Earlier this year, I was chatting to a farmer in Carlow and the subject of conacre came up. I resisted the temptation to enquire about the rental cost but I did enquire about the status of the soil and other agronomic questions.
The farmer was forthcoming with the information as he pointed to the good health of the soil (although he had no soil sample results) and that there were no other problems, as far as he knew.
As I had a little local knowledge of the farm and the general area, I asked if he had a plan for the weeds on the farm.
“No problem” was the immediate reply, “sure they must be the usual spring-type weeds,” he added.
I enquired about his knowledge of ALS-resistant corn marigold and his experience to date with the problem. I asked this because I suspected his newly acquired land would have a resistant population, just like many neighbouring farms in that area. As expected, this prompted further discussion on the topic.
Spring weed control in cereals is one of the jobs that is taken for granted on many farms. Once the herbicide is applied, most assume that all will be well.
Picking a specific herbicide product for an individual field is often an efficiency compromise for getting the job done on the entire crop.
To a degree, this is entirely understandable but this type of approach will select out certain weeds and can leave a more difficult weed spectrum over time.
There are lots of examples of this happening in Irish fields throughout the country.
Weeds like fumitory, knotgrass, bindweed, groundsel, poppy, etc., can build up in fields following years of use of a product with the same broad weed selection.
In time, this means weed control problems, but this is mainly due to inability to control rather than resistance to herbicides (see ALS or SU herbicide resistance comments later in the article). Frequently, the weeds that build up in numbers are ones that the herbicide could never control or ones that the active had a poor control rating against.
Poor control of these weeds can be amplified when factors such as spraying conditions, growing conditions and weed size are not accounted for in the rate of product used and these factors could equally increase or decrease control efficiency depending on whether they are favourable or not.
Spring weed control trials
At this stage, it is pertinent to review some research work to refresh people’s memories about the rates and timings which can be used to secure good weed control.
Research work carried out by Teagasc in the early 1990s tested herbicides at lower rates than normal and monitored the resulting yields, quality parameters and the weed control.
In one trial, Dr Brian Mitchell tested Cameo (at full, half and quarter rates) plus Duplosan 0.5 litres/ha applied at GS30/31.
The results showed that all treatments gave good weed control (90% to 100%) and yields were similar across all treatments.
At that time, the quarter-rate Cameo had a similar yield and weed control score to the higher Cameo rates. However, knotgrass and bindweed survived where the quarter-rate treatments were used but these weeds were uncompetitive and did not affect yield.
Although it was not reported by the researcher, in both trials the half-rate Cameo yielded higher than the full rate Cameo (although not significantly).
I suspect environmental conditions and crop growth at the time of application may well have been factors in these findings, as we experienced similar findings in recent trials.
The crop may have been under some stress at the time of application and the full rate affected the crop (and the weeds) at the time of application and this knocked a little off yields.
Parts of these trials were re-examined during 2009 to 2011 through work in the Teagasc BETTER Farm programme. The trials looked at weed control with reduced rates of herbicides. They also tested whether the addition of a wild oat herbicide, Pinoxaden (plus Adigor), to the herbicide mix would influence the control of broad-leafed weeds.
Ally Max plus Galaxy was the herbicide mixture used and this was applied at full, half, and quarter rates. One set of treatments included the wild oat herbicide (Axial at 83% of label rate plus full rate Adigor) while the other did not. The herbicide treatments were applied at either an early timing (four-leaf to early tillering) or a late timing (before GS30).
So these trials allowed us to compare different rates of herbicide, at different timings, with or without a wild oat herbicide. We also included two treatments targeting annual meadow grass control (IPU or Hussar), which were both applied at the early timing. Full details of the individual treatments and timing are shown in Table 1.
The trials were carried out at Teagasc Research Centre in Oak Park and at the BETTER Farm in Wexford. The Oak Park trial was sown in mid-March with excellent crop establishment but high weed pressure (see Figure 1).
The BETTER Farm trial (Wexford) was sown in mid-April and, while the crop emerged well, a dry spell after emergence slowed crop growth. This trial site had a very low weed challenge, with less than 10 weeds/m2 present.
As seen in Figure 1, Oak Park had a very high weed density with an average of over 75 weeds/m2. Poppy, charlock, speedwell, chickweed and knotgrass were the most prevalent weeds in this trial.
At the Oak Park site, the untreated plot yields averaged 5.28t/ha compared to the early herbicide timing treatments, which yielded 7.26t/ha, and the later timing treatments yielded 6.37t/ha. The LSD (5%) in this trial was 0.3t/ha so the yield differences were statistically significant.
