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Non-herbicide weed control in the Northern region

Diversity in cropping systems and diversity in weeds in the northern GRDC grains region of NSW and Queensland calls for diversity in weed management solutions, including non-herbicide tactics.

Survey work in the region has identified over 70 different weed species that impact on grain production and over 10 per cent of these weed species have confirmed populations within Australia that are resistant to glyphosate and several other chemical modes of action (MOA).

DAF weeds researchers Annie van de Meulen and Michael Widderick.

DAF weeds researchers Annie van de Meulen and Michael Widderick.

Confirmed herbicide resistance in weed populations found in NSW and Queensland.
Source: adapted from a table prepared by M Widderick, DAF.

Mode of Action Resistant weeds
A (fops, dims, dens) wild oats, paradoxa grass, annual ryegrass
B (SUs, imis etc) annual ryegrass, wild oats, paradoxa grass, Indian hedge mustard, charlock, wild radish, turnip weed, African turnip weed, common sowthistle, black bindweed
C (triazines, ureas, amides etc) awnless barnyard grass, liverseed grass
D (DNAs, benzamides etc) annual ryegrass
I (phenoxys, pyridines etc) wild radish
L (bipyridyls i.e. diquat, paraquat) flaxleaf fleabane
M (glycines i.e. glyphosate) annual ryegrass, awnless barnyard grass, liverseed grass, windmill grass, feathertop Rhodes grass, sweet summer grass, flaxleaf fleabane, common sowthistle
Z (dicarboxylic acids etc) wild oats

A recent survey of common sowthistle determined populations as glyphosate resistant if treated seedlings were surviving and reshooting 21 days after glyphosate application. In this testing, glyphosate was applied at the upper label rate for small sized plants (up to 5 leaf).

While the majority of common sowthistle samples collected from central Queensland to central NSW were still susceptible to label rates of glyphosate applied to small seedlings, resistant populations were found throughout the study area, showing that this is not a localised problem but rather the inevitable result of over-reliance on a particular herbicide.

Most northern region weeds are self-pollinated so resistant plants will produce resistant seed. To reduce the likelihood of resistance, a key approach is to use multiple tactics to maintain low weed numbers. While weed numbers are low so too is the risk of resistance genes being present in the population.

To keep these ‘difficult to control’ weeds in check will clearly require other, non-herbicide, tactics to reduce germination and weed seed set. Queensland Department of Agriculture and Fisheries (DAF) researchers in Queensland have been studying common weeds, particularly feathertop Rhodes grass, barnyard grass and common sowthistle, to find weaknesses in each weed’s ecology to help identify non-chemical control tactics that could be part of an effective management system.

Dr Michael Widderick and the DAF weed research team are investigating non-chemical options, including various cover crops, crop competition, strategic tillage, strategic burning and harvest weed seed control options.

He says that although growers are making good use of chemical strategies such as double knock, residual herbicides, spot spraying and weed sensing technology to preserve herbicide efficacy, there is an urgent need to investigate non-chemical options that can be added to a weed management program to target resistant weeds in the northern region, as outlined in the WeedSmart 10 Point Plan.

Strategic tillage

“Most growers are keen to preserve their zero or minimum tillage farming systems that have delivered significant benefits and so are very reluctant to re-introduce cultivation for weed control purposes,” says Dr Widderick. “We are currently researching ways to use cultivation that will have maximum effect on driving down weed numbers while having least impact on the min-till farming system.”

The aim of this research is to investigate the impact of different tillage operations in situations where the weed population has blown out and intensive patch or paddock-scale management is required.

“The key is to understand weed ecology, particularly how seed in the soil seed bank responds to different types of cultivation,” he says.

The team used small plots to determine the effect of burial at different depths on weed seed persistence (long-term viability) and emergence. They also conducted experiments to determine the displacement of seed (glass beads were used to represent the seed) throughout the cultivated zone using four different types of machine—harrows, gyral, off-set discs and one-way discs—compared to the zero till control treatment.

Sowthistle emergence occurs primarily from seeds close to the soil surface with up to 30% of viable seeds emerging over 5 months. Seed can emerge from a depth of up to 2 cm with approximately 4% emergence after 6 months. Seed buried below 5 cm is unable to emerge and can persist at depth.

