Key points

• NSW Department of Primary Industries research shows that every dollar spent on controlling summer weeds returns $8/hectare
• Effective weed control during summer and autumn also rids paddocks of crop volunteers, which reduces disease pathotype carryover from one season to the next
• Increased costs and rising weed resistance to herbicide have prompted calls to revise best-practice summer and autumn glyphosate application tactics

When seeking to optimise summer and autumn spraying, weed management specialist Mark Congreve says several factors influence the efficacy of glyphosate.

Mr Congreve, who works for Independent Consultants Australia Network, says glyphosate works best when a lethal dose enters the leaf and moves to the roots.

“There are several factors that users can consider to maximise the return on their investment for summer and autumn spraying,” he says.

“Some of these include targeting small and non-stressed weeds, knowing how resistance develops, using ammonium sulfate to improve water quality, understanding leaf uptake, and using very coarse droplets.

“Effective glyphosate use is also important due to its increased cost during the past 12 months and a rise in weed resistance to glyphosate.”

Mr Congreve says confirmed resistance to glyphosate continues to rise across Australia (see Table 1) in many weed species.

Table 1: First confirmation of glyphosate resistance in Australia.

Adapted from Heap, I. The International Herbicide-Resistant Weed Database. Online. Monday, 3 October 2022. Available via www.weedscience.org

“While resistance is widespread nationally, the frequency of populations is heavily dominated by summer weeds in northern Australia, as this is where the most frequent applications have occurred over many years,” he says.

“While glyphosate resistance is now increasingly common, this does not always mean that glyphosate cannot be used.

“For several species, it is common for glyphosate to be ‘dose-responsive’, meaning that it may still work should enough glyphosate enter the plant and move (translocate) to the site of activity. This is a combination of both application rate and how we elect to apply glyphosate.

“For example, see the resistance test data from the Grains Orana Alliance trial sites in Table 2, where the Forbes and Peak Hill populations could be controlled by increasing application rate when combined with excellent application.

"However, it should be noted that for some species such as annual ryegrass, if complete control is not achieved for every application, then it is expected that resistance will further evolve.

“Additionally, the weed population will become extremely difficult to control, even with high rates (such as the Coolah population in Table 2). This highlights the need to ensure 100 per cent control in all situations to delay the selection of ‘strong’ resistance.”

Table 2: Grain Orana Alliance resistance testing in 2021 from three sites.

Source: Street and O’Brien (2022). Killing glyphosate-resistant ryegrass? Application does matter. GRDC Updates.

Mr Congreve says increasing the application rate of glyphosate is often an initial tactic when battling weed populations with emerging herbicide resistance.

“One strategy that may be effective on these populations is using optical sprayers that permit the application of much-higher rates on individual weeds,” he says.

“However, there are also other tactics growers can use in the early stages of resistance management.

“It is critical that growers ensure no weeds are allowed to set seed after a glyphosate application. If survivors are not allowed to set seed, then at least the problem will not become worse from year to year.”

Other tactics

Mr Congreve says glyphosate binds tightly to soil on contact, which is one reason why it does not work as a residual herbicide.

“So, if you use dirty water or water with small soil particles in your spray tank, glyphosate will instantly bind to these particles, reducing the amount of glyphosate available for plant uptake.

“A quick way to check water quality is to fill a 10-litre bucket and add a 10-cent piece. If you cannot see the 10-cent piece, the water is too dirty for glyphosate and paraquat.

“Additionally, we often see reduced effectiveness of glyphosate in wheel tracks, and this is likely to be a combination of compacted conditions affecting weed growth, poor spray deposition behind the wheels and often dust, where glyphosate has contacted dust and been de-activated.”

Water quality

For most herbicides, he says water with a pH of 5 to 8.5 is suitable for herbicide application and generally does not require adjusting.

“Where water pH is more than 8.5, look to understand why the pH is high. For those using bore water, high pH generally indicates water high in either bicarbonates (total alkalinity) or cations (total hardness). This needs to be addressed before use (see Table 2).”

Table 2: Water quality benchmarks

Source: Mark Congreve, Independent Consultants Australia Network.

Mr Congreve says an extensive survey of bore water in the NSW Central West shows that about 80 per cent of samples were of poor quality for either bicarbonates or cations or both.

“If you are using rainwater, there is a high likelihood the water quality will be good enough for spraying,” he says.

“Grain Orana Alliance research showed that bore water used with a weakly glyphosate-resistant population was associated with a higher density of annual ryegrass surviving. The work also demonstrated that if you are using poor-quality water, one of the worst things you can do is add more poor-quality water into your spray tank.”

Grain Orana Alliance research showed that just using an acidifying adjuvant (LI700 in this case) did not improve annual ryegrass control when using poor-quality bore water.

“Interestingly, putting the poor-quality water through a reverse osmosis filter improved the quality of the bore water to a level that was similar to rainwater quality and then did not adversely affect glyphosate performance.

