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Investigate adverse experiences when using herbicides

There are many reasons why you might get an unexpected result when using herbicides. Good records and proper investigation of all possibilities can help hone in on the cause.

A shuttle of glyphosate applied over the top of a Roundup Ready cotton crop was recently shown to also contain a damaging level of 2,4-D impurity, resulting in significant crop injury and yield loss.

The grower involved did not accept the suggestion that the crop damage was due to poor sprayer decontamination or spray drift from a fallow application of 2,4-D, and he was able to prove the problem was due to product impurity.

2,4-D herbicide injury in cotton after the crop was sprayed with glyphosate product contaminated with 2,4-D.

Other shuttles of the same batch may have been applied to fallow weeds where the residual 2,4-D in the glyphosate would have gone unnoticed. Full rate 2,4-D in glyphosate is known to compromise glyphosate efficacy, but studies of low-rate 2,4-D impurity in glyphosate could not be found.

Where can impurities come from?

While the agricultural chemical manufacture and supply chain in Australia is considered first-class and is highly regulated, there is an acceptance that the nil-impurity requirement for the manufacture of agricultural chemicals is unattainable in facilities that use multi-purpose equipment for synthesis, formulation and packaging of products.

Companies therefore apply their own quality assurance standards before releasing products for distribution and sale. If the level of risk posed by certain residual impurities in a product is underestimated, there is potential for instances of crop injury, pesticide residue in produce or poor performance of the product on the intended target weed, fungus or pest.

Mistakes can and do happen within the manufacturing process and chemical supply and distribution chain. To ensure that risks of contamination are minimised and that quality assurance protocols are followed carefully, it is important that any breaches or errors are identified quickly, reported and investigated.

Keep good records of each spray event, including batch numbers of applied product, to help identify the cause of adverse experiences with herbicides.

There are two important things to note: firstly, the current regulations specify that crop protection products must contain nil impurities (other than manufacturing impurities listed in the APVMA standard); and secondly, companies are required to recall product batches when contamination issues are identified. The Australian Pesticide and Veterinary Medicines Authority (APVMA) oversees a highly regulated system of registration, compliance and enforcement on crop protection products.

Assess potential application issues

When misapplication (wrong product applied, incorrect mixing, contaminated product etc) occurs, symptoms of affected plants are usually uniform throughout the treated area. It is often suggested that poor application technique or poor sprayer decontamination is the reason for crop injury or poor weed control results – suggesting a grower ‘own-goal’. Such potential errors must be considered, but if best practice spray techniques and spray rig decontamination procedures have been followed, product impurity should also be considered and investigated.

The chemistry of the product will determine the risk of residues being held within the tank and spray lines of the application rig. This is why there are differences in the sprayer hygiene requirements after using particular products.

Most modern spray rigs have impervious rubber and plastic, or stainless steel components, drastically reducing the risk of chemical absorption and subsequent extraction. Residues on the rubber surfaces are the main concern, and all registered cleaners will physically remove residues when used as directed, but cracked rubber components can present a contamination risk. All filters/strainers must be cleaned and all actuators and taps musts be cycled as the cleaner is run through the spray boom and tank loading system, agitators and tank.

Crop injury or poor weed control that is associated with just one sprayer tank load would suggest sprayer contamination. Effects from contaminated tanks are usually worse at the beginning of the spray run, with damage diminishing with spraying and tank reloading. The field pattern can provide clues to the sprayer filling routine in the field where the crop damage occurred.

The other major reason commonly cited for crop injury in spray drift. Although there is always some small amount of drift when agricultural chemicals are sprayed from a ground rig, the amount is down to ‘virtually safe’ levels within a few tens of metres. If the conditions are very windy, or the boom is too high, or the droplet size too small, spray could drift a few hundred metres from the application ground rig.

Spray droplets may travel a few feet to several kilometres from the targeted area, depending on weather conditions and spray application; but the potential for drift damage decreases with distance because droplets are deposited or become diluted in the atmosphere. The pattern of injury is normally seen most prominently on the section of the field closest to the sprayer that generated the spray drift, and decreases across the field.

During inversion conditions, a similar amount of product is subject to drift, but the drifting product will not dilute as much in the air, so concentrations at specific locations can be higher than expected in non-inversion conditions.

