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For an effective kill, weed management specialists recommend targeting weeds when they are small.
Photo: Nicole Baxter

Weed management specialists are encouraging growers to spray weeds early in their growth stage to improve kill rates before sowing.

Dr Peter Boutsalis, a specialist in herbicide resistance at Plant Science Consulting, says wet conditions during autumn 2020 in eastern Australia caused rapid early growth and poor pre-season control of annual ryegrass, which is further exacerbated by a continual increase in weeds with resistance to herbicides.

“A lot of plants were sent in for testing because they had been sprayed with glyphosate and did not die,” Dr Boutsalis says.

“From the samples sent in from South Australia, Victoria and Western Australia, about 50 to 60 per cent have been confirmed as glyphosate-resistant. From New South Wales, the figure is higher at about 80 per cent.”

Target small weeds

A factor that is likely to have contributed to poor weed control, Dr Boutsalis says, is applying herbicide to weeds that are too large, especially where there is also some level of glyphosate resistance present.

Glyphosate is translocated, he says, which means it moves to root tips and shoots to kill the growing plant. “The larger the plant, the more herbicide is needed for an effective kill. A weed with a large taproot sprayed at a low rate is likely to recover.”

He says this is commonly observed in sowthistle. By way of example, he pointed to a grower who requested a resistance test on sowthistle at an early and late growth stage.

“The grower’s sowthistle had a low level of glyphosate resistance, but we also checked a glyphosate-susceptible sowthistle population and a glyphosate-resistant sowthistle population,” he says.


Timing of weed control is important, with complete control of resistant plants possible if sprayed early. Back row: Glyphosate resistant sowthistle. Middle row: Suspect resistant sowthistle. Front row: Glyphosate susceptible sowthistle. Glyphosate provided 100 per cent control of sowthistle resistant to glyphosate at this young growth stage. Source: Peter Boutsalis

“When glyphosate was sprayed at the early rosette stage, all three populations were killed, even the glyphosate-resistant population.”

One month later, at bolting, Dr Boutsalis says, applying glyphosate resulted in sowthistle survival even in the glyphosate-susceptible population. Leaf burning was evident in the susceptible population but each of the sowthistle populations recovered.

“Targeting herbicide applications to weeds early in their development will produce a more effective kill, especially for broadleaf weeds.”

Do not wait

Waiting for a whole paddock of weeds to germinate is false economy, he says. “You’re better off targeting weeds when they’re small and applying another knockdown if there’s another flush of weeds. This is especially important as temperatures rise in southern Australia during summer, because glyphosate works more effectively in cooler conditions.”

Weeds specialist Mark Congreve, of Independent Consultants Australia Network, suggests avoiding spraying when temperatures are more than 30°C to 35°C because, as temperature increases, two things happen. “Firstly, the plant modifies the leaf cuticle to reduce its water loss from transpiration. This makes it even slower for glyphosate to enter the leaf,” he says.

“At the same time, hotter temperatures increase the rate of droplet evaporation. This combination means that less glyphosate gets into the leaf when applied under hot conditions.”

Use a double knock

A double-knock application of a contact herbicide such as paraquat applied soon after the glyphosate is a useful tool to improve consistency of results. However, timing of the double knock is important.

“If glyphosate has been applied and paraquat is sprayed two to three weeks later, when weeds are larger, control of the double knock is also likely to be poor,” Dr Boutsalis says. “Applying paraquat between one and seven days after glyphosate is ideal, with timing dependent on plant size and temperature.”

Set-up and timing

For summer applications, Mr Congreve says it is harder to get the glyphosate into the leaf before droplet evaporation.

He says glyphosate requires many hours on the leaf surface for full penetration, so applications before rainfall should be avoided.

“Application in the middle of the day when it is hot will result in much faster droplet evaporation. Larger droplets (very coarse or larger) will take longer to evaporate. Consider if these large droplets are also appropriate for any other products in the tank mix.

“Fast travel speeds, with high boom heights, also generally reduce weed coverage.

“High-quality glyphosate formulations will have the right surfactant built into the formulation; however, poor-quality glyphosate products may have a low amount or the wrong type of surfactant included, which can reduce uptake.

“Generally, glyphosate isopropylamine formulations often tend to be more robust than single-potassium salt formulations under summer conditions and are normally more compatible with other products in a tank mix.”

Use ammonium sulphate

When using glyphosate, Dr Boutsalis encourages the use of ammonium sulfate as per label recommendations.

Ammonium sulfate should be added to both soften hard water and to help glyphosate move through plants.

Mr Congreve adds that if glyphosate is added to hard water (containing high levels of calcium or other cations) the formulated glyphosate will preferentially break apart in the spray tank and reform as a calcium salt of glyphosate. This calcium salt is insoluble and has poor ability to penetrate the leaf.

“Adding ammonium sulfate will precipitate free positively charged ions, but it has to be added to the water first and left for a short period before glyphosate is added,” Dr Boutsalis says.

“Ammonium can also improve glyphosate activity within the plant, by helping glyphosate move across cell membranes.”

Carefully consider mixtures

Mr Congreve says some tank mixes could improve control on certain weeds. However, a number of mixtures can also reduce glyphosate performance, especially under summer conditions or where low-level glyphosate resistance is present.

For example, adding a second mode of action such as a Group G herbicide may improve control of weeds such as fleabane and sowthistle. But Mr Congreve says Group G herbicide mixtures may reduce the performance of glyphosate on grass weeds.

He says other tank mixtures that may reduce glyphosate activity on summer weeds in certain situations include phenoxy herbicides (for example 2,4-D), triazines (for example atrazine), paraquat and spraying oils.

“Seek advice from a local agronomist for the best herbicide mixtures for particular weed species,” he says.

Test escapes

If weeds survive an application of glyphosate, Dr Boutsalis encourages growers and agronomists to send living plants collected from paddocks between autumn and early spring for resistance testing (’Quick Test’). Additionally, seeds collected at harvest time can be sent for testing over summer and reporting in early autumn.

Further information is available from Plant Science Consulting in Adelaide or the Charles Sturt University Plant Interactions Research Group.

“A lot of growers have requested the test of more than one rate of glyphosate,” Dr Boutsalis says.

“Sometimes using a higher label rate of glyphosate applied improves control marginally, but in other cases it is very effective, and so a resistance test will help determine the level of herbicide resistance they’re facing.”

Further reading

More information: Peter Boutsalis, 0400 664 460Planet Science Consulting; Mark Congreve, 0427 209 234,; weed management and herbicide application is available from GRDC's Herbicide Behaviour page and WeedSmart Portal.

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