“As reverse osmosis filtration is not commonly available, another practical option can be to pre-treat lower-quality bore water with ammonium sulfate before it is used for glyphosate.”

Ammonium sulfate:

• ‘fixes’ hard water;
• partially dissipates bicarbonates;
• improves tank mix compatibility; and
• assists cell membrane transfer when glyphosate is inside the leaf.

Nonetheless, he says, ammonium sulfate needs time to work.

“It needs to be fully dissolved before you load glyphosate into the tank, and then it needs an additional five to 10 minutes to ‘condition’ the water after it has dissolved,” he says.

“Some granular ammonium sulfate formulations dissolve more quickly than others. And all granular forms dissolve faster when the water is warmer. If you are in a hurry, use a liquid formulation to reduce the time needed for the ammonium sulfate to dissolve.

“A problem that can sometimes be encountered, often due to being in a rush to complete the job, is that ammonium sulfate is only partially dissolved when 2,4-D amine is added to the tank.

“Should this happen, the solution can become like toothpaste, and the next few hours will be spent cleaning out the tank.”

Leaf uptake

Glyphosate takes a relatively long time to move into the leaf compared with some other herbicides, Mr Congreve says.

“Glyphosate is a water-based herbicide, but weeds have waxy cuticles on their leaves, which slows leaf entry.

“Under hot and low-humidity summer conditions, the plant further changes the leaf cuticle composition, making glyphosate uptake even slower.

“Additionally, droplets will evaporate very quickly under these conditions. So glyphosate uptake in summer is even more challenging. This is why we typically apply higher rates for summer applications, to attempt to compensate for the relatively lower amount of glyphosate entering the leaf.

“If a storm comes through an hour after glyphosate has been applied, only a small amount of the product will have already entered the leaf, with the remainder potentially washed off and lost. It takes several hours for most glyphosate to move into a weed, even with the best-quality formulations.”

Rule of thumb

Mr Congreve’s rule of thumb for maximum uptake of glyphosate is a Delta T of four to six for four to six hours after application.

“However, this is generally not realistic for summer spraying because conditions often run at a Delta T of six to 10, or even higher,” he says. “This means not as much glyphosate enters the plant compared with spraying during less evaporative conditions such as during autumn.

“Adjuvants might help a little, but nothing is available to deliver all the applied glyphosate into the plant within an hour or so.

“Adding the ‘wrong’ adjuvant to glyphosate often further reduces leaf uptake, so only use the type of adjuvant recommended on the label and in the situation where it is recommended.

“Before glyphosate resistance emerged, the general strategy was to increase application rates when faced with ‘difficult’ conditions. However, with resistant populations, users need to consider more than just increasing the application rate.”

Droplet size

In an ideal world, Mr Congreve says the best tactic to use is larger droplets, very coarse (VC) or greater, and higher concentrations (robust herbicide rate plus lower water volume). This is because larger droplets and higher concentrations:

• increase droplet survival time;
• reduce off-target losses (less drift); and
• assist cuticle uptake.

However, he says small upright grasses can be difficult to hit with very coarse droplets.

“The weed target is either missed completely, or the target is hit, but the large droplet bounces off.

“So, when using very large droplets on small targets, water rates may need to be increased to about 80 to 100L.

“While a medium-coarse spray quality will improve coverage to these small weed targets, these smaller droplets are less likely to survive during hot and dry conditions during summer.

“Additionally, an increasing number of glyphosate labels now require the use of coarse (or larger droplets).”

Accordingly, he recommends using very coarse or larger droplets for:

• all 2,4-D mixes;
• reducing drift;
• situations as stated on some labels;
• when spraying weeds with leaf surfaces to which droplets will easily ‘stick’;
• summer applications; and
• high stubble environments.

He says a medium to coarse droplet might be more appropriate when applying:

• Group 14 (G) mixes;
• Group 1 (A) mixes; and
• herbicides to surfaces that are difficult to wet (weeds with hairy, small or upright leaves such as grasses).

Glyphosate movement

It is important to remember that glyphosate requires two to three days to fully move (translocate) from a leaf’s surface to the roots.

“Glyphosate translocation is reduced when weeds are stressed in very dry or very wet (waterlogged) conditions,” Mr Congreve says.

“Also consider the impacts of tank mix partners such as plant hormones, which disrupt glyphosate movement through a plant, and fast-acting mix partners, which destroy the vascular bundle and reduce glyphosate translocation.

“Plant hormones include Group 4 (I) herbicides such as 2,4-D, dicamba and fluroxypyr.

“Fast-acting herbicides include paraquat, glufosinate and Group 14 (G) herbicides.

“Fast ‘brownout’ (or cell death) is not desirable when using glyphosate mixes. Plant cells must be alive to move a lethal dose from the leaves to the roots.

“Antagonism is frequently more noticeable in weed populations with glyphosate resistance when the tank mix partner is used at a higher rate, and during summer conditions.”

More information: Mark Congreve, mark@icanrural.com.au