In the BETTER Farm trial, weed control could be considered to be commercially acceptable from all treatments. However, in the Oak Park trial, reduced weed control was observed where quarter rates of herbicide were applied at the later timing. This is a common observation where weed control becomes more challenging as weeds get bigger.
The trial results in Figure 2 show a substantial difference between the herbicide treatments and the sites. In the Oak Park site, where the weed challenge was high, an average yield response of 1.27t/ha was recorded at the early timings compared with 0.47t/ha from the later timing.
Why did this happen? Weeds that were left uncontrolled (until GS30) competed with the barley and reduced tiller numbers, which resulted in less shoots and eventually fewer grains produced per square metre.
At the BETTER Farm site, the weed challenge was low and the yield benefit from the herbicide treatments was less clear. The early application timings gave an average yield benefit of 0.01t/ha relative to the untreated but the late applications reduced yields by, on average, 0.07t/ha.
It should be noted this Wexford crop suffered due to the later planting date and the subsequent dry conditions. At this site, many of the treatments resulted in visible additional stress to the treated plots.
Although the control of weeds in this trial did not show financial benefit, it must be remembered that part of the purpose of weed control is to prevent seed return to the soil so as to help prevent long-term problems also.
These results proved similar to those from previous trials, where full herbicide rates reduced yield (in the BETTER Farm) compared to the half or quarter rates in the early timings due to the detrimental effect of the herbicide rate on a stressed crop.
A significantly lower yield than the untreated plots was also observed in the Ally Max + Galaxy + IPU and the Hussar + Galaxy treatments. Again, this is a consequence of the crop being under stress at the time of the herbicide application.
The addition of a wild oat herbicide did not affect the broad-leaved weed control or yields with the early or late application timings. There was little or no wild oats at the trial sites and so actual control of wild oats could not be assessed.
The messages from these trials are:
Apply herbicides early (GS14-21) rather than late (GS30 or later) for best results.Applying a reduced rate of herbicide early gave a higher yield than a full rate (or any rate) at a late timing. A higher return of spend can be expected where a higher weed burden is present.Avoid applying herbicides to crops under stress. Ideally, wait for crops to recover and aim to apply the herbicide after three good growth days.Consider including a wild oat herbicide with broad-leaf herbicides.Controlling weeds in your spring cereals
Choice of herbicide or herbicide combinations is obviously important for the successful control of weeds. But the following points are also very important for the successful control of the target weeds.
Crop densityHerbicides have different strengths and weaknesses for the control of specific weeds and products give ratings on the label as to their ability to control specific weeds.
Labels frequently show the weeds a product will control well, only partially control or have no effect on. This needs to be used to guide on the best product choice for a specific weed spectrum. Where the herbicide only manages to partially control the weed (to the point where it is stunted and hardly growing), the crop itself then becomes part of the control process if it grows to compete with and smother out the weed.
Heavier crop densities help to reduce the light and nutrient availability to the recovering weeds and this helps to subdue them. So aim to produce a dense crop as a thin cereal crop is less competitive and more likely to let poorly controlled weeds recover.
Weather conditionsPlants increase the intensity of their waxy leaf layer as part of their defence mechanism when under stress. This can happen when the weather is harsh, but the defensive wax layer reduces again when the plant is actively growing.
Herbicides work more effectively when the waxy layer is reduced as the active ingredient(s) can penetrate into the plant cells more easily.
Ideally, apply herbicides after three growth days and, if possible, where good growth conditions are likely for the following two to three days post-application. This weather pattern will help to create the best conditions for the herbicide to penetrate the weed and have the desired effect on weeds. In such conditions, the herbicide will have minimal effect on the cereal plant.
Other issuesOther specific issues, such as droplet size, water type, etc., can also affect the weed control outcome.
Herbicide selection for
broad-leaved weeds
Many growers are excellent at recording field management history but few are really good at mapping out the most common weeds in a field. A note of the weeds present should be kept when walking the field in spring before herbicide is applied, with a follow-up comment in the run up to harvest.
Arguably, the weeds still actively growing before harvest are the most worrying and these should be to the forefront of the herbicide selection in the following spring.
Once the weed spectrum is established, choosing the herbicide should be relatively straightforward.
All herbicides carry a weed guide as part of their label, which can help in selection, but generally this list is limited.
Additional information can be obtained from your local adviser. Table 2 outlines some of the major herbicides available for spring cereals and the susceptibility of the weeds.
Careful selection of a combination of herbicide actives is vital to achieve coverage of the majority of broad-leaved weeds. For the majority of crops, a combination of sulfonylurea and hormone-based herbicides should be used.
Annual meadow grass
For many growers, annual meadow grass (AMG) remains a major problem in spring cereals. Heavy land (moisture retentive), combined with wet springs/summers and thin crops, can all increase AMG plant numbers and these can negatively affect crop yields.
Hussar will control AMG up to GS30, which is a small AMG plant. IPU – Fieldguard (PCS 02458), Arelon (PCS 02457) and Croplink IPU (PCS 04498) – was cleared for use on spring barley last year.
When applied to the crop early (target the AMG @ 2-3 leaf stage), IPU at 1.0 litre/ha can give very effective control with some persistence and aid subsequent broad leaf control.
If using IPU on spring barley, the crop itself needs to be growing strongly, and avoid using it if heavy rain is forecast in the days following application. This is because IPU can wash down to the roots and cause damage to the crop.
ALS-resistant weeds
A herbicide-resistant weed is a weed which can survive (and produce seeds) following application of an appropriate rate of herbicide that would normally have killed it.
The main herbicide-resistance type we encounter is known as ALS resistance. ALS herbicides target the enzyme acetolactate synthase (ALS) in the weed plants and, where a change in this enzyme in a weed becomes resistant to the herbicide, the plant is no longer killed.
ALS inhibitor weeds are an increasing problem over the past number of years. ALS-resistant corn marigold and chickweed are the most common weeds found across the country, but ALS-resistant poppy has also been identified. These resistant weeds affect the entire family of ALS herbicides, which includes Ally, Cameo, Hussar, Pacifica, etc.
Although there are differences as to how an ALS-resistant weed will react to different ALS herbicides, practical weed control necessitates that reliable weed control requires a non-ALS herbicide to be used with the ALS.
Tim O’Donovan carried out pot experiments in recent years and found different levels of ALS resistance. But where a non-ALS herbicide was used, control was good to excellent (ruling out cross resistance).
Table 3 gives options to consider when controlling ALS-resistant weeds or the options can be used as an anti-resistant strategy.
Earlier this year, I was chatting to a farmer in Carlow and the subject of conacre came up. I resisted the temptation to enquire about the rental cost but I did enquire about the status of the soil and other agronomic questions.
The farmer was forthcoming with the information as he pointed to the good health of the soil (although he had no soil sample results) and that there were no other problems, as far as he knew.
As I had a little local knowledge of the farm and the general area, I asked if he had a plan for the weeds on the farm.
“No problem” was the immediate reply, “sure they must be the usual spring-type weeds,” he added.
I enquired about his knowledge of ALS-resistant corn marigold and his experience to date with the problem. I asked this because I suspected his newly acquired land would have a resistant population, just like many neighbouring farms in that area. As expected, this prompted further discussion on the topic.
Spring weed control in cereals is one of the jobs that is taken for granted on many farms. Once the herbicide is applied, most assume that all will be well.
Picking a specific herbicide product for an individual field is often an efficiency compromise for getting the job done on the entire crop.
To a degree, this is entirely understandable but this type of approach will select out certain weeds and can leave a more difficult weed spectrum over time.
There are lots of examples of this happening in Irish fields throughout the country.
Weeds like fumitory, knotgrass, bindweed, groundsel, poppy, etc., can build up in fields following years of use of a product with the same broad weed selection.
In time, this means weed control problems, but this is mainly due to inability to control rather than resistance to herbicides (see ALS or SU herbicide resistance comments later in the article). Frequently, the weeds that build up in numbers are ones that the herbicide could never control or ones that the active had a poor control rating against.
Poor control of these weeds can be amplified when factors such as spraying conditions, growing conditions and weed size are not accounted for in the rate of product used and these factors could equally increase or decrease control efficiency depending on whether they are favourable or not.
Spring weed control trials
At this stage, it is pertinent to review some research work to refresh people’s memories about the rates and timings which can be used to secure good weed control.
Research work carried out by Teagasc in the early 1990s tested herbicides at lower rates than normal and monitored the resulting yields, quality parameters and the weed control.
In one trial, Dr Brian Mitchell tested Cameo (at full, half and quarter rates) plus Duplosan 0.5 litres/ha applied at GS30/31.
The results showed that all treatments gave good weed control (90% to 100%) and yields were similar across all treatments.
At that time, the quarter-rate Cameo had a similar yield and weed control score to the higher Cameo rates. However, knotgrass and bindweed survived where the quarter-rate treatments were used but these weeds were uncompetitive and did not affect yield.
Although it was not reported by the researcher, in both trials the half-rate Cameo yielded higher than the full rate Cameo (although not significantly).
I suspect environmental conditions and crop growth at the time of application may well have been factors in these findings, as we experienced similar findings in recent trials.
The crop may have been under some stress at the time of application and the full rate affected the crop (and the weeds) at the time of application and this knocked a little off yields.
Parts of these trials were re-examined during 2009 to 2011 through work in the Teagasc BETTER Farm programme. The trials looked at weed control with reduced rates of herbicides. They also tested whether the addition of a wild oat herbicide, Pinoxaden (plus Adigor), to the herbicide mix would influence the control of broad-leafed weeds.
Ally Max plus Galaxy was the herbicide mixture used and this was applied at full, half, and quarter rates. One set of treatments included the wild oat herbicide (Axial at 83% of label rate plus full rate Adigor) while the other did not. The herbicide treatments were applied at either an early timing (four-leaf to early tillering) or a late timing (before GS30).
So these trials allowed us to compare different rates of herbicide, at different timings, with or without a wild oat herbicide. We also included two treatments targeting annual meadow grass control (IPU or Hussar), which were both applied at the early timing. Full details of the individual treatments and timing are shown in Table 1.
The trials were carried out at Teagasc Research Centre in Oak Park and at the BETTER Farm in Wexford. The Oak Park trial was sown in mid-March with excellent crop establishment but high weed pressure (see Figure 1).
The BETTER Farm trial (Wexford) was sown in mid-April and, while the crop emerged well, a dry spell after emergence slowed crop growth. This trial site had a very low weed challenge, with less than 10 weeds/m2 present.
As seen in Figure 1, Oak Park had a very high weed density with an average of over 75 weeds/m2. Poppy, charlock, speedwell, chickweed and knotgrass were the most prevalent weeds in this trial.
At the Oak Park site, the untreated plot yields averaged 5.28t/ha compared to the early herbicide timing treatments, which yielded 7.26t/ha, and the later timing treatments yielded 6.37t/ha. The LSD (5%) in this trial was 0.3t/ha so the yield differences were statistically significant.
In the BETTER Farm trial, weed control could be considered to be commercially acceptable from all treatments. However, in the Oak Park trial, reduced weed control was observed where quarter rates of herbicide were applied at the later timing. This is a common observation where weed control becomes more challenging as weeds get bigger.
The trial results in Figure 2 show a substantial difference between the herbicide treatments and the sites. In the Oak Park site, where the weed challenge was high, an average yield response of 1.27t/ha was recorded at the early timings compared with 0.47t/ha from the later timing.
Why did this happen? Weeds that were left uncontrolled (until GS30) competed with the barley and reduced tiller numbers, which resulted in less shoots and eventually fewer grains produced per square metre.
At the BETTER Farm site, the weed challenge was low and the yield benefit from the herbicide treatments was less clear. The early application timings gave an average yield benefit of 0.01t/ha relative to the untreated but the late applications reduced yields by, on average, 0.07t/ha.
It should be noted this Wexford crop suffered due to the later planting date and the subsequent dry conditions. At this site, many of the treatments resulted in visible additional stress to the treated plots.
Although the control of weeds in this trial did not show financial benefit, it must be remembered that part of the purpose of weed control is to prevent seed return to the soil so as to help prevent long-term problems also.
These results proved similar to those from previous trials, where full herbicide rates reduced yield (in the BETTER Farm) compared to the half or quarter rates in the early timings due to the detrimental effect of the herbicide rate on a stressed crop.
A significantly lower yield than the untreated plots was also observed in the Ally Max + Galaxy + IPU and the Hussar + Galaxy treatments. Again, this is a consequence of the crop being under stress at the time of the herbicide application.
The addition of a wild oat herbicide did not affect the broad-leaved weed control or yields with the early or late application timings. There was little or no wild oats at the trial sites and so actual control of wild oats could not be assessed.
The messages from these trials are:
Apply herbicides early (GS14-21) rather than late (GS30 or later) for best results.Applying a reduced rate of herbicide early gave a higher yield than a full rate (or any rate) at a late timing. A higher return of spend can be expected where a higher weed burden is present.Avoid applying herbicides to crops under stress. Ideally, wait for crops to recover and aim to apply the herbicide after three good growth days.Consider including a wild oat herbicide with broad-leaf herbicides.Controlling weeds in your spring cereals
Choice of herbicide or herbicide combinations is obviously important for the successful control of weeds. But the following points are also very important for the successful control of the target weeds.
Crop densityHerbicides have different strengths and weaknesses for the control of specific weeds and products give ratings on the label as to their ability to control specific weeds.
Labels frequently show the weeds a product will control well, only partially control or have no effect on. This needs to be used to guide on the best product choice for a specific weed spectrum. Where the herbicide only manages to partially control the weed (to the point where it is stunted and hardly growing), the crop itself then becomes part of the control process if it grows to compete with and smother out the weed.
Heavier crop densities help to reduce the light and nutrient availability to the recovering weeds and this helps to subdue them. So aim to produce a dense crop as a thin cereal crop is less competitive and more likely to let poorly controlled weeds recover.
Weather conditionsPlants increase the intensity of their waxy leaf layer as part of their defence mechanism when under stress. This can happen when the weather is harsh, but the defensive wax layer reduces again when the plant is actively growing.
Herbicides work more effectively when the waxy layer is reduced as the active ingredient(s) can penetrate into the plant cells more easily.
Ideally, apply herbicides after three growth days and, if possible, where good growth conditions are likely for the following two to three days post-application. This weather pattern will help to create the best conditions for the herbicide to penetrate the weed and have the desired effect on weeds. In such conditions, the herbicide will have minimal effect on the cereal plant.
Other issuesOther specific issues, such as droplet size, water type, etc., can also affect the weed control outcome.
Herbicide selection for
broad-leaved weeds
Many growers are excellent at recording field management history but few are really good at mapping out the most common weeds in a field. A note of the weeds present should be kept when walking the field in spring before herbicide is applied, with a follow-up comment in the run up to harvest.
Arguably, the weeds still actively growing before harvest are the most worrying and these should be to the forefront of the herbicide selection in the following spring.
Once the weed spectrum is established, choosing the herbicide should be relatively straightforward.
All herbicides carry a weed guide as part of their label, which can help in selection, but generally this list is limited.
Additional information can be obtained from your local adviser. Table 2 outlines some of the major herbicides available for spring cereals and the susceptibility of the weeds.
Careful selection of a combination of herbicide actives is vital to achieve coverage of the majority of broad-leaved weeds. For the majority of crops, a combination of sulfonylurea and hormone-based herbicides should be used.
Annual meadow grass
For many growers, annual meadow grass (AMG) remains a major problem in spring cereals. Heavy land (moisture retentive), combined with wet springs/summers and thin crops, can all increase AMG plant numbers and these can negatively affect crop yields.
Hussar will control AMG up to GS30, which is a small AMG plant. IPU – Fieldguard (PCS 02458), Arelon (PCS 02457) and Croplink IPU (PCS 04498) – was cleared for use on spring barley last year.
When applied to the crop early (target the AMG @ 2-3 leaf stage), IPU at 1.0 litre/ha can give very effective control with some persistence and aid subsequent broad leaf control.
If using IPU on spring barley, the crop itself needs to be growing strongly, and avoid using it if heavy rain is forecast in the days following application. This is because IPU can wash down to the roots and cause damage to the crop.
ALS-resistant weeds
A herbicide-resistant weed is a weed which can survive (and produce seeds) following application of an appropriate rate of herbicide that would normally have killed it.
The main herbicide-resistance type we encounter is known as ALS resistance. ALS herbicides target the enzyme acetolactate synthase (ALS) in the weed plants and, where a change in this enzyme in a weed becomes resistant to the herbicide, the plant is no longer killed.
ALS inhibitor weeds are an increasing problem over the past number of years. ALS-resistant corn marigold and chickweed are the most common weeds found across the country, but ALS-resistant poppy has also been identified. These resistant weeds affect the entire family of ALS herbicides, which includes Ally, Cameo, Hussar, Pacifica, etc.
Although there are differences as to how an ALS-resistant weed will react to different ALS herbicides, practical weed control necessitates that reliable weed control requires a non-ALS herbicide to be used with the ALS.
Tim O’Donovan carried out pot experiments in recent years and found different levels of ALS resistance. But where a non-ALS herbicide was used, control was good to excellent (ruling out cross resistance).
Table 3 gives options to consider when controlling ALS-resistant weeds or the options can be used as an anti-resistant strategy.
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