Glyphosate: Avoid the resistance risk!

Glyphosate is an essential component of IWM and a resistance management tool. We must protect its efficacy to ensure we can continue to control critical grass weeds in the future, writes Vijaya Bhaskar of Teagasc Oak Park.

Glyphosate is a non-selective herbicide used to control annual and perennial weeds. Worldwide over 50 species of weeds have had populations with glyphosate resistance. In Europe, glyphosate resistance has typically been selected in permanent perennial crops such as orchards and vineyards. But glyphosate-resistant ryegrass populations have also been found in cereal crops in Italy.  

Glyphosate, when applied correctly, remains an essential component of integrated weed management (IWM), preventing challenging grass weeds taking over fields. It is particularly relied upon in stale seedbed systems which are essential for any non-inversion tillage system and is good practice in plough-based systems too.

Glyphosate rates and application timings are critical in avoiding resistance build-up. 

In this article, I summarise the results of the efficacy of glyphosate on various grass weeds and found some interesting results with both rates and products, especially when dealing with resistance-prone grasses like blackgrass and Italian ryegrass. 

Glyphosate efficacy

Glyphosate products primarily differ in the formulation of glyphosate (which include an acid, potassium salt, ammonium salt, isopropylamine salt, etc.) and in the built-in surfactant package (which facilitate movement of glyphosate into plants), which can impact weed control efficacy or product performance.

Other factors which 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

Field stubble or pre-sowing

The rates indicated on product labels differ with the situation, such as stubbles or pre-sowing, etc, and have a recommended and maximum total dose.

The maximum total dose of glyphosate allowed in stubble is 1440 g/ha (ie. 4 L/ha of 360 g/L glyphosate) in any 12 month period.

The recommended guidance rate for stubble or pre-sowing vary depending on the types and sometimes the density of weeds present. The recommended product rate is generally 540 g/ha (ie. 1.5 L/ha of 360 g/L glyphosate) to control most annual grass weeds. Annual grasses should have at between 5 and 10 cm of green leaf length when spraying. Up to 1440 g/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 540 g/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 540 g/ha continuously may allow survivors to gradually build glyphosate resistance within a population leading to complex multiple resistance making future weed control extremely challenging. 

UK recommendations

In the UK, where herbicide-resistant blackgrass is present, the weed resistance action group (WRAG) recommends glyphosate at 720 g/ha for control of blackgrass and Italian ryegrass, but the rate may vary depending on the size of the target weed.

A maximum of two glyphosate applications is recommended for a stale seedbeds to prevent resistance development.

Recent research in the UK, revealed some populations of sterile brome with decreased glyphosate sensitivity.

Pre-sowing glyphosate use in Ireland

In a survey of 103 tillage farms:

• Pre-sowing glyphosate was used by 82% growers, with 59% using generic (eg. Mizr, Gallup, Rattler, Clearall, etc.) and 41% using Roundup products (Figure 1).
• Overall, the glyphosate application rate varied from 540 g/ha to 1080 g/ha, with some farms applying it as low as 360 g/ha or as high as 1440 g/ha.

Growers who used Roundup products, mostly applied from 540 g/ha to 720 g/ha (Figure 1). More than 35% of generic glyphosate users applied greater than 720 g/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?

Figure 1: Glyphosate-based herbicide use to manage grass weeds before sowing (n=84)

Glyphosate efficacy research

Different grass-weed populations with different sensitivity statuses were used in glasshouse to evaluate: two generics (Monsanto-Bayer and another manufacturer) with 360 g/L glyphosate, and three Roundup (Monsanto-Bayer) products with 360 g/L, 480 g/L or 720 g/kg concentrations.

Plants were sprayed at 2-4 leaf (GS 12-14) or tillering (>GS 21) stage. At the early stage, annual meadow grass had 5 cm of leaves, with other grasses >10 cm. Assessments were carried out 25 days post-spraying.   

