COVID-19 looms heavily in all media at present. While the epidemic of herbicide resistance may not garner as much attention, there are lessons for crop producers from the public health problems we are currently facing. Coronavirus appears to have been carried from heavily affected areas, such as Wuhan in China and then from northern Italy, by people travelling to and from those areas.
Having returned from severely affected areas, those people are responsible for community transmission by unknowingly infecting others to cause an outbreak in their own countries.
Community transmission is an interesting terminology in which to consider the problem that Ireland and other countries face with the issue of herbicide resistance.
The two commonly used wild oat herbicides, Foxtrot and Axial, have the same herbicidal mode of action – they are both ACCase inhibitors. They are both widely used to control a range of grass weed species in cereal crops in Ireland, including wild oats. As many readers may be aware, resistance to both herbicides has been noted previously in research carried out by Teagasc in Oak Park.
The questions that are addressed in this article are:
How prevalent is herbicide resistance?How is herbicide resistance spreading?Continuous annual use
It is widely believed that resistance only occurs when farmers spray the same herbicides in fields where the same crops are grown year after year with no rotation. While it must be stressed that this type of crop management should be avoided to reduce the risk of herbicide resistance developing, it is not the only way in which resistance develops.
Take a grower who has operated in this way for many years. Eventually, he/she notices patches of weeds that are surviving herbicide application in their fields. These growers can be thought of like the first patients to contract COVID-19.
When these farmers sell their straw, or a contractor cuts their crops or bales their straw, or they lend machinery to their neighbours, it is likely that they will spread herbicide resistant weed seeds onto these neighbouring farms, just as the people travelling from northern Italy transmitted COVID-19.
All of a sudden, the initial case of herbicide resistance becomes multiple cases and we have a mini-epidemic on our hands. More and more, this spread of resistance by physical transfer is being thought of as the main way in which herbicide resistance spreads throughout the countryside. So it is increasingly important that farmers, researchers and advisers factor this into their work and practice.
How much resistance?
In order to determine how much resistance is prevalent in Irish wild oat populations, researchers in Teagasc sampled weeds from 102 fields throughout Co Wexford in the summer of 2017. Wexford was chosen because: It is one of the most important grain-growing counties in the country.It produces large amounts of spring barley so spring-germinating grass weeds such as wild oats are abundant there.We had found some resistant populations there in a previous study.The researchers looked at each population’s response to the field rate of Axial and Foxtrot. They found that over half of all the populations tested had greater than 20% survival when treated with Foxtrot, indicating that the majority of wild oat populations included in this study appeared to be developing or had symptoms of herbicide resistance.
Fewer of the populations examined appeared to have developed resistance to Axial, but any population that displayed Axial resistance was also resistant to Foxtrot.
Patterns in occurrence
Of perhaps greater interest is the fact that the researchers looked to see, if possible, where these resistant populations had come from. Ultimately, they were looking for possible patterns in where they were located in Co Wexford.
They found that resistant populations appeared to be grouped relatively closer to one another to form a cluster of cases of herbicide resistance in a relatively small area, very like what we are now seeing with coronavirus. This suggests that herbicide resistance started from one or more initial resistant population and this was then being spread through the movement of seed in a local area by some means.
Further to this, the researchers scored each field for the severity of the infestation of wild oats. They found that the more weeds that were in a field, the more likely it was that there were resistant weeds also present in that field. This can be looked at in two ways:
The bigger the infestation density in a patch, the more weeds there are in that area. And the more weeds that are present, the higher the chance that one of them will be carrying a mutation that allows it to survive the herbicide. Think of herbicide resistance as the weed’s lottery ticket. More weeds in the patch equals more chances of developing herbicide resistance. The same is true for everything in nature.Resistant weeds spread naturally in the area in which they develop as the seeds shed from a surviving resistant plant will produce new plants that are resistant. So when these are sprayed next time they are resistant too and they survive to produce more seeds. Therefore, the patch of resistant weeds grows. What can growers do?
Again, if we look at the spread of coronavirus, the key message is to focus on hygiene and stop the spread. Growers too can heed this same advice. Despite the pressure that growers are under at harvest time, it is important to regularly blow down machinery between fields, jobs or farms.
Weed spread may be also caused by the movement of farmyard manure, seed stocks or straw. People who suspect they may have COVID-19 have been told to get tested and “self-isolate”. Farmers can also learn from this. If you know you have a difficult patch of weeds, get them tested for herbicide resistance and, in the meantime, spot-spray the patch while it is small. Or better still, hand-rogue the weeds and carefully dispose of them.
Herbicide resistance, like any epidemic, will require everyone working together to overcome the problem. Like the current viral outbreak, we would advise against hysteria, and advise farmers to stick to the best principles of integrated weed management.
Use a sound crop rotation strategy. Include winter- and spring-sown crops regularly in a given field. Avoid spraying the same herbicide (and herbicide family) on the same patch of weeds year after year. Pay great attention to spraying the corners of fields. Increase seeding rate in areas where weeds are abundant. Regular field inspection remains a critical part of good field management.
All these integrated weed management techniques will help you to control all weeds, while also acting as an effective anti-resistance strategy.
Ronan Byrne was a Walsh Fellow researcher at Teagasc, Oak Park, who did all the early work on grassweed resistance.
COVID-19 looms heavily in all media at present. While the epidemic of herbicide resistance may not garner as much attention, there are lessons for crop producers from the public health problems we are currently facing. Coronavirus appears to have been carried from heavily affected areas, such as Wuhan in China and then from northern Italy, by people travelling to and from those areas.
Having returned from severely affected areas, those people are responsible for community transmission by unknowingly infecting others to cause an outbreak in their own countries.
Community transmission is an interesting terminology in which to consider the problem that Ireland and other countries face with the issue of herbicide resistance.
The two commonly used wild oat herbicides, Foxtrot and Axial, have the same herbicidal mode of action – they are both ACCase inhibitors. They are both widely used to control a range of grass weed species in cereal crops in Ireland, including wild oats. As many readers may be aware, resistance to both herbicides has been noted previously in research carried out by Teagasc in Oak Park.
The questions that are addressed in this article are:
How prevalent is herbicide resistance?How is herbicide resistance spreading?Continuous annual use
It is widely believed that resistance only occurs when farmers spray the same herbicides in fields where the same crops are grown year after year with no rotation. While it must be stressed that this type of crop management should be avoided to reduce the risk of herbicide resistance developing, it is not the only way in which resistance develops.
Take a grower who has operated in this way for many years. Eventually, he/she notices patches of weeds that are surviving herbicide application in their fields. These growers can be thought of like the first patients to contract COVID-19.
When these farmers sell their straw, or a contractor cuts their crops or bales their straw, or they lend machinery to their neighbours, it is likely that they will spread herbicide resistant weed seeds onto these neighbouring farms, just as the people travelling from northern Italy transmitted COVID-19.
All of a sudden, the initial case of herbicide resistance becomes multiple cases and we have a mini-epidemic on our hands. More and more, this spread of resistance by physical transfer is being thought of as the main way in which herbicide resistance spreads throughout the countryside. So it is increasingly important that farmers, researchers and advisers factor this into their work and practice.
How much resistance?
In order to determine how much resistance is prevalent in Irish wild oat populations, researchers in Teagasc sampled weeds from 102 fields throughout Co Wexford in the summer of 2017. Wexford was chosen because: It is one of the most important grain-growing counties in the country.It produces large amounts of spring barley so spring-germinating grass weeds such as wild oats are abundant there.We had found some resistant populations there in a previous study.The researchers looked at each population’s response to the field rate of Axial and Foxtrot. They found that over half of all the populations tested had greater than 20% survival when treated with Foxtrot, indicating that the majority of wild oat populations included in this study appeared to be developing or had symptoms of herbicide resistance.
Fewer of the populations examined appeared to have developed resistance to Axial, but any population that displayed Axial resistance was also resistant to Foxtrot.
Patterns in occurrence
Of perhaps greater interest is the fact that the researchers looked to see, if possible, where these resistant populations had come from. Ultimately, they were looking for possible patterns in where they were located in Co Wexford.
They found that resistant populations appeared to be grouped relatively closer to one another to form a cluster of cases of herbicide resistance in a relatively small area, very like what we are now seeing with coronavirus. This suggests that herbicide resistance started from one or more initial resistant population and this was then being spread through the movement of seed in a local area by some means.
Further to this, the researchers scored each field for the severity of the infestation of wild oats. They found that the more weeds that were in a field, the more likely it was that there were resistant weeds also present in that field. This can be looked at in two ways:
The bigger the infestation density in a patch, the more weeds there are in that area. And the more weeds that are present, the higher the chance that one of them will be carrying a mutation that allows it to survive the herbicide. Think of herbicide resistance as the weed’s lottery ticket. More weeds in the patch equals more chances of developing herbicide resistance. The same is true for everything in nature.Resistant weeds spread naturally in the area in which they develop as the seeds shed from a surviving resistant plant will produce new plants that are resistant. So when these are sprayed next time they are resistant too and they survive to produce more seeds. Therefore, the patch of resistant weeds grows. What can growers do?
Again, if we look at the spread of coronavirus, the key message is to focus on hygiene and stop the spread. Growers too can heed this same advice. Despite the pressure that growers are under at harvest time, it is important to regularly blow down machinery between fields, jobs or farms.
Weed spread may be also caused by the movement of farmyard manure, seed stocks or straw. People who suspect they may have COVID-19 have been told to get tested and “self-isolate”. Farmers can also learn from this. If you know you have a difficult patch of weeds, get them tested for herbicide resistance and, in the meantime, spot-spray the patch while it is small. Or better still, hand-rogue the weeds and carefully dispose of them.
Herbicide resistance, like any epidemic, will require everyone working together to overcome the problem. Like the current viral outbreak, we would advise against hysteria, and advise farmers to stick to the best principles of integrated weed management.
Use a sound crop rotation strategy. Include winter- and spring-sown crops regularly in a given field. Avoid spraying the same herbicide (and herbicide family) on the same patch of weeds year after year. Pay great attention to spraying the corners of fields. Increase seeding rate in areas where weeds are abundant. Regular field inspection remains a critical part of good field management.
All these integrated weed management techniques will help you to control all weeds, while also acting as an effective anti-resistance strategy.
Ronan Byrne was a Walsh Fellow researcher at Teagasc, Oak Park, who did all the early work on grassweed resistance.
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