At a recent herbicide meeting organised by Dow AgroSciences and Whelehan Crop Protection it was apparent that there are far more issues with herbicide resistance in Irish fields than we would want. Resistance now appears to be present in a growing range of weeds and there are concerns that some of these may be showing a level of resistance to a number of herbicide families.
For the past few decades, weed control in crops, and especially in cereals, was a relatively straightforward process. For a long time new actives and products arrived to tackle the evolving challenges facing growers.
Two unrelated points must be noted from that era. One is that the majority of new actives to arrive were acetolactate synthase (ALS) inhibitors from the sulfonylurea (SU) herbicide camp. The second was the increasing number of troublesome grassweeds that emerged during this period.
The increasing reliance of SU products, especially for spring cereals, inevitably lead to the build-up of ALS resistance in a number of weed species. Sulfonylurea actives were frequently prescribed and used on their own with no alternative mode of action to catch any off-types that were less well controlled. The consequence was inevitable – a rapid build-up of any resistant variants of that weed in that field.
Control versus resistance
We have known of the existence of control problems for many years at field level, but there is quite a gap between knowing that control of a specific weed is bad and the existence of resistance. For this reason, there is a lack of acceptance of the presence of resistance at the scientific level and we do not feature much in the international database of resistant weeds.
Formal acceptance of the existence of resistance is dependent on the harvesting of seeds from the surviving plants with the feared resistance. These seeds must then be germinated in controlled conditions and the plants sprayed with the product in question. This process must be gone through to verify the existence of resistance in a weed population. It must be tested in the offspring of surviving weeds rather than just accepting the possibility of poor initial control conditions in the crop.
ALS resistance, associated with sulfonylurea and other herbicide groups, happens as a result of the presence of the ALS enzyme in a plant. This enzyme denatures the herbicide active within the target plant (weed) and thus prevents it from being killed. The speed at which this process can happen in a weed population is uncertain, but it may be relatively slow and it may happen in only a small proportion of the weed population in a field. However, if a plant which has developed this resistance is allowed to survive, it will quickly multiply to become the dominant type of that weed in that patch or field.
Development and spread
Unlike the development of resistance in fungal diseases, resistant weeds will only develop in fields that are being treated with ALS herbicides. However, seeds from these resistant weeds are carried into other fields by birds, machinery, slurry, straw, farmyard manure, animals or even humans.
However, on balance, the expansion of a resistant population should be more predictable and measured, as should all actions to help prevent the movement of resistant seeds from field to field.
All farmers, and not just tillage farmers, need to aware of herbicide resistance. A proportion of resistant weed seeds will get the opportunity to leave the field through straw in particular. And in many instances this straw can move 160km north or west to carry a problem like resistant chickweed onto long-term grassland. And for this reason it is conceivable that problems will arise in grassland where SU herbicides are used alone.
In this country many advisers and consultants are of the opinion that ALS resistance is present in populations of chickweed (and this has been verified by Teagasc), corn marigold, and poppy. However, only chickweed has been documented as resistant in Ireland in the international database. This is not to suggest that many other weeds are not resistant – it is just a statement that it has not been formally proven and registered.
Get your weeds checked
In an effort to formalise the existence of herbicide resistance in weeds in Ireland, Dow AgroSciences has undertaken to help farmers verify the presence, or not, of resistance in different weeds in Irish fields. One of the biggest impediments to this process in the past has been the fact that seeds must be collected from the suspected resistant weeds and these must then be sent away for verification. This can be difficult to do and it is especially difficult to collect the seeds of some species.
Because of this, Dow AgroSciences has developed a technique which enables surviving plants to be taken from the treated fields and sent for verification. This whole-plant option make the process must easier and should be much more appealing to farmers and advisers.
Staff at Dow AgroSciences then propagate the plants to produce the seeds for testing. These are germinated and the resulting plants are sprayed with the appropriate chemistry for testing. So, if you think you have weeds in some of your fields that are resistant to ALS herbicides, it would be very useful if you could get samples of surviving plants to a Whelehan Crop Protection representative (Dow agents in Ireland).
If we can do this in a broad spectrum of fields in a controlled manner, we can begin to get a handle on the extent of broad-leaved weed resistance to ALS herbicides in this country. The feeling on the ground is that the problem is bigger and more widespread than is being suggested and may even include more weeds.
At this point in time, whether you have known ALS resistant weeds or not, you must assume you have resistance because the development of this resistance is only a year or two away from your fields unless you are preventing resistant weeds from expanding. The main tool to do this is to use a second or third mode of action in the herbicide mix so that any weeds that might survive the SU active in the mix can be killed by a different mode of action.
What you include with an SU will depend on the weeds that are in the field and especially those that are thought to be suspect. There are many actives that can help in this regard but hormone actives, pyridine chemistry such as clopyralid or fluroxypyr or HBN herbicides will cover a relatively broad range of weeds.
The main risk weeds will be chickweed, corn marigold and poppy, so a mix must contain at least one alternative mode of action against each of these species. This will prevent resistance development as the second mode of action should still kill any plants that escape a SU active if they have ALS resistance.
Another twist in the tale is that resistance to ALS-herbicides is predominantly due to target site resistance and the resistance mutation profile is not the same for all herbicide families within this ALS group.
The Proline197 mutation confers resistance to the SUs and only moderate or no resistance to the triazolopyrimidine herbicides such as florasulam (Spitfire).Tryptophan574 confers resistance to both the SUs and triazolopyrimidine herbicides.At present Proline197 would appear to be the more common mutation and is currently found in chickweed.
It remains important to state that resistance development is not confined to ALS resistance. There is considerable talk of reduced efficacy in other families to specific weeds.
To put it simply, any family that is continually exposed to a single family of actives will inevitable crack at some point so it is important to realise that all chemical interventions need a helping hand.
The main ways we do this is by rotation, which enables different families of actives to be used in different crops, and by the use of stubble cultivation or stale seedbeds.
Any action which helps weed seeds to germinate in a non-crop situation will help get rid of this seed from the weed bank. This could help eliminate the only two resistant weeds in your field and so help prevent a resistance problem.
At a recent herbicide meeting organised by Dow AgroSciences and Whelehan Crop Protection it was apparent that there are far more issues with herbicide resistance in Irish fields than we would want. Resistance now appears to be present in a growing range of weeds and there are concerns that some of these may be showing a level of resistance to a number of herbicide families.
For the past few decades, weed control in crops, and especially in cereals, was a relatively straightforward process. For a long time new actives and products arrived to tackle the evolving challenges facing growers.
Two unrelated points must be noted from that era. One is that the majority of new actives to arrive were acetolactate synthase (ALS) inhibitors from the sulfonylurea (SU) herbicide camp. The second was the increasing number of troublesome grassweeds that emerged during this period.
The increasing reliance of SU products, especially for spring cereals, inevitably lead to the build-up of ALS resistance in a number of weed species. Sulfonylurea actives were frequently prescribed and used on their own with no alternative mode of action to catch any off-types that were less well controlled. The consequence was inevitable – a rapid build-up of any resistant variants of that weed in that field.
Control versus resistance
We have known of the existence of control problems for many years at field level, but there is quite a gap between knowing that control of a specific weed is bad and the existence of resistance. For this reason, there is a lack of acceptance of the presence of resistance at the scientific level and we do not feature much in the international database of resistant weeds.
Formal acceptance of the existence of resistance is dependent on the harvesting of seeds from the surviving plants with the feared resistance. These seeds must then be germinated in controlled conditions and the plants sprayed with the product in question. This process must be gone through to verify the existence of resistance in a weed population. It must be tested in the offspring of surviving weeds rather than just accepting the possibility of poor initial control conditions in the crop.
ALS resistance, associated with sulfonylurea and other herbicide groups, happens as a result of the presence of the ALS enzyme in a plant. This enzyme denatures the herbicide active within the target plant (weed) and thus prevents it from being killed. The speed at which this process can happen in a weed population is uncertain, but it may be relatively slow and it may happen in only a small proportion of the weed population in a field. However, if a plant which has developed this resistance is allowed to survive, it will quickly multiply to become the dominant type of that weed in that patch or field.
Development and spread
Unlike the development of resistance in fungal diseases, resistant weeds will only develop in fields that are being treated with ALS herbicides. However, seeds from these resistant weeds are carried into other fields by birds, machinery, slurry, straw, farmyard manure, animals or even humans.
However, on balance, the expansion of a resistant population should be more predictable and measured, as should all actions to help prevent the movement of resistant seeds from field to field.
All farmers, and not just tillage farmers, need to aware of herbicide resistance. A proportion of resistant weed seeds will get the opportunity to leave the field through straw in particular. And in many instances this straw can move 160km north or west to carry a problem like resistant chickweed onto long-term grassland. And for this reason it is conceivable that problems will arise in grassland where SU herbicides are used alone.
In this country many advisers and consultants are of the opinion that ALS resistance is present in populations of chickweed (and this has been verified by Teagasc), corn marigold, and poppy. However, only chickweed has been documented as resistant in Ireland in the international database. This is not to suggest that many other weeds are not resistant – it is just a statement that it has not been formally proven and registered.
Get your weeds checked
In an effort to formalise the existence of herbicide resistance in weeds in Ireland, Dow AgroSciences has undertaken to help farmers verify the presence, or not, of resistance in different weeds in Irish fields. One of the biggest impediments to this process in the past has been the fact that seeds must be collected from the suspected resistant weeds and these must then be sent away for verification. This can be difficult to do and it is especially difficult to collect the seeds of some species.
Because of this, Dow AgroSciences has developed a technique which enables surviving plants to be taken from the treated fields and sent for verification. This whole-plant option make the process must easier and should be much more appealing to farmers and advisers.
Staff at Dow AgroSciences then propagate the plants to produce the seeds for testing. These are germinated and the resulting plants are sprayed with the appropriate chemistry for testing. So, if you think you have weeds in some of your fields that are resistant to ALS herbicides, it would be very useful if you could get samples of surviving plants to a Whelehan Crop Protection representative (Dow agents in Ireland).
If we can do this in a broad spectrum of fields in a controlled manner, we can begin to get a handle on the extent of broad-leaved weed resistance to ALS herbicides in this country. The feeling on the ground is that the problem is bigger and more widespread than is being suggested and may even include more weeds.
At this point in time, whether you have known ALS resistant weeds or not, you must assume you have resistance because the development of this resistance is only a year or two away from your fields unless you are preventing resistant weeds from expanding. The main tool to do this is to use a second or third mode of action in the herbicide mix so that any weeds that might survive the SU active in the mix can be killed by a different mode of action.
What you include with an SU will depend on the weeds that are in the field and especially those that are thought to be suspect. There are many actives that can help in this regard but hormone actives, pyridine chemistry such as clopyralid or fluroxypyr or HBN herbicides will cover a relatively broad range of weeds.
The main risk weeds will be chickweed, corn marigold and poppy, so a mix must contain at least one alternative mode of action against each of these species. This will prevent resistance development as the second mode of action should still kill any plants that escape a SU active if they have ALS resistance.
Another twist in the tale is that resistance to ALS-herbicides is predominantly due to target site resistance and the resistance mutation profile is not the same for all herbicide families within this ALS group.
The Proline197 mutation confers resistance to the SUs and only moderate or no resistance to the triazolopyrimidine herbicides such as florasulam (Spitfire).Tryptophan574 confers resistance to both the SUs and triazolopyrimidine herbicides.At present Proline197 would appear to be the more common mutation and is currently found in chickweed.
It remains important to state that resistance development is not confined to ALS resistance. There is considerable talk of reduced efficacy in other families to specific weeds.
To put it simply, any family that is continually exposed to a single family of actives will inevitable crack at some point so it is important to realise that all chemical interventions need a helping hand.
The main ways we do this is by rotation, which enables different families of actives to be used in different crops, and by the use of stubble cultivation or stale seedbeds.
Any action which helps weed seeds to germinate in a non-crop situation will help get rid of this seed from the weed bank. This could help eliminate the only two resistant weeds in your field and so help prevent a resistance problem.
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