireland, weed control in spring cereals, and a large area of winter cereals, is based on a family of herbicides known as ALS herbicides. Common names from this family include Ally Max, Cameo Max, Pacifica, Boxer etc. This family of herbicides has been very successful worldwide but it is also the grouping with the most reported cases of resistant weeds (see Figure 1). Luckily many of the ALS-resistant weeds do not grow in Ireland but this should serve as a warning that these herbicides must be protected to ensure they remain effective for a long time to come.
Normally, we would expect that if a weed develops resistance to one ALS herbicide (eg Ally), then it would be resistant to other ALS herbicides (eg Pacifica, Boxer etc). However, detailed genetic studies in the UK have identified that there are differences between the ALS-resistant weeds and how they are controlled by ALS herbicides.
Initial Teagasc studies have also found this to be the case among Irish samples of chickweed but in practical terms you still need non-ALS herbicides for reliable control as you cannot tell (by looking) what genetic mutation has occurred in your fields.
Do we have herbicide
resistance in Ireland?
Yes. Teagasc, DuPont and Dow have confirmed resistance to ALS herbicides (Ally etc) in common chickweed, corn marigold and common poppy. This work has also shown that there are differences between some ALS herbicides in their resistance status.
In an effort to establish the extent of ALS-resistant chickweed, Teagasc conducted a survey of 20 cereal fields from the main tillage areas in 2010. Chickweed collected from 16 of the farms exhibited some reduced level of control with Ally, with seven farms meeting the criteria of having resistant types. Two other chickweed samples came from grassland fields and these were fully controlled by Ally, even at reduced rates.
All of the fields with reduced sensitivity to Ally had been in tillage for greater than 10 years and the farmers had being using ALS herbicides each year, albeit in conjunction with other non-ALS herbicides.
The full results from this screening test are shown in Figure 2. For this work the weight of 10 sprayed chickweed plants was divided by the weight of 10 unsprayed chickweed plants for each sample source – the higher the bar in the chart, the more growth occurred after the herbicide was applied, indicating resistance. Importantly, all populations were fully controlled by CMPP (at full rate).
Other tests (not shown) confirmed that Fluroxypyr, IPU and Calaris gave good control of chickweed, irrespective of whether or not it was resistant to Ally. Remember that these tests were carried out in a glasshouse where temperatures were higher and spraying conditions were better than in most field situations.
How can I control
resistant weeds?
Selection pressure is probably the most important factor determining the evolution of herbicide resistance. Coming back to an earlier point, the more often we use the same herbicide, the more likely we will find resistant weeds. The weather also plays a part here.
In the Teagasc chickweed screen, resistance developed even though alternative herbicides were used. However, the alternative was used at low rates and possibly in sub-optimal conditions. None of the farmers I collected the chickweed from would dare use a half pint of CMPP in cold weather to control chickweed. But that is what they added to Ally over many years.
1. Observe and record unusual weeds: This is the starting point for control, but is mostly overlooked as a control option. A keen eye from the combine cab and a quick phone call to a merchant/adviser will prevent a lot of future problems. I often say to farmers who have developed weed problems that if the two of us could wind the clock back a few years, we would prevent the problem from escalating. The sayings “a stitch in time saves nine” and “one year’s seeding is seven year’s weeding” should be the mottos for dealing with herbicide-resistant weeds.
2. Use alternative herbicides: For the majority of tillage farmers, an effective herbicide is still the preferred solution. So, taking the three ALS-resistant weeds, what are the solutions? From the Teagasc glasshouse studies we see no cross-resistance with other herbicide groups, and CMPP and fluroxypyr are still fully effective against chickweed.
Tank-mixing alternative chemistry with ALS herbicides is an anti-resistance strategy, but ensure the alternative is used correctly as it is now doing most of the heavy lifting (see table below). Re-assess weed control two to four weeks after spraying.
$ALS herbicides include: Ally Max, Eagle, and Pacifica etc.
*Florasulam (in Spitfire) is an ALS but is not affected by dominant ALS mutation in the Irish chickweed population.
3. Use cultural controls: This is using your knowledge of the weed and how it grows to reduce its numbers and vigour. For example, corn marigold is acid loving and has been reduced in experiments from 148 plants/m2 to one plant/m2 by using a combination of liming, fallowing and herbicides. Poppy seed production is halved as winter wheat density is doubled up to 200 crop plants per m2.
