It is now evident that an insufficient glyphosate application rate for a specific weed species can increase problems with grass weed control.
In addition, not all glyphosate products work equally, and may not be adequate in all situations.
It is now evident that reduced sensitivity or tolerance to glyphosate is developing in Italian ryegrass and blackgrass populations found in Ireland.
Unlike target-site resistance, which is action-specific and metabolism-based, non-target-site resistance affects multiple herbicide modes of action, including glyphosate and needs further investigation.
Growers should note that if they continuously use rates that leave survivors, these are the plants that could develop into resistant population.
So, glyphosate use must be supplemented with cultivation and herbicide applications (pre- and where necessary, post-emergence) to eradicate any possible survivors.
Glyphosate
Glyphosate is an essential component of integrated weed management (IWM) procedures and a resistance management tool, according to Teagasc research scientist, Vijaya Bhaskar.
He said: “We must protect its efficacy to ensure we can continue to control critical grass weeds in the future.”
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.
Glyphosate-resistant ryegrass populations have also been found in cereal crops in Italy.
Glyphosate, when applied correctly, remains an essential component of 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.
Glyphosate application
Glyphosate rates and application timings are critical in avoiding resistance build-up.
The maximum total dose of glyphosate allowed in stubble is 1,440g/ha (i.e., 4L/ha of 360g/L glyphosate) in any 12-month period according to Teagasc.
Vijaya Bhaskar continued: “The recommended guidance rate for stubble or pre-sowing varies depending on the types and sometimes the density of weeds present.
“Annual grasses should have at between 5cm and 10cm 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.”
Currently, the recommended rate for glyphosate containing products is generally 540g/ha (i.e., 1.5L/ha of 360g/L glyphosate) to control most annual grass weeds.
Teagasc 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 leading to complex multiple resistance making future weed control extremely challenging.
In a recent survey of 103 Irish tillage farmers, pre-sowing glyphosate was used by 82% of the participants, with 59% using generic products.
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.