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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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