1) Spraying at the 2-4 leaf stage

• Weed control efficacy is affected by using sub-optimal product rates (< 540 g/ha)
• Roundup and generics performed equally well on annual meadow grass and spring wild oats, regardless of their sensitivity status, and 540 g/ha achieved 100% control (Table 1).
• Roundup products gave 100% control of bromes and canary grass at 540 g/ha (Table 1). But 720 g/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 2 and 3).
• Italian ryegrass (Figure 2):
• 540 g/ha (Roundup) or 720 g/ha (generics) achieved 100% control of sensitive population.
• But 720 g/ha of all products resulted in 90-100% control of the two ACCase/ALS-resistant populations. 1080 g/ha of Monsanto-Bayer products achieved 100% control but a generic from another manufacturer only gave 96% control.
• Blackgrass (Figure 3):
• 720 g/ha (Monsanto-Bayer) or 1080 g/ha (other manufacturer) achieved 100% control of ACCase/ALS-resistant population.
• But 540 g/ha (Monsanto-Bayer) or 720 g/ha (other manufacturer) achieved 100% control of ACCase-resistant population.
• Overall, effective rates differed for controlling sensitive or single-site resistant strains relative to ACCase/ALS-resistant strains. 720 or > 540 g/ha (Monsanto-Bayer) or 1080 or > 720 g/ha (other manufacturer) are the most effective treatments for all populations of blackgrass and Italian ryegrass, regardless of their sensitivity status (Table 1).

Figure 2: Symptoms of blackgrass populations following application of different glyphosate products and rates at the 2-4 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 3: Symptoms of Italian ryegrass populations following application of different glyphosate products and rates at the 2-4 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.

2) Spraying at the tillering stage

In fields with varying growth of grass weeds, glyphosate must control grasses at a range of growth stages.

In general, larger the weed size (typically stubbles in January or February with large and dense mixed weed populations following a spring crop, see Figure 4, the higher the rate required for effective control.

Figure 4: Large and dense populations of sterile brome mixed with other annual weeds in a stubble field

In situations where 540 and 720 g/ha glyphosate were not effective at the 2-4 leaf stage, higher rates would be needed for more advanced plants.

We evaluated one Round-up and two generic glyphosate products at 1080 g/ha application rate. All products achieved 100% control of all grass weed populations (Table 1). The control of blackgrass and Italian ryegrass plants from different populations when sprayed at 1080 g/ha glyphosate at the tillering stage are shown in Figures 5 and 6.

Figure 5: Symptoms of blackgrass populations following application of different glyphosate products to larger tillering plants at a rate of 1080 g/ha. Herbicide-controlled plants showing 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.

Figure 6: Symptoms of Italian ryegrass with different sensitive status following application of different glyphosate products to larger tillering plants at a rate of 1080 g/ha. Herbicide-controlled plants showing chlorosis (yellowing) and necrosis (browning) of foliage. 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 or fully resistant only to ALS herbicides.

3) What rates should you apply?

Table 1 shows glyphosate rates for achieving effective control of all populations of grass weeds.

Based on our glasshouse research, the yellow highlighted cells indicate that a higher glyphosate is necessary when targeting smaller grass-weed populations. For larger and dense target grass-weed populations, 1080 g/ha is necessary.

Growers should note that the rates specified in this table are for controlling annual grass weeds growing from seeds in a pre-sowing context only. In fields where annual grass weeds are present alongside scutch, a higher rate (based on scutch density) specified on the product label is necessary.

Growers should assess the field situation (eg. weed types present, population size, plant size, etc.) to determine effective rates and be careful not to exceed the maximum total dose (1440 g/ha) outlined on the label over the season.

*Field performance and efficacy may vary depending on the product.

**Evolved ACCase/ALS-resistant strains or populations that have developed reduced sensitivity to glyphosate, may be able to tolerate higher product rate than specified in the table.

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 540 g/ha 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 and need higher product rates (> 720 g/ha). 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. 

1080 g/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 noted.

Growers should note that if we continuously use rates that leave survivors, these are the plants that could develop into resistant populations that could make the 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.