Having a competitive crop is fundamental to reducing herbicide selection pressure so seedbed preparation, reducing compaction, avoiding weed contamination of seeds and machinery, seed rate, and fertiliser management all contribute to this. Oats is very competitive against most weeds and reduces weed growth significantly, again reducing the pressure on the herbicide to perform.
4. Crop rotation: Having a reasonable crop rotation in a field enables you to use alternative herbicides and cultural controls together. Residual herbicides used in oilseed rape, beans, maize and beet are not safe to use in cereals. Therefore, the target weed will not be exposed to them if you only grow continuous spring barley. Alternating between an early-drilled autumn crop (eg oilseed rape) and a late-drilled spring crop (eg maize) will also change the dominant weeds that germinate in a field as opposed to drilling the last week of March each year.
What weeds will become
resistant in the future?
UK research found that weeds that produce high numbers of seeds which can reproduce more than once per season pose the highest risk of herbicide resistance.
If herbicide resistance has already been found in a similar situation (crop, climate, etc) this also should act as a warning.
The most important chemical factors include the herbicide mode of action, mode of use (ie alone or in mixtures and sequences or multiple applications in a season), its intrinsic activity and its residual activity. Cultural control continues to have a major role to play in herbicide resistance management strategies.
Table 1 contains a summary of some Irish tillage weeds and whether herbicide resistance has been confirmed in these in other countries.
While herbicide resistance has not been a dominant factor in Ireland the recent confirmation of a number of specific weeds should serve as a warning that resistance can happen here and that it can get much worse. But it is manageable and the key factor is to identify an incidence early and then take appropriate action. Chemicals always need an alternative complementary method of control and this can either be a separate herbicide family or the help of a range of cultural control techniques.
The best advice that can be given is to try and prevent resistance from occurring.
Groundsel
Groundsel is a common weed in winter cereals as it can escape the traditional autumn herbicides. Excellent control of groundsel is achieved in the spring with ALS herbicides such as Cameo Max etc. However, ALS-resistant groundsel has recently been confirmed in wheat fields in France. To prevent/delay this problem occurring here, growers and advisers need to mix alternative herbicides with ALS when tidying up weed control in the spring and consider appropriate cultural options. The French research can be found in Pest Management Science 2015.
To read the full Crop Protection Focus Supplement, click here.
In the past few weeks, two decisions have highlighted the precarious nature of the list of chemicals tillage farmers have to protect crops from weeds and pests. Chlorpyrifos (better known as Dursban, Clinch etc) was taken off the market with almost immediate effect (31 March 2016) due to a health and safety risk to the sprayer operator. Glyphosate (better known as Roundup, Gallup etc) had its re-registration decision referred to a political forum even though a comprehensive body of scientific evidence supports its continued use as a herbicide.
While growers and advisers can only play with the hand they are dealt, our aim should be to utilise pesticides as efficiently as possible. This is as simple as following the label but also involves keeping up to date with new research and avoiding practices that may affect future performance. One such area is herbicide resistance.
What is herbicide resistance?
Herbicide resistance describes a weed that can survive (and produce seeds) after being treated with a rate of a herbicide that would normally kill it. For example, herbicide A normally controls weed B, but due to continued use over time, weed B adapts and can now de-toxify herbicide A and survive a robust rate.
Herbicide resistance is not poor control of weeds as a result of difficult spraying conditions or a poor choice of herbicide being applied etc. In practice, I find most complaints on poor weed control can be attributed to weather factors and/or errors in application technique. However, for a small number of weeds – namely chickweed, marigold and poppy – I find that herbicide resistance is increasingly to blame for control failures.
Some confusion can also arise from the weed charts written on herbicide labels. Manufacturers use the term ‘resistant’ on their product labels to guide the user as to which weeds are not controlled by a herbicide. For example, DuPont deems cleavers to be resistant to Ally Max and does not claim control on the label. It is naturally resistant rather than having developed resistance.
How does resistance occur?
This is a simple question but without a simple answer. However, this knowledge is crucial to help prevent herbicide resistance becoming unmanageable. Scientists are very good at confirming the presence of resistance and explaining the genetic and biochemical reasons as to why a weed can survive a herbicide spray. However, there is no consensus as to how this resistance occurs.
While being careful not to attempt to over-simplify the issue, resistance generally occurs in one of two ways.
1. A very small number of mutant weeds (single gene alterations) exists in the population, either before or after a herbicide is introduced. When the herbicide is applied over a period of time the mutant weeds can survive and reproduce, thus increasing their proportion in the population.
2. Weeds (multiple gene alterations) slowly develop partial resistance over many generations. This is less dramatic in the field and harder to confirm in experiments.
In either case, there is no convincing evidence to suggest that herbicides cause such mutations but rather select out the resistant weeds over time. Despite this, resistance is not inevitable and it is probable that resistant genes simply do not exist in certain weed species. Irrespective of how resistance occurs, the more you control weeds with herbicides, the more you select and the more likely you will find resistant weeds in your fields.
How can I spot resistance?
In my experience, growers or advisers will not spot herbicide-resistant weeds at low levels in a field. It’s only when the weed becomes noticeable that any action is taken. One of the best times to spot a low-level problem is during the harvest – from the combine cab. If you see some weeds that were poorly controlled, or are occurring in patches (not coinciding with a sprayer run), it is time to be suspicious and talk to your adviser.
Best practice guidelines to spot herbicide resistance are as follows:
A gradual decline in control level over several years.Healthy plants beside dead plants of the same species.Poor weed control leading to discrete weed patches.Poor control of one susceptible species when other susceptible species are well controlled.How common is herbicide
resistance worldwide?
There is no absolute answer to that question because it is not a static phenomenon. However, data on the subject is being continuously updated and Dr Ian Heap, a weed researcher in the US, edits an excellent website (www.weedscience.org) on the subject. According to this source, there are currently 249 weeds (144 broad-leaved and 105 grass weeds) that have developed resistance to 160 different herbicides worldwide.
Most of the scientifically proven resistant weed cases have occurred in the US, Canada and Australia, primarily in wheat, maize and cotton. Spain, France and Italy have the most reported cases of herbicide resistance in the EU, but these are mainly in horticultural crops.
ALS inhibitor herbicides
In Ireland, weed control in spring cereals, and a large area of winter cereals, is based on a family of herbicides known as ALS herbicides. Common names from this family include Ally Max, Cameo Max, Pacifica, Boxer etc. This family of herbicides has been very successful worldwide but it is also the grouping with the most reported cases of resistant weeds (see Figure 1). Luckily many of the ALS-resistant weeds do not grow in Ireland but this should serve as a warning that these herbicides must be protected to ensure they remain effective for a long time to come.
Normally, we would expect that if a weed develops resistance to one ALS herbicide (eg Ally), then it would be resistant to other ALS herbicides (eg Pacifica, Boxer etc). However, detailed genetic studies in the UK have identified that there are differences between the ALS-resistant weeds and how they are controlled by ALS herbicides.
Initial Teagasc studies have also found this to be the case among Irish samples of chickweed but in practical terms you still need non-ALS herbicides for reliable control as you cannot tell (by looking) what genetic mutation has occurred in your fields.
Do we have herbicide
resistance in Ireland?
Yes. Teagasc, DuPont and Dow have confirmed resistance to ALS herbicides (Ally etc) in common chickweed, corn marigold and common poppy. This work has also shown that there are differences between some ALS herbicides in their resistance status.
In an effort to establish the extent of ALS-resistant chickweed, Teagasc conducted a survey of 20 cereal fields from the main tillage areas in 2010. Chickweed collected from 16 of the farms exhibited some reduced level of control with Ally, with seven farms meeting the criteria of having resistant types. Two other chickweed samples came from grassland fields and these were fully controlled by Ally, even at reduced rates.
All of the fields with reduced sensitivity to Ally had been in tillage for greater than 10 years and the farmers had being using ALS herbicides each year, albeit in conjunction with other non-ALS herbicides.
The full results from this screening test are shown in Figure 2. For this work the weight of 10 sprayed chickweed plants was divided by the weight of 10 unsprayed chickweed plants for each sample source – the higher the bar in the chart, the more growth occurred after the herbicide was applied, indicating resistance. Importantly, all populations were fully controlled by CMPP (at full rate).
Other tests (not shown) confirmed that Fluroxypyr, IPU and Calaris gave good control of chickweed, irrespective of whether or not it was resistant to Ally. Remember that these tests were carried out in a glasshouse where temperatures were higher and spraying conditions were better than in most field situations.
How can I control
resistant weeds?