Seed persistence (the percentage of viable seed after burial) in fleabane was most reduced when seed was buried to a depth of 2 cm and not disturbed for at least two years. Seed buried to a depth of 10 cm remained viable for over 3 years. Feathertop Rhodes grass seed persisted for only 12 months regardless of being left on the surface or buried to 10 cm depth. Barnyard grass however, persisted on the soil surface for up to 2 years and when buried to 10 cm depth remained viable for over three years.

The Gyral machine placed the majority of weed seed in the 0–2 cm and 2–5 cm zones while the offset discs and one-way discs achieving burial of about half the seed below 5 cm depth.

“All species responded to increased tillage intensity with reduced germinations,” says Dr Widderick. “The message here is that infrequent but intense cultivation can be a useful weed management tool within an otherwise zero tillage farming system. Generally, once a deep cultivation has been done there should be no cultivation of that area or paddock for at least four years to avoid the risk of bringing seed back to the soil surface.”

Strategic burning

Feathertop Rhodes grass is known to colonise around mature plants and potentially spread to form distinct weedy patches. Killing the large plant at the centre of the colony is usually not possible using chemical treatments.

Strategic burning of early infestations of this weed can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface from 7500 seeds per m2 to less than 500 seeds per m2.

Growers have made effective use of a flame-thrower to burn large feathertop Rhodes grass plants during the fallow.

Strategic burning of early infestations of feathertop Rhodes grass in a fallow can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface.

Strategic burning of early infestations of feathertop Rhodes grass in a fallow can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface.

Crop competition

Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane.

“Our experiment looked at the effect of crop competition on its own, however, in commercial situations crop competition would be used in conjunction with herbicide applications,” says Dr Widderick. “Narrowing wheat rows from 50 cm to 25 cm spacing had the most marked effect on fleabane seedhead production with an additive advantage if the crop density is also increased from 50 plants per m2 to 100 plants per m2.”


Fleabane seedhead production (Source: M Widderick, DAF)

Project work is continuing to investigate the options for increasing crop competitiveness in sorghum, winter and summer pulses and wheat.

“We are particularly keen to identify ways to improve the competitiveness of sorghum crops, which are often a weak link in northern farming systems,” he says. “Highly competitive summer grasses that also have high tolerance and or resistance to herbicide can gain the upper hand in sorghum crops that often do not achieve canopy closure.”

Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane.

Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane.

Cover crops

Summer fallow periods are heavily reliant on glyphosate for summer grass control. Preliminary research has explored the potential role of cover crops in place of a chemical fallow for control of summer grass weeds. Summer cover crops such as cowpea, lablab and French millet have the potential to smother summer growing weeds, particularly barnyard grass and feathertop Rhodes grass and return large amounts of organic biomass to the soil.

French millet planted on its own, or in combination with the legumes, increased the amount of biomass produced. The higher the biomass production the greater the suppression of weeds. Cover crops will tend to use fallow stored moisture so the team investigated the effect of two termination dates on both subsequent crop yield and on weed numbers.

“Feathertop Rhodes grass germination was minimal after all the cover crop treatments, and the yield of the following wheat crop was comparable to the chemical fallow control and no yield differences were found between treatments,” says Dr Widderick. “For barnyard grass, late termination of the cover crop reduced weed emergences before and after the following wheat crop, however there was a trend toward slightly reduced wheat yield compared to the early termination treatments that tended to boost yield compared to the chemical fallow control.”

“The reduction in barnyard grass emergence and wheat yield are both likely due to reduced soil water following the late terminated cover crops,” he says. “Much more work is required to identify suitable cover crops and define the parameters for their use as a weed management tactic.”

Harvest weed seed control

Harvest weed seed control (HWSC) is known to be an effective strategy and is widely adopted in Western Australia and increasingly in South Australia, Victoria and southern NSW. The efficacy of the current tools such as narrow windrow burning, chaff carts, tramlining or chaff decks, bale direct systems and the new integrated Harrington Seed Destructor to control weeds found in the northern growing region is largely unknown.

Trial work in winter crops to date have revealed that these tools can effectively collect and concentrate brassica weeds growing in wheat and chickpea crops in the northern region. The DAF weed research team is conducting further HWSC trials this winter with a focus on collecting wild oats seed. They are also keen to hear from growers planning to plant sorghum this summer where the team will be assessing the efficacy of various HWSC tools against barnyard grass and feathertop Rhodes grass.

More information – Read the full published paper here: Non-chemical tactics for improved control of key northern region weeds and listen in to the webinar recording with Michael Widderick and Annie van de Meulen.

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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. 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You an follow Jana on Twitter and keep up to date with the HRZ here.

Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

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