Read time: 8 minutes

Remove the fetters from crops and they’ll trample the weeds

Weeds can exploit situations where crops fail to germinate or grow less vigorously. This does not usually mean that the weeds prefer soils that have constraints such as acidity, compaction or low nutrition status.

While crop responses to changes in soil pH are extensively researched, there is far less research available that quantifies the impact of the amelioration of soil acidity on weed growth.

Gauz Asam and Catherine Borger

Gauz Azam and Catherine Borger

To help fill this knowledge gap, research scientists Catherine Borger, Gaus Azam, Chris Gazey, Andrew van Burgel and Craig Scanlan from the Department of Primary Industries and Regional Development, Western Australia (DPIRD), have recently published the results from long-term studies measuring the impact of ameliorating soil acidity on the growth of annual (rigid) ryegrass (Lolium rigidum) in wheat.

Practical tips:

  • In acidic soils, the application of lime increases soil pH and improves the crop’s competitive ability against annual ryegrass.
  • Lime applications increase initial growth of both wheat and ryegrass.
  • The application of lime in previous years reduced ryegrass density, biomass, and seed production in wheat crops in 2018.
  • Lime increased wheat tiller number and, at one location, increased yield.
  • Crop and weed establishment may be poor in the season following soil amelioration. The crop often ‘catches up’ later in the season.
  • Reacidification is common. An ongoing liming program is likely to be required to maintain the competitive edge of crops over weeds such as annual ryegrass.

Most crops and pastures grow best in soils with a pH between 5.5 and 8, but some crops, such as barley, are more sensitive to soil pH than others. Similarly, some weeds are able to grow in hostile environments but will often grow better when the pH is in the optimal range for crop growth. For example, annual ryegrass competes very strongly with wheat in low pH soils, but actually grows best in the same pH range as crops. On the other hand, there is some evidence that wild radish prefers acidic soils.

Identifying soil constraints can involve detailed investigations and there are commonly multiple constraints at play. With approximately half of the agricultural soils in Australia having a surface pH of 5.5 or less, this constraint alone can be responsible for significant yield loss. Conversely, South Australian farmers are more likely to have to contend with high pH soil constraints, with 60 per cent of agricultural soils in that state being highly alkaline.

Experimental design features

These experiments were conducted at field sites in the Merredin and Wongan Hills shires in Western Australia. The scope of this research included two experiments:

  1. A field experiment was conducted from 2016 to 2018 at DPIRD’s Merredin Research Facility on naturally acidic soil to investigate the effect of crop rotation (continuous wheat and wheat–chemical fallow), lime incorporation (nil and to 15 cm) and lime rate (0, 2, 4 and 6 t/ha). Wheat and annual ryegrass production was measured in the 2018 season.
  2. A field experiment at DPIRD’s Wongan Hills Research Facility was established in 1994 on soil with low pH as a result of agricultural practices. The trial investigated the long-term effect of lime rate (0, 0.5, 1, 2 and 4 t/ha applied in 1994) and top-up applications of 0 or 1.5 t/ha in 1998 and 0 or 3 t/ha in 2014. In 2018 soil was cultivated to a depth of 0, 15 or 25 cm prior to seeding. Wheat and annual ryegrass production was measured in the 2018 season.

Detailed findings

Crop rotation and lime at Merredin

Within the continuous wheat rotation at Merredin, increasing rates of lime increased surface soil pH (0–5 cm) from 4.9 to 6.0 and pH at depth (10–15 cm) from 4.3 to 4.7 with no incorporation. Increasing rates of lime reduced density, biomass, and seed production of ryegrass and increased wheat tiller number and yield.

Incorporation of lime had no significant effect on wheat yield or ryegrass biomass, even though incorporation increased pH at depth (10–15 cm) from 4.2 to 5.1.

A wheat-fallow rotation reduced ryegrass density, biomass and seed production and increased yield compared to the continuous wheat system. Lime rate and incorporation within the wheat-fallow system increased soil pH (0–5 cm) from 4.9 to 5.8, but had no effect on ryegrass due to uniformly low weed pressure. Fallowing is a very effective weed control measure, but is unlikely to be a profitable option unless weed pressure is very high.

Long-term effects of lime application at Wongan Hills

Cumulative lime application at the Wongan Hills site increased soil pH from 5.6 to 6.4 (0–10 cm), 4.6 to 5.4 (10–20 cm), and 4.1 to 4.9 (20–30 cm).

Lime applications in 1994 and 2014 had long-lasting impact on weed growth, resulting in reduced ryegrass density, biomass and seed production in the 2018 crop. The lower rates applied in 1998 had no significant impact on ryegrass density and seed production.

Wheat density was not affected by lime, but tiller number increased with increasing rates of lime applied in 1994 and 2014. The slight increase to wheat yield following application of lime was not significant and incorporation of lime in 2018 did not affect ryegrass or wheat production.

Deep tillage increased pH at depth (20–30 cm) from 4.2 to 5.2. The interaction between lime application in 2014 and incorporation of lime in 2018 was significant for ryegrass, with weed density, biomass and seed production decreasing with increasing depth of tillage in those plots where lime was not applied in 2014 (0 t/ha treatment). Deep tillage did not significantly affect ryegrass in plots where 3 t/ha of lime was applied in 2014, as ryegrass density was already very low across all tillage treatments. By 2018, the lime applied in 2014 had already done the heavy lifting in terms of reducing weed pressure in the 3 t/ha plots.

WeedSmart conclusion

Applying and incorporating lime is the best way to increase the pH of acidic soils, but it usually takes several years before a surface lime application has a measurable effect on soil pH at depth. Incorporation is the best way to speed up the process and also releases other soil nutrients to boost crop growth.

By default, the incorporation of lime by tillage or inversion also buries weed seed, placing at least a portion of the seed bank deeper in the soil profile and prohibiting germination. Annual ryegrass seed has optimal emergence from a depth of 1 or 2 cm. Emergence reduces with increasing depth and ryegrass does not emerge from depths of 10 cm or more. Even when buried, some seed can remain viable and emerge if the next sowing operation brings the seed back near the soil surface.

This trial work confirms the importance of crop competition in a diverse weed control program. Addressing soil constraints, such as low pH (and the associated aluminium toxicity), enables the crop to compete strongly with weeds such as annual ryegrass – reducing weed growth and seed production.

This research was conducted by researchers from the Department of Primary Industries and Regional Development, Western Australia and was supported by the Grains Research and Development Corporation, a WeedSmart financial partner, through the Soil Constraints Initiative—Innovative Approaches to Managing Subsoil Acidity (DAW00252) project.

References

Borger CPD, Azam G, Gazey C, van Burgel A, Scanlan CA (2020) Ameliorating soil acidity–reduced growth of rigid ryegrass (Lolium rigidum) in wheat. Weed Sci. 68: 426–433. doi: 10.1017/wsc.2020.38

Related Articles

View all
Article
News

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
News

What to expect at WeedSmart Week 2021

Big 6 at WeedSmart Week 2021 – Double knock to protect glyphosate
The WeedSmart Forum is set for Tuesday 17 August, 2021 at the Civic Centre in Esperance WA. The program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
Greg Warren from Farm and General in Esperance will be sharing his thoughts on the control of weeds like summer-germinating ryegrass, marshmallow, fleabane and portulaca.
He says the growers around Esperance are tackling glyphosate resistance in annual ryegrass, along with brome and barley grass and other emerging weeds using a range of integrated control tactics. The double knock plays a key role in preserving glyphosate (and soil moisture) and providing a clean seed bed for planting crops.

 
Big 6 at WeedSmart Week 2021 – Increase crop competition
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the farms hosting a visit during WeedSmart Week is Warrakirri’s 12,800 cropping operation at Condingup. Farm manager Con Murphy has implemented a variety of tactics to combat their main weeds – annual ryegrass and wild radish. Since 2015 the farm has undergone an intensive soil amelioration program to improve the drainage and ameliorate the sandy soils across the farm.
Con says the benefits have been seen in better germination and establishment that sets their cereal, pulse and canola crops up to compete strongly with weeds. There is also a benefit at the end of the season where rain in August or September enters the soil profile without causing waterlogging, and providing a better finish for their crops.
Since 2016-17 about 80% of the farm has been ripped and a portion has been ripped 2 or 3 times because the sandy soils tend to slump after substantial rainfall events, recreating the hardpan.
Con will be showing the WeedSmart tour group how their ripping, drainage, liming and spading program has helped grow more crop and less weeds!
Listen to the podcast with Warrakirri’s Con Murphy talking about the impact of improved drainage on crop competition

Big 6 at WeedSmart Week 2021 – Implement harvest weed seed control
The WeedSmart Week machinery display is set for Wednesday 18 August, 2021 at Dave Campbell’s shed near Esperance WA. The 3-day WeedSmart Week program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
We’ve saved the harvest weed seed control discussion for the machinery session on Wednesday 18 August. Ben White from Kondinin Group will host the machinery session with spray and harvesting gear on display including impact mills from Seed Terminator, Redekop and iHSD (both hydraulic and belt-driven), Emar chaff deck, and spray technologies including Goldacres’ G6 Crop Cruiser series 2, and weed detection technologies using drones and weed identifying cameras (green on green).
Ben White, Kondinin Group (Photo: Melissa Powell, courtesy of GRDC)
Growers doing the WeedSmart Big 6
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the growers who will open up their farm for a visit is Adrian Perks who farms at Condingup, 70 km north-east of Esperance. Adrian runs a continuous cropping program on his 4300 ha property, growing canola, wheat, barley, faba beans and lupins. This diverse rotation allows him to mix and rotate both chemical and non-chemical weed control tactics. Over half of Adrian’s farm is sandplain, on which he has implemented a soil amelioration program to address non-wetting to increase the competitiveness of his crops. He currently uses chaff decks for harvest weed seed control and is introducing an impact mill this season. Adrian monitors the tramtracks for weed growth and if he feels the weed pressure is too high, he uses a shielded sprayer to reduce seed set. The bus tour will include four farm visits and a machinery display.
Listen to Adrian on the Regional Update podcast.
Adrian Perkins, Condingup WA
 

Article
News

WeedSmart agronomist set to tackle high rainfall zone weeds

Every locality has its own spectrum of weeds, and growers face different opportunities and challenges regarding the control tactics they can employ.
The WeedSmart Big 6 approach is a practical way to ensure that an integrated weed management program is put in place that disrupts weed seed production and the evolution of herbicide resistance.
Commencing in January 2021, Jana Dixon has joined the WeedSmart team of extension agronomists, with a focus on applying the Big 6 to manage weeds in the high rainfall cropping systems of southern Australia – from Esperance in WA to south-eastern SA, Tasmania and south-western Victoria.
Jana will add to the dedicated and experienced extension agronomists on the WeedSmart team with Peter Newman in the Western region, Chris Davey in the South, Greg and Kirrily Condon in the East and Paul McIntosh in the North.
Jana Dixon has joined the WeedSmart team of extension agronomists, with a focus on applying the Big 6 to manage weeds in the high rainfall cropping systems of southern Australia – from Esperance in WA to south-eastern SA, Tasmania and south-western Victoria.
Jana hails from the Mid North of SA, and began working at Pinion Advisory (previously Rural Directions) while she was studying agriculture at the University of Adelaide. She has been employed full-time at Pinion Advisory since January 2019 as an agribusiness consultant, based in Clare, and spends most of her time delivering agronomy and farm business advice to clients from a wide range of cropping regions in South Australia.
Pinion Advisory is a foundation WeedSmart sponsor and Jana has been involved in two WeedSmart Week events already – the first as a participant and grower group organiser at the Horsham event in 2019 and then as the local organiser for WeedSmart Week 2020 in Clare.
In welcoming her to the WeedSmart team, program manager Lisa Mayer says Jana brings energy, commitment and insight to deliver communications focussed on the southern region’s high rainfall regions.
“Growers in the southern high rainfall zones are facing some serious issues with herbicide resistance influencing their farming decisions,” says Ms Mayer. “Jana will be engaging with agronomists, growers and researchers in each of the distinct high rainfall zones to understand the complexities and look for practical ways to apply the WeedSmart Big 6 in various cropping scenarios.”
“We plan to deliver WeedSmart Week in Esperance, part of Western Australia’s high rainfall cropping zone, in August 2021 and Jana will play a key role in the planning and delivering of our annual 3-day flagship event.”
Jana says her experience with the WeedSmart program has been very positive and she has been particularly impressed with the support the program has from all sectors of the grains industry.
Newly appointed WeedSmart extension agronomist, Jana Dixon (green cap) leading discussions with farm visit host, Ben Marshman, Owen SA, and growers and agronomists attending WeedSmart Week 2020 in Clare.
“I have spoken to many growers and agronomists who have found real value in the information that the WeedSmart program delivers,” she says. “For many it is as much about considering another operator’s philosophy on dealing with weeds, and taking a fresh look at their own systems, rather than just learning about a new tactic or the traits of a new herbicide in isolation from the big picture.”
She says the high calibre of industry people who contribute their time and expertise to the program is testament to the value WeedSmart has to agribusiness, growers, agronomists and researchers alike.
In taking on the responsibility for delivering information tailored for the high rainfall zones Jana says she is pleased to have an extensive network of contacts through Pinion Advisory, with offices in a number of high rainfall areas to provide easy access to local agronomists and growers. She is also aware that there are major differences in weed spectrums and farming systems in each high rainfall zone and plans to take full advantage of the opportunity this role presents to expand her understanding of different approaches to weed management.
“The long and favourable growing season and the associated prolonged periods of weed germination, is a key factor that I see potentially impacting on a grower’s weed management strategies in these regions,” she says. “On the other hand, access to highly diverse rotations and a focus on crop competition are two strategies that can play an important role in achieving excellent weed management in these regions.”
“I am keen to engage with anyone working and farming in the high rainfall zones to build my knowledge and understanding,” she says. “And to create opportunities to develop and extend the WeedSmart Big 6 strategies, both herbicide and non-herbicide, that work in each area and in different situations.”
WeedSmart is the industry voice delivering science-backed weed control solutions with support from the Grains Research and Development Corporation (GRDC), major herbicide, machinery and seed companies, and university and government research partners, all of whom have a stake in sustainable farming systems.
You an follow Jana on Twitter and keep up to date with the HRZ here.

Subscribe to the WeedSmart Newsletter