Selection pressure is probably the most important factor determining the evolution of herbicide resistance. Coming back to an earlier point, the more often we use the same herbicide, the more likely we will find resistant weeds. The weather also plays a part here.
In the Teagasc chickweed screen, resistance developed even though alternative herbicides were used. However, the alternative was used at low rates and possibly in sub-optimal conditions. None of the farmers I collected the chickweed from would dare use a half pint of CMPP in cold weather to control chickweed. But that is what they added to Ally over many years.
1. Observe and record unusual weeds: This is the starting point for control, but is mostly overlooked as a control option. A keen eye from the combine cab and a quick phone call to a merchant/adviser will prevent a lot of future problems. I often say to farmers who have developed weed problems that if the two of us could wind the clock back a few years, we would prevent the problem from escalating. The sayings “a stitch in time saves nine” and “one year’s seeding is seven year’s weeding” should be the mottos for dealing with herbicide-resistant weeds.
2. Use alternative herbicides: For the majority of tillage farmers, an effective herbicide is still the preferred solution. So, taking the three ALS-resistant weeds, what are the solutions? From the Teagasc glasshouse studies we see no cross-resistance with other herbicide groups, and CMPP and fluroxypyr are still fully effective against chickweed.
Tank-mixing alternative chemistry with ALS herbicides is an anti-resistance strategy, but ensure the alternative is used correctly as it is now doing most of the heavy lifting (see table below). Re-assess weed control two to four weeks after spraying.
$ALS herbicides include: Ally Max, Eagle, and Pacifica etc.
*Florasulam (in Spitfire) is an ALS but is not affected by dominant ALS mutation in the Irish chickweed population.
3. Use cultural controls: This is using your knowledge of the weed and how it grows to reduce its numbers and vigour. For example, corn marigold is acid loving and has been reduced in experiments from 148 plants/m2 to one plant/m2 by using a combination of liming, fallowing and herbicides. Poppy seed production is halved as winter wheat density is doubled up to 200 crop plants per m2.
Having a competitive crop is fundamental to reducing herbicide selection pressure so seedbed preparation, reducing compaction, avoiding weed contamination of seeds and machinery, seed rate, and fertiliser management all contribute to this. Oats is very competitive against most weeds and reduces weed growth significantly, again reducing the pressure on the herbicide to perform.
4. Crop rotation: Having a reasonable crop rotation in a field enables you to use alternative herbicides and cultural controls together. Residual herbicides used in oilseed rape, beans, maize and beet are not safe to use in cereals. Therefore, the target weed will not be exposed to them if you only grow continuous spring barley. Alternating between an early-drilled autumn crop (eg oilseed rape) and a late-drilled spring crop (eg maize) will also change the dominant weeds that germinate in a field as opposed to drilling the last week of March each year.
What weeds will become
resistant in the future?
UK research found that weeds that produce high numbers of seeds which can reproduce more than once per season pose the highest risk of herbicide resistance.
If herbicide resistance has already been found in a similar situation (crop, climate, etc) this also should act as a warning.
The most important chemical factors include the herbicide mode of action, mode of use (ie alone or in mixtures and sequences or multiple applications in a season), its intrinsic activity and its residual activity. Cultural control continues to have a major role to play in herbicide resistance management strategies.
Table 1 contains a summary of some Irish tillage weeds and whether herbicide resistance has been confirmed in these in other countries.
While herbicide resistance has not been a dominant factor in Ireland the recent confirmation of a number of specific weeds should serve as a warning that resistance can happen here and that it can get much worse. But it is manageable and the key factor is to identify an incidence early and then take appropriate action. Chemicals always need an alternative complementary method of control and this can either be a separate herbicide family or the help of a range of cultural control techniques.
The best advice that can be given is to try and prevent resistance from occurring.
Groundsel
Groundsel is a common weed in winter cereals as it can escape the traditional autumn herbicides. Excellent control of groundsel is achieved in the spring with ALS herbicides such as Cameo Max etc. However, ALS-resistant groundsel has recently been confirmed in wheat fields in France. To prevent/delay this problem occurring here, growers and advisers need to mix alternative herbicides with ALS when tidying up weed control in the spring and consider appropriate cultural options. The French research can be found in Pest Management Science 2015.
To read the full Crop Protection Focus Supplement, click here.
This is a subscriber-only article
SHARING OPTIONS: