If you feel caught because you cannot grow rape in a specific field due to brassica weed infection, you now have a control option to consider. The introduction of commercial Clearfield technology provides varieties that are resistant to herbicides, which can kill problem brassicas like charlock but not damage these specific varieties.
Clearfield is a form of herbicide tolerance bred into rape through standard conventional plant breeding, so it is not a GM technology. This technology is being developed since the 1980s, initially by Cyanamid, when imazamox tolerance was first observed in sunflowers in the US.
As a result of this finding, Cyanamid began to screen other crops to see if a similar natural mutation existed in other species as had been discovered in sunflowers. The trait was found in oilseed rape and now imazamox (an ALS or SU type herbicide) can be used on these tolerant varieties to kill many difficult brassica and other weeds.
A production system
Basically, Clearfield is an oilseed rape (OSR) production system, which can be used to help control difficult problems in individual fields. The technology has been used in OSR seed production here for the past few years and became available commercially this year. It is really a completely different approach to weed control.
In its early development phase, there was a significant yield drag associated with the technology , which meant lower yield in varieties which carried the herbicide tolerant (HT) trait. This situation has now changed significantly and breeding has now virtually removed the yield drag leaving current varieties up with the best of the rest.
There are now up to 14 different varieties with this in-built technology from five breeding companies. Two of these will be sold in Ireland this autumn – these are Phoenix CL and Impression CL. All Clearfield varieties are hybrids and they carry either CL or IM in their name.
The area sown to these varieties in the UK has been growing massively in recent years with 60,000ha planted last year and an expected 120,000ha plus this autumn.
Erucic acid
One major requirement for the need for this technology hinges around the erucic acid content of its oil when it is being extracted. This is now said to have human health implications and so permissible levels are being forced down in traded product. The EU is expected to decrease its maximum limit from 5% to 2% in the near future.
Levels have been bred down close to zero in commercial varieties in recent decades but the acid has been increasing in rape oil in recent years. This is said to be due to contamination from seeds of brassica weeds such as charlock and runch. So control of these weeds has become increasingly important.
Cleranda herbicide
Using Clearfield means that you choose to use a specific herbicide called Cleranda, post emergence when weeds are still small. This contains 17.5g/l of imazamox plus 375g/l of metazachlor to apply double these amounts at the recommended rate of 2.0 l/ha. The mix has a good broad-leaved weed spectrum plus the difficult brassica weeds. It is also good on volunteer cereals, as well as bromes, wild oats and ryegrasses. The herbicide must be used with an adjuvant called Dash at 1.0 l/ha.
The process sounds simple but it is not always. It is essential that Cleranda be sprayed when the target weeds are dry. Broad-leaved weeds must be sprayed between one-to-four true leaves and grass weeds between two to three true leaves. This timing will inevitably occur during a busy planting time and effectiveness will be reduced if application is late. At least two hours drying is required post application to ensure uptake. Good spray coverage is also essential.
So Clearfield is not a simple solution but it is a real solution to an otherwise big challenge where brassica weeds are bad. It may not be needed in every field but its use should be confined to whole fields because the Cleranda spray will kill standard rape if there is over-spray, drift or contamination.
Volunteers
One of the big issues for HT technology is the potential for a new problem from volunteer plants. Seeds are viable and they can persist for years following the crop. And the presence of the tolerance gene means that volunteers will not be killed by ALS or SU herbicides. For this reason, herbicide mixtures containing hormonal or contact actives should be used in crops following a Clearfield crop.
While Clearfield volunteers will confer tolerance to imazamox, any out-crossing with wild species will result in variable tolerance to imazamox because they will be produced from Clearfield and non-Clearfield parents. Such a resultant population will not have complete tolerance. It must also be noted that imazamox tolerance genes are different to those, which confer ALS resistance.
Spray timing
The challenge with timing is to have the target weeds emerged whilst still not having a big weed canopy so the herbicide can still hit the ground. This is important because the imazamox is largely contact in its activity and this hits the problem brassicas, while the methazachlor is almost exclusively residual and so must be put on the soil. For this reason it is essential that weeds cannot be too big, as stated previously.
Imazamox kills by inhibiting amino acid production at the growing points. It is taken to the growing points along with sugars and other nutrients in the xylem during growth. Small actively growing weeds move most sugars and so most imazamox to the growing points. Larger weeds take longer to die and have greater resources to fuel regrowth.
Charlock infested patches in rape flower much earlier in spring and their seeds help to raise the erucic acid content of oilseed rape seed.
Early germinating weeds such as chickweed, mayweed, poppy, cranesbill etc get big very quickly and can be difficult to kill with delayed treatments. Brassicas like runch develop a large tap root quickly, as well as a big canopy, and so it is best to spray at the 2-3 leaf stage.
The herbicide is strong on cruciferous weeds like charlock, runch, hedge mustard and white mustard. It also has good activity on other crucifers but the maximum growth stage for control can differ (max 4-leaves for shepherd’s purse). It is also strong on dead nettles, fumitory, fat hen, pale persicaria, redshank, speedwells, volunteer cereals, wild oats, brome and ryegrass.
It is not good against field pansy and annual meadowgrass and is only moderate against mayweeds, groundsel, cleavers, cranesbill and poppy.
- Clearfield is a new technology using herbicide tolerance which enables the application of specific herbicide actives which are capable of killing troublesome brassica weeds in the crop.
- The associated herbicide called Cleranda will kill standard oilseed rape varieties.
- Application timing for the herbicide and the weather at spraying are very important.