What to do if your crop is damaged or weeds don’t die as expected?

Along with several other possible causes, unintended application of contaminated product should be considered as a potential explanation for crop injury or poor weed control.

Keep in mind that if product impurity is the problem, it is most likely due to a low-dose effect that may be difficult to diagnose or may take longer to express in the target weeds or susceptible crops.

Finding the cause of an ‘adverse experience’ with herbicide is one of the most important reasons to keep accurate and detailed spray records.

If a problem occurs:

  • Take detailed, time-stamped photographs of the crop or weeds and record everything you know about the crop or fallow management, weather conditions in the weeks prior to the damage being seen, spray history of the field etc. If possible, geotag the photos so they can be easily associated with the correct field.
  • Record the relevant batch numbers of the chemicals used, which can be checked against the retention samples at the factory if necessary. Collect samples from drums of product used prior to the injury being observed (up to 14 days prior to symptoms being obvious). When you take samples, make sure there are witnesses who can vouch for the voracity of the evidence you have collected. Testing for one impurity (e.g. 2,4-D in glyphosate) costs less than $500 per sample.
  • Document the injury over time. For example, injury in cotton from low rates of 2,4-D will grow out in two weeks, but injury from higher rates, could last three to four weeks and are the most likely to result in yield loss. Similarly with weeds although the impact may be more difficult to document.
  • Mark out the affected area in the field to help assess crop yield loss at the end of the season. Note the pattern and intensity of the problem across the field.
  • Eliminate as many possible causes as you can. Re-assess the application technique and equipment, consider the pattern of damage in the field, look at the weather conditions for the relevant period of time and so on.
  • Test for herbicide resistance in weeds.
  • Report the crop damage or poor weed control. The APVMA administers the Adverse Experience Reporting Program, which allows anyone to report a problem with an agricultural chemical, including crop and plant damage, for example, plant death, severe stunting or significant yield loss. This is also the way to report poor weed control outcomes.

The APVMA acknowledges there is likely under-reporting of adverse experiences. The magnitude of under-reporting is unknown and provides limitations in quantifying product risk.

Investigations of spray drift are conducted by the relevant state government body, for example: NSW EPA (call Environment Line: 131-555), Biosecurity Queensland (call 132-523) and Chemical Standards Officer (Victoria) (call 03 5430 4463). Industry organisations will also support growers impacted by chemical damage to crops.

If the damage is due to factors other than spray drift, the affected party will need to take legal action and seek compensation themselves.

Related resources

Is poor weed control due to herbicide resistance?

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Can multi-species planting provide effective weed control?

Crop competition is one of the most effective weed control tools available to growers, but some crops simply don’t have a competitive edge.
Dr Andrew Fletcher, a farming systems scientist with CSIRO, says companion planting and intercropping is an option that growers can consider to bolster the competitiveness of an otherwise uncompetitive but valuable crop in the rotation. International research suggests that it can!
Andrew Fletcher, CSIRO farming systems researcher sees potential for multi-species plantings to compete with weeds. Photo: GRDC
“When two or more species are grown together they can occupy ecological niches that might otherwise be taken up by weeds,” he says. “Multi-species plantings have several potential benefits including increased crop yield and improved soil health, but the right combination can also reduce weed biomass by over 50 per cent.”
Multi-species plantings can be quite challenging to integrate into a grain cropping rotation but are more easily used in mixed grain and livestock operations and in intensive pastures for dairy cattle. International research suggests there is a significant untapped opportunity to increase the use of these systems in Australian grain production systems. However, relevant Australian data is scarce and more research is required to understand this untapped potential in Australian systems.    
A mixed-species cover crop can provide multiple soil health benefits, grazing and fodder for livestock and weed control through crop competition and stopping weed seed set.
“Crop competition is a non-herbicide pillar in the WeedSmart Big 6, with the potential to do some serious heavy lifting in terms of weed control,” says Andrew. “Intercropping and companion planting offers a means to bolster the competitiveness of some crops and to keep them in the rotation without risking a weed blow-out.”
What is intercropping, companion planting and mixed-species planting?
In brief: These systems all involve planting two or more crop species together. The combinations are almost limitless.
The details: Intercropping involves planting two or more species together and harvesting the grain of multiple crops. This generally relies on the grain species having different size seed and compatible harvest times.
Companion planting involves two or more species planted together with the intention to harvest grain from one species only after grazing or terminating the other species before seed set.
Sowing a low-growing species like clover between the rows of cereal can compete with weeds in the inter-row area, fix nitrogen and provide the basis of a pasture after the cereal grain is harvested. This is one example of companion planting.
Mixed-species planting is used to describe plantings of several species grown together primarily for the soil health benefits, and that may have potential for grazing and or forage conservation.
How do these systems suppress weed growth?
In brief: These multi-species systems are designed to take up the ecological space that might otherwise present and opportunity for weeds to fill.
The details: Intercropping and companion planting provide additional weed control in situations where one of the species is a relatively poor competitor as a sole crop. By maximising competition, weed growth is suppressed by up to 58 per cent compared to the least competitive species grown on its own. If a competitive crop such as barley is sown in the most competitive configuration possible, there is little additional benefit from adding a second species.
The downside of using this multi-species strategy for weed control is that in-crop herbicide options the choice of herbicides is limited. This is mainly due to the common combinations being a grass crop with a legume or brassica, meaning grass and broadleaf herbicide options can’t be used, except for when one species is terminated. This needs to be factored into decisions around intercropping and companion cropping.  
What are the best-bet combinations for enhanced weed control?
In brief: It depends on the farming system and the other reasons for considering a multi-species planting.
The details: If the aim is to produce grain, the species selected should have easily separated seed. A well-known example is peaola (field pea plus canola). A recent review of historical trials showed that the median yield increase was 31 per cent compared to sole crops of peas and canola, but the weed control effects of peaola in Australia are unquantified.
An effective companion planting combination is wheat undersown with tillage radish and a legume. The broadleaf companions are sprayed out at stem elongation, leaving the cereal to mature through to harvest.
If there is livestock in the farming system, dual purpose combinations such as grazing canola plus vetch and oats can provide excellent weed suppression. This mix could be grazed and then terminated as hay or silage at stem elongation.
Multi-species plantings add a layer of complexity to the farming system, but many growers have taken on the challenge and are reaping the rewards in crop yield, soil health and weed suppression.

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Investigate adverse experiences when using herbicides

A shuttle of glyphosate applied over the top of a Roundup Ready cotton crop was recently shown to also contain a damaging level of 2,4-D impurity, resulting in significant crop injury and yield loss.
The grower involved did not accept the suggestion that the crop damage was due to poor sprayer decontamination or spray drift from a fallow application of 2,4-D, and he was able to prove the problem was due to product impurity.
2,4-D herbicide injury in cotton after the crop was sprayed with glyphosate product contaminated with 2,4-D.
Other shuttles of the same batch may have been applied to fallow weeds where the residual 2,4-D in the glyphosate would have gone unnoticed. Full rate 2,4-D in glyphosate is known to compromise glyphosate efficacy, but studies of low-rate 2,4-D impurity in glyphosate could not be found.
Where can impurities come from?
While the agricultural chemical manufacture and supply chain in Australia is considered first-class and is highly regulated, there is an acceptance that the nil-impurity requirement for the manufacture of agricultural chemicals is unattainable in facilities that use multi-purpose equipment for synthesis, formulation and packaging of products.
Companies therefore apply their own quality assurance standards before releasing products for distribution and sale. If the level of risk posed by certain residual impurities in a product is underestimated, there is potential for instances of crop injury, pesticide residue in produce or poor performance of the product on the intended target weed, fungus or pest.
Mistakes can and do happen within the manufacturing process and chemical supply and distribution chain. To ensure that risks of contamination are minimised and that quality assurance protocols are followed carefully, it is important that any breaches or errors are identified quickly, reported and investigated.
Keep good records of each spray event, including batch numbers of applied product, to help identify the cause of adverse experiences with herbicides.
There are two important things to note: firstly, the current regulations specify that crop protection products must contain nil impurities (other than manufacturing impurities listed in the APVMA standard); and secondly, companies are required to recall product batches when contamination issues are identified. The Australian Pesticide and Veterinary Medicines Authority (APVMA) oversees a highly regulated system of registration, compliance and enforcement on crop protection products.
Assess potential application issues
When misapplication (wrong product applied, incorrect mixing, contaminated product etc) occurs, symptoms of affected plants are usually uniform throughout the treated area. It is often suggested that poor application technique or poor sprayer decontamination is the reason for crop injury or poor weed control results – suggesting a grower ‘own-goal’. Such potential errors must be considered, but if best practice spray techniques and spray rig decontamination procedures have been followed, product impurity should also be considered and investigated.
The chemistry of the product will determine the risk of residues being held within the tank and spray lines of the application rig. This is why there are differences in the sprayer hygiene requirements after using particular products.
Most modern spray rigs have impervious rubber and plastic, or stainless steel components, drastically reducing the risk of chemical absorption and subsequent extraction. Residues on the rubber surfaces are the main concern, and all registered cleaners will physically remove residues when used as directed, but cracked rubber components can present a contamination risk. All filters/strainers must be cleaned and all actuators and taps musts be cycled as the cleaner is run through the spray boom and tank loading system, agitators and tank.
Crop injury or poor weed control that is associated with just one sprayer tank load would suggest sprayer contamination. Effects from contaminated tanks are usually worse at the beginning of the spray run, with damage diminishing with spraying and tank reloading. The field pattern can provide clues to the sprayer filling routine in the field where the crop damage occurred.
The other major reason commonly cited for crop injury in spray drift. Although there is always some small amount of drift when agricultural chemicals are sprayed from a ground rig, the amount is down to ‘virtually safe’ levels within a few tens of metres. If the conditions are very windy, or the boom is too high, or the droplet size too small, spray could drift a few hundred metres from the application ground rig.
Spray droplets may travel a few feet to several kilometres from the targeted area, depending on weather conditions and spray application; but the potential for drift damage decreases with distance because droplets are deposited or become diluted in the atmosphere. The pattern of injury is normally seen most prominently on the section of the field closest to the sprayer that generated the spray drift, and decreases across the field.
During inversion conditions, a similar amount of product is subject to drift, but the drifting product will not dilute as much in the air, so concentrations at specific locations can be higher than expected in non-inversion conditions.
What to do if your crop is damaged or weeds don’t die as expected?
Along with several other possible causes, unintended application of contaminated product should be considered as a potential explanation for crop injury or poor weed control.
Keep in mind that if product impurity is the problem, it is most likely due to a low-dose effect that may be difficult to diagnose or may take longer to express in the target weeds or susceptible crops.
Finding the cause of an ‘adverse experience’ with herbicide is one of the most important reasons to keep accurate and detailed spray records.
If a problem occurs:

Take detailed, time-stamped photographs of the crop or weeds and record everything you know about the crop or fallow management, weather conditions in the weeks prior to the damage being seen, spray history of the field etc. If possible, geotag the photos so they can be easily associated with the correct field.
Record the relevant batch numbers of the chemicals used, which can be checked against the retention samples at the factory if necessary. Collect samples from drums of product used prior to the injury being observed (up to 14 days prior to symptoms being obvious). When you take samples, make sure there are witnesses who can vouch for the voracity of the evidence you have collected. Testing for one impurity (e.g. 2,4-D in glyphosate) costs less than $500 per sample.
Document the injury over time. For example, injury in cotton from low rates of 2,4-D will grow out in two weeks, but injury from higher rates, could last three to four weeks and are the most likely to result in yield loss. Similarly with weeds although the impact may be more difficult to document.
Mark out the affected area in the field to help assess crop yield loss at the end of the season. Note the pattern and intensity of the problem across the field.
Eliminate as many possible causes as you can. Re-assess the application technique and equipment, consider the pattern of damage in the field, look at the weather conditions for the relevant period of time and so on.
Test for herbicide resistance in weeds.
Report the crop damage or poor weed control. The APVMA administers the Adverse Experience Reporting Program, which allows anyone to report a problem with an agricultural chemical, including crop and plant damage, for example, plant death, severe stunting or significant yield loss. This is also the way to report poor weed control outcomes.

The APVMA acknowledges there is likely under-reporting of adverse experiences. The magnitude of under-reporting is unknown and provides limitations in quantifying product risk.
Investigations of spray drift are conducted by the relevant state government body, for example: NSW EPA (call Environment Line: 131-555), Biosecurity Queensland (call 132-523) and Chemical Standards Officer (Victoria) (call 03 5430 4463). Industry organisations will also support growers impacted by chemical damage to crops.
If the damage is due to factors other than spray drift, the affected party will need to take legal action and seek compensation themselves.
Related resources
Is poor weed control due to herbicide resistance?

Article
Ask an Expert

What can I do at harvest to reduce my future weed burden?

As crops mature and harvesters begin reaping, consider the potential fate of seeds ripening on weeds that escaped in-crop control measures.
Peter Newman, WeedSmart’s western extension agronomist, says harvest time is an important opportunity to assess weed burden across the farm and be proactive about driving down the weed seed bank.
“Harvest can either be a super-spreader or a weed suppressing event,” he says. “Small patches of weeds can quickly expand when seed is blown out the back of the harvester. On the other hand, the harvester can be a powerful weed management tool if any one of the harvest weed seed control options are implemented.”
WeedSmart’s western extension agronomist, Peter Newman says efforts made to reduce the spread of weed seed at harvest will soon pay off for growers.
Australian growers have led the world in inventing and adopting harvest weed seed control tools such as impact mills, chaff carts, chaff decks and chaff lining, all of which can reliably destroy over 90 per cent of the weed seed that enters the front of the harvester.    
“In addition to harvest weed seed control there are several other actions in the WeedSmart Big 6 that growers can implement just prior to, during and immediately after harvest that will make a measurable difference to the weed burden in future growing seasons,” says Peter. “The WeedSmart Big 6 tactics are scientifically-proven to reduce the risk of herbicide resistance through diverse herbicide use and cultural control to prevent weed seed set.”
What can I do before harvest to manage late emerged weeds?
In brief: Scout for and map weedy patches. Consider sacrificing small areas of high density weeds. Swathing can be a very effective way to stop seed set of late emerged or resistant weeds. Collect weed seeds for herbicide susceptibility testing.
The details: Growers across Australia use a variety of methods to map weeds – from the simple to the sublime. ‘Dropping a pin’ using the tractor’s GPS mapping system as you travel through a weedy section when spraying or harvesting is easy and provides useful information about the distribution of weeds in the paddock. Many growers have their own drones and use them the collect images or video footage of the crop that can be viewed or analysed to identify high density weed patches.
Collect seed for herbicide susceptibility testing – knowing what still works is vital information for planning next season’s herbicide program. There are three herbicide testing facilities in Australia that are equipped to test weed seed samples – Plant Science Consulting, CSU Herbicide Resistance Testing and UWA Herbicide Resistance Testing.
Collecting weed seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest, collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest, it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis.
Keep samples from different locations separate and details noted on the bag. Only use paper bags (double layer) to collect and send seed samples. Ensure bags are sealed so that the samples don’t mix during transit.
Which harvest weed seed control tool is best for my situation?
In brief: There are six harvest weed seed control tools used in Australia – impact mills, chaff decks, chaff lining, chaff carts, bale direct and narrow windrow burning. Choose the one that best suits your system and budget.
The details: Impact mills are best suited to continuous cropping situations. Residues are retained and evenly spread. Chaff decks have lower capital cost and are well-suited to controlled traffic situations. Chaff carts are popular with grain producers who also run livestock. Bale direct is also expensive but has a good fit in locations where there is access to straw markets. Chaff lining is currently the best ‘entry level’ system and can be used in CTF or non-CTF systems, with best results where the harvester runs on the same track each year. Chaff lining has essentially superseded narrow windrow burning, overcoming the time required and risks involved in burning and reducing the loss of nutrients from the system.
If you haven’t used harvest weed seed control tools before, it doesn’t take long to build and fit a chaff lining chute ready for use this harvest season.
What should I be ready to do straight after harvest?
In brief: Spraying weeds immediately after harvest is fairly common practice. Weeds present may be close to maturity or fresh germinations of summer-active weed species.
The details: Some growers get in early with knockdown herbicide applied under the cutter bar when swathing barley or canola. Consider using the double knock strategy, heavy grazing pressure and possibly a soil residual herbicide that is compatible with your planned crop rotation. Pay particular attention to any weedy patches identified before or during harvest. Stopping seed set at every opportunity is the crux of an effective weed management program.
Give some thought to what might be the underlying cause of weedy patches – fixing problems such as pH and soil nutrition imbalances, waterlogging and spray practices that routinely deliver low doses of herbicide.

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