Glyphosate is a non-selective herbicide used to control annual and perennial weeds. Reduced sensitivity or resistance is building to the product across the world.
In Italy, glyphosate-resistant ryegrass populations have been found in cereal crops. However, not all glyphosate products are performing as they should be, and product formulation may be impacting this.
Correct glyphosate rates and application timings are critical in avoiding resistance build-up.
Glyphosate efficacy
Glyphosate products differ in how they are mixed or formulated.
Formulations can include an acid, potassium salt, ammonium salt, isopropylamine salt, etc, and they have a built-in surfactant package, which facilitates the movement of glyphosate into plants, which can impact weed control efficacy or product performance.
Factors that can improve
glyphosate efficacy include:
Using conditioners to reduce water hardness.Spraying conditions: – Warm conditions will result in active plant growth and herbicide uptake.
– Dry plants will have less risk of loss with subsequent rain.
– Calm conditions will help accurate spraying.
Glyphosate controlling grass weeds
The rates indicated on product labels differ with the situation, such as stubbles or presowing, etc.
The recommended guidance rates for stubble or presowing vary depending on the types and density of weeds present.
The recommended product rate is generally 540g/ha (1.5L/ha of 360g/L glyphosate) to control most annual grass weeds. Annual grasses should have five to 10 cm of green-leaf length when spraying.
Up to 1,440g/ha is necessary to control scutch, depending on plant density, which can be treated in combination with annual grass weeds.
Our glasshouse studies have shown that glyphosate, at 540g/ha, did not achieve 100% control of certain blackgrass or Italian ryegrass populations, which are resistant to ACCase/ALS herbicides, even though these use a different mode of action to glyphosate.
With these weeds in fields, use of 540g/ha continuously may allow survivors to gradually build glyphosate resistance within a population. This could lead to the weeds having resistance to all herbicide control options – making control extremely challenging.
Presowing glyphosate use in Ireland
In a survey of 103 tillage farms:
Presowing glyphosate was used by 82% of growers, with 59% using generic products (eg, Mizr, Gallup, Rattler, Clearall, etc) and 41% using Round-up (original product). Generic products are basically copies of the original product. They can be produced once the patent protection goes out of date, but are generally not the exact same as the original product.
Overall, the glyphosate application rate varied from 540g/ha to 1,080g/ha, with some farms applying it as low as 360g/ha or as high as 1,440g/ha. Growers who used Round-up products mostly applied from 540g/ha to 720g/ha. More than 35% of generic glyphosate users applied greater than 720g/ha.
The use of higher rates with generics may be influenced by their low cost. But does the use of low rates bring weed control challenges?
Researching efficacy of glyphosate
Different grass-weed populations with different sensitivity statuses were used in the glasshouse to evaluate: two generic glyphosate products (Monsanto-Bayer and another manufacturer) with 360g/L glyphosate, and three Round-up (Monsanto-Bayer) products with 360g/L, 480g/L or 720g/kg concentrations.
Plants were sprayed at the two to four leaf stage (GS12-14) or ‘tillering’ stage (>GS21).
At the early stage, annual meadow grass had 5cm of leaves, with other grasses at over 10cm. Assessments were carried out 25 days post-spraying.
1. Spraying at
the two to four
leaf stage
Weed control efficacy was affected by using sub-optimal product rates (<540g/ha).Round-up and generics performed equally well on annual meadow grass and spring wild oats, regardless of their sensitivity status, and 540g/ha achieved 100% control (Table 1). Round-up products gave 100% control of bromes and canary grass at 540g/ha (Table 1). But 720g/ha was necessary when using generics from Monsanto-Bayer (for brome) or another manufacturer (for bromes and canary grass).Product efficacy as well as weed control rates varied, depending on the sensitivity status of Italian ryegrass or blackgrass populations (Figures 1 and 2).
Figure 1: Symptoms of blackgrass populations following application of different glyphosate products and rates at the two- or four-leaf stage. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square. For glyphosate evaluations, we used populations that were resistant to both ACCase (Falcon, Stratos Ultra) and ALS (Pacifica) herbicides or only to ACCase herbicides.
Figure 2: Symptoms of Italian ryegrass populations following application of different glyphosate products and rates at the two- to four-leaf stage. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square.
For glyphosate evaluations, we used populations that were fully susceptible to ACCase (Axial, Falcon, Stratos Ultra) and ALS (Pacifica, Broadway) herbicides or fully resistant to ACCase/ALS herbicides.
– 540g/ha (Round-up) or 720g/ha (generics) achieved 100% control of the sensitive population;
– But 720g/ha of all products resulted in 90% to 100% control of the two ACCase/ALS-resistant populations. A total of 1,080g/ha of Monsanto-Bayer products achieved 100% control, but a generic from another manufacturer only gave 96% control.
Figure 2: Symptoms of Italian ryegrass populations following application of different glyphosate products and rates at the two- to four-leaf stage. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square.
For glyphosate evaluations, we used populations that were fully susceptible to ACCase (Axial, Falcon, Stratos Ultra) and ALS (Pacifica, Broadway) herbicides or fully resistant to ACCase/ALS herbicides.
– 720g/ha (Monsanto-Bayer) or 1,080g/ha (other manufacturer) achieved 100% control of ACCase/ALS-resistant population;
– But 540g/ha (Monsanto-Bayer) or 720g/ha (other manufacturer) achieved 100% control of ACCase-resistant populations.
Overall, effective rates differed for controlling sensitive or single-site resistant strains relative to ACCase/ALS-resistant strains. 720 or >540g/ha (Monsanto-Bayer) or 1,080g/ha or >720g/ha (other manufacturer) are the most effective treatments for all populations of blackgrass and Italian ryegrass, regardless of their sensitivity status (Table 1).2. Spraying at the tillering stage
In fields with varying growth of grass weeds, glyphosate must control grasses across a range of growth stages. In general, the larger the weed size, the higher the rate required for effective control. In situations where 540g/ha and 720g/ha glyphosate were not effective at the two- to four-leaf stage, higher rates would be needed for more advanced plants.
We evaluated one Round-up and two generic glyphosate products at the 1,080g/ha application rate. All products achieved 100% control of all grass weed populations (Table 1).
Figure 3: Symptoms of blackgrass populations following application of different glyphosate products to larger tillering plants at a rate of 1,080g/ha. Herbicide-controlled plants showed chlorosis (yellowing) and necrosis (browning) of foliage. For glyphosate evaluations, we used populations that were resistant to both ACCase (Falcon, Stratos Ultra) and ALS (Pacifica) herbicides or only to ACCase herbicides.
The control of blackgrass and Italian ryegrass plants from different populations when sprayed at 1,080g/ha glyphosate at the tillering stage are shown in Figures 3 and 4.
What rate should you apply?
Table 1 shows glyphosate rates for achieving effective control of all populations of grass weeds.
Based on our glasshouse research, the highlighted cells indicate that a higher glyphosate rate is necessary for effective control when targeting smaller grass-weed populations.
When targeting larger and denser grass-weed populations, 1,080g/ha of glyphosate is necessary.
Growers should note that the rates specified in this table are for controlling annual grass weeds growing from seeds in a stubble or presowing context only.
Where scutch is present, a higher rate is needed and should be based on scutch density and label rates.
Farmers should assess weed types, population size, plant size, etc, to determine effective rates and be careful not to exceed the maximum total dose (1,440g/ha or 4L/ha of 360g Glyphosate) outlined on the label.
Summary
An insufficient glyphosate product rate for the specific weed species or weed size can increase problems with grass-weed control. Not all products work equally, and 540g/ha glyphosate (1.5L/ha of 360g/L glyphosate) may not be adequate in all situations.
Reduced sensitivity or tolerance to glyphosate is developing in our Italian ryegrass and blackgrass populations that are resistant to ACCase/ALS modes of action. These plants need higher product rates (>720g/ha) depending on target size.
Unlike target-site resistance, which is mode of action-specific, metabolism-based, non-target-site resistance affects multiple herbicide modes of action, including glyphosate, and needs further investigation. A total of 1,080g/ha of any product is effective for grass-weed control of all populations when sprayed on larger tillering plants. At that rate and weed size, no weed control differences between products were noted. Growers should note, if we continuously use rates that leave survivors, these are the plants that could develop into resistant populations, which could make any glyphosate product ineffective in the future. Glyphosate use must be supplemented with cultivation and herbicide applications (pre- and, where necessary, post-emergence) to eradicate any possible survivors.
Glyphosate is a non-selective herbicide used to control annual and perennial weeds. Reduced sensitivity or resistance is building to the product across the world.
In Italy, glyphosate-resistant ryegrass populations have been found in cereal crops. However, not all glyphosate products are performing as they should be, and product formulation may be impacting this.
Correct glyphosate rates and application timings are critical in avoiding resistance build-up.
Glyphosate efficacy
Glyphosate products differ in how they are mixed or formulated.
Formulations can include an acid, potassium salt, ammonium salt, isopropylamine salt, etc, and they have a built-in surfactant package, which facilitates the movement of glyphosate into plants, which can impact weed control efficacy or product performance.
Factors that can improve
glyphosate efficacy include:
Using conditioners to reduce water hardness.Spraying conditions: – Warm conditions will result in active plant growth and herbicide uptake.
– Dry plants will have less risk of loss with subsequent rain.
– Calm conditions will help accurate spraying.
Glyphosate controlling grass weeds
The rates indicated on product labels differ with the situation, such as stubbles or presowing, etc.
The recommended guidance rates for stubble or presowing vary depending on the types and density of weeds present.
The recommended product rate is generally 540g/ha (1.5L/ha of 360g/L glyphosate) to control most annual grass weeds. Annual grasses should have five to 10 cm of green-leaf length when spraying.
Up to 1,440g/ha is necessary to control scutch, depending on plant density, which can be treated in combination with annual grass weeds.
Our glasshouse studies have shown that glyphosate, at 540g/ha, did not achieve 100% control of certain blackgrass or Italian ryegrass populations, which are resistant to ACCase/ALS herbicides, even though these use a different mode of action to glyphosate.
With these weeds in fields, use of 540g/ha continuously may allow survivors to gradually build glyphosate resistance within a population. This could lead to the weeds having resistance to all herbicide control options – making control extremely challenging.
Presowing glyphosate use in Ireland
In a survey of 103 tillage farms:
Presowing glyphosate was used by 82% of growers, with 59% using generic products (eg, Mizr, Gallup, Rattler, Clearall, etc) and 41% using Round-up (original product). Generic products are basically copies of the original product. They can be produced once the patent protection goes out of date, but are generally not the exact same as the original product.
Overall, the glyphosate application rate varied from 540g/ha to 1,080g/ha, with some farms applying it as low as 360g/ha or as high as 1,440g/ha. Growers who used Round-up products mostly applied from 540g/ha to 720g/ha. More than 35% of generic glyphosate users applied greater than 720g/ha.
The use of higher rates with generics may be influenced by their low cost. But does the use of low rates bring weed control challenges?
Researching efficacy of glyphosate
Different grass-weed populations with different sensitivity statuses were used in the glasshouse to evaluate: two generic glyphosate products (Monsanto-Bayer and another manufacturer) with 360g/L glyphosate, and three Round-up (Monsanto-Bayer) products with 360g/L, 480g/L or 720g/kg concentrations.
Plants were sprayed at the two to four leaf stage (GS12-14) or ‘tillering’ stage (>GS21).
At the early stage, annual meadow grass had 5cm of leaves, with other grasses at over 10cm. Assessments were carried out 25 days post-spraying.
1. Spraying at
the two to four
leaf stage
Weed control efficacy was affected by using sub-optimal product rates (<540g/ha).Round-up and generics performed equally well on annual meadow grass and spring wild oats, regardless of their sensitivity status, and 540g/ha achieved 100% control (Table 1). Round-up products gave 100% control of bromes and canary grass at 540g/ha (Table 1). But 720g/ha was necessary when using generics from Monsanto-Bayer (for brome) or another manufacturer (for bromes and canary grass).Product efficacy as well as weed control rates varied, depending on the sensitivity status of Italian ryegrass or blackgrass populations (Figures 1 and 2).
Figure 1: Symptoms of blackgrass populations following application of different glyphosate products and rates at the two- or four-leaf stage. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square. For glyphosate evaluations, we used populations that were resistant to both ACCase (Falcon, Stratos Ultra) and ALS (Pacifica) herbicides or only to ACCase herbicides.
Figure 2: Symptoms of Italian ryegrass populations following application of different glyphosate products and rates at the two- to four-leaf stage. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square.
For glyphosate evaluations, we used populations that were fully susceptible to ACCase (Axial, Falcon, Stratos Ultra) and ALS (Pacifica, Broadway) herbicides or fully resistant to ACCase/ALS herbicides.
– 540g/ha (Round-up) or 720g/ha (generics) achieved 100% control of the sensitive population;
– But 720g/ha of all products resulted in 90% to 100% control of the two ACCase/ALS-resistant populations. A total of 1,080g/ha of Monsanto-Bayer products achieved 100% control, but a generic from another manufacturer only gave 96% control.
Figure 2: Symptoms of Italian ryegrass populations following application of different glyphosate products and rates at the two- to four-leaf stage. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square.
For glyphosate evaluations, we used populations that were fully susceptible to ACCase (Axial, Falcon, Stratos Ultra) and ALS (Pacifica, Broadway) herbicides or fully resistant to ACCase/ALS herbicides.
– 720g/ha (Monsanto-Bayer) or 1,080g/ha (other manufacturer) achieved 100% control of ACCase/ALS-resistant population;
– But 540g/ha (Monsanto-Bayer) or 720g/ha (other manufacturer) achieved 100% control of ACCase-resistant populations.
Overall, effective rates differed for controlling sensitive or single-site resistant strains relative to ACCase/ALS-resistant strains. 720 or >540g/ha (Monsanto-Bayer) or 1,080g/ha or >720g/ha (other manufacturer) are the most effective treatments for all populations of blackgrass and Italian ryegrass, regardless of their sensitivity status (Table 1).2. Spraying at the tillering stage
In fields with varying growth of grass weeds, glyphosate must control grasses across a range of growth stages. In general, the larger the weed size, the higher the rate required for effective control. In situations where 540g/ha and 720g/ha glyphosate were not effective at the two- to four-leaf stage, higher rates would be needed for more advanced plants.
We evaluated one Round-up and two generic glyphosate products at the 1,080g/ha application rate. All products achieved 100% control of all grass weed populations (Table 1).
Figure 3: Symptoms of blackgrass populations following application of different glyphosate products to larger tillering plants at a rate of 1,080g/ha. Herbicide-controlled plants showed chlorosis (yellowing) and necrosis (browning) of foliage. For glyphosate evaluations, we used populations that were resistant to both ACCase (Falcon, Stratos Ultra) and ALS (Pacifica) herbicides or only to ACCase herbicides.
The control of blackgrass and Italian ryegrass plants from different populations when sprayed at 1,080g/ha glyphosate at the tillering stage are shown in Figures 3 and 4.
What rate should you apply?
Table 1 shows glyphosate rates for achieving effective control of all populations of grass weeds.
Based on our glasshouse research, the highlighted cells indicate that a higher glyphosate rate is necessary for effective control when targeting smaller grass-weed populations.
When targeting larger and denser grass-weed populations, 1,080g/ha of glyphosate is necessary.
Growers should note that the rates specified in this table are for controlling annual grass weeds growing from seeds in a stubble or presowing context only.
Where scutch is present, a higher rate is needed and should be based on scutch density and label rates.
Farmers should assess weed types, population size, plant size, etc, to determine effective rates and be careful not to exceed the maximum total dose (1,440g/ha or 4L/ha of 360g Glyphosate) outlined on the label.
Summary
An insufficient glyphosate product rate for the specific weed species or weed size can increase problems with grass-weed control. Not all products work equally, and 540g/ha glyphosate (1.5L/ha of 360g/L glyphosate) may not be adequate in all situations.
Reduced sensitivity or tolerance to glyphosate is developing in our Italian ryegrass and blackgrass populations that are resistant to ACCase/ALS modes of action. These plants need higher product rates (>720g/ha) depending on target size.
Unlike target-site resistance, which is mode of action-specific, metabolism-based, non-target-site resistance affects multiple herbicide modes of action, including glyphosate, and needs further investigation. A total of 1,080g/ha of any product is effective for grass-weed control of all populations when sprayed on larger tillering plants. At that rate and weed size, no weed control differences between products were noted. Growers should note, if we continuously use rates that leave survivors, these are the plants that could develop into resistant populations, which could make any glyphosate product ineffective in the future. Glyphosate use must be supplemented with cultivation and herbicide applications (pre- and, where necessary, post-emergence) to eradicate any possible survivors. 
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