If you feel caught because you cannot grow rape in a specific field due to brassica weed infection, you now have a control option to consider. The introduction of commercial Clearfield technology provides varieties that are resistant to herbicides, which can kill problem brassicas like charlock but not damage these specific varieties.
Clearfield is a form of herbicide tolerance bred into rape through standard conventional plant breeding, so it is not a GM technology. This technology is being developed since the 1980s, initially by Cyanamid, when imazamox tolerance was first observed in sunflowers in the US.
As a result of this finding, Cyanamid began to screen other crops to see if a similar natural mutation existed in other species as had been discovered in sunflowers. The trait was found in oilseed rape and now imazamox (an ALS or SU type herbicide) can be used on these tolerant varieties to kill many difficult brassica and other weeds.
A production system
Basically, Clearfield is an oilseed rape (OSR) production system, which can be used to help control difficult problems in individual fields. The technology has been used in OSR seed production here for the past few years and became available commercially this year. It is really a completely different approach to weed control.
In its early development phase, there was a significant yield drag associated with the technology , which meant lower yield in varieties which carried the herbicide tolerant (HT) trait. This situation has now changed significantly and breeding has now virtually removed the yield drag leaving current varieties up with the best of the rest.
There are now up to 14 different varieties with this in-built technology from five breeding companies. Two of these will be sold in Ireland this autumn – these are Phoenix CL and Impression CL. All Clearfield varieties are hybrids and they carry either CL or IM in their name.
The area sown to these varieties in the UK has been growing massively in recent years with 60,000ha planted last year and an expected 120,000ha plus this autumn.
Erucic acid
One major requirement for the need for this technology hinges around the erucic acid content of its oil when it is being extracted. This is now said to have human health implications and so permissible levels are being forced down in traded product. The EU is expected to decrease its maximum limit from 5% to 2% in the near future.
Levels have been bred down close to zero in commercial varieties in recent decades but the acid has been increasing in rape oil in recent years. This is said to be due to contamination from seeds of brassica weeds such as charlock and runch. So control of these weeds has become increasingly important.
Cleranda herbicide
Using Clearfield means that you choose to use a specific herbicide called Cleranda, post emergence when weeds are still small. This contains 17.5g/l of imazamox plus 375g/l of metazachlor to apply double these amounts at the recommended rate of 2.0 l/ha. The mix has a good broad-leaved weed spectrum plus the difficult brassica weeds. It is also good on volunteer cereals, as well as bromes, wild oats and ryegrasses. The herbicide must be used with an adjuvant called Dash at 1.0 l/ha.
The process sounds simple but it is not always. It is essential that Cleranda be sprayed when the target weeds are dry. Broad-leaved weeds must be sprayed between one-to-four true leaves and grass weeds between two to three true leaves. This timing will inevitably occur during a busy planting time and effectiveness will be reduced if application is late. At least two hours drying is required post application to ensure uptake. Good spray coverage is also essential.
So Clearfield is not a simple solution but it is a real solution to an otherwise big challenge where brassica weeds are bad. It may not be needed in every field but its use should be confined to whole fields because the Cleranda spray will kill standard rape if there is over-spray, drift or contamination.
Volunteers
One of the big issues for HT technology is the potential for a new problem from volunteer plants. Seeds are viable and they can persist for years following the crop. And the presence of the tolerance gene means that volunteers will not be killed by ALS or SU herbicides. For this reason, herbicide mixtures containing hormonal or contact actives should be used in crops following a Clearfield crop.
While Clearfield volunteers will confer tolerance to imazamox, any out-crossing with wild species will result in variable tolerance to imazamox because they will be produced from Clearfield and non-Clearfield parents. Such a resultant population will not have complete tolerance. It must also be noted that imazamox tolerance genes are different to those, which confer ALS resistance.
Spray timing
The challenge with timing is to have the target weeds emerged whilst still not having a big weed canopy so the herbicide can still hit the ground. This is important because the imazamox is largely contact in its activity and this hits the problem brassicas, while the methazachlor is almost exclusively residual and so must be put on the soil. For this reason it is essential that weeds cannot be too big, as stated previously.
Imazamox kills by inhibiting amino acid production at the growing points. It is taken to the growing points along with sugars and other nutrients in the xylem during growth. Small actively growing weeds move most sugars and so most imazamox to the growing points. Larger weeds take longer to die and have greater resources to fuel regrowth.
Charlock infested patches in rape flower much earlier in spring and their seeds help to raise the erucic acid content of oilseed rape seed.
Early germinating weeds such as chickweed, mayweed, poppy, cranesbill etc get big very quickly and can be difficult to kill with delayed treatments. Brassicas like runch develop a large tap root quickly, as well as a big canopy, and so it is best to spray at the 2-3 leaf stage.
The herbicide is strong on cruciferous weeds like charlock, runch, hedge mustard and white mustard. It also has good activity on other crucifers but the maximum growth stage for control can differ (max 4-leaves for shepherd’s purse). It is also strong on dead nettles, fumitory, fat hen, pale persicaria, redshank, speedwells, volunteer cereals, wild oats, brome and ryegrass.
It is not good against field pansy and annual meadowgrass and is only moderate against mayweeds, groundsel, cleavers, cranesbill and poppy.
- Clearfield is a new technology using herbicide tolerance which enables the application of specific herbicide actives which are capable of killing troublesome brassica weeds in the crop.
- The associated herbicide called Cleranda will kill standard oilseed rape varieties.
- Application timing for the herbicide and the weather at spraying are very important.
This is a subscriber-only article
SHARING OPTIONS: