With pulses an essential component of systems that have high-break-crop-intensity, growers and advisers are cautioned that adoption of alternative weed management practices will be essential to reduce weed herbicide resistance issues.

Research into sustainable herbicide use has been a focus of a strategic joint investment of GRDC and the South Australian Research and Development Institute (SARDI).

SARDI senior research agronomist (Weed Ecology), Dr Navneet Aggarwal, says herbicide-tolerant pulse crops have become an important part of rotations due to: their high market price; ability to fix nitrogen and provide a break in the disease cycle; and the fact they offer other herbicide options.

The research has demonstrated that herbicide longevity can be enhanced through an integrated approach which includes:

• considered use of tolerance traits;
• better rotation of chemistries;
• tactics such as wick wiping and clipping; and
• increased seeding rates to increase crop competition and reduce weed seed establishment.

However, complacency in herbicide usage could be affecting the success of pulses as break crops, with research finding concerning levels of herbicide resistance.

"Initially, ryegrass control in break crops relied on Group A chemistry and its overuse contributed to increased herbicide resistance," Dr Aggarwal says.

"Management then moved to herbicides with different modes of action, such as groups D, J and K. Unfortunately, we are now seeing resistance to these groups as well."

Paddock surveys

The research team surveyed 36 paddocks across South Australia to investigate the weed species and herbicide resistance issues in high-break-crop-intensity rotations.

"The selected paddocks had imidazolinone-tolerant break crops, such as PBA Hurricane XT lentils (PBR), or Clearfield® canola, which had been sown at least twice in the past five to six years, as a dominant break crop," Dr Aggarwal says.

"In addition, paddocks with two non-imidazolinone break crops, such as conventional lentils, conventional or triazine-tolerant canola, field peas, chickpeas, faba beans and lupins were also included."

At each site, seeds from ryegrass and two dominant broadleaf weed species were collected and screened for resistance. The results were alarming, Dr Aggarwal says.

Key findings

"We found ryegrass and common sowthistle to be the dominant weed species in the high-break-crop-intensity focus paddocks," Dr Aggarwal says.

"Resistance screening detected ryegrass resistance to sulfonylurea and pinoxaden-cloquintocet-mexyl (Axial®) herbicides in almost all paddocks.

"Ryegrass resistance to imidazolinone herbicides was observed in both paddocks with high history of imidazolinone use and paddocks with no history of imidazolinone use.

"In these paddocks, 46 per cent of ryegrass populations were found to be resistant and 21 per cent of populations had started developing resistance to clethodim.

"Resistance to the Group J and Group K herbicides prosulfocarb + S-metolachlor (Boxer Gold®) and pyroxasulfone (Sakura®) was also confirmed in ryegrass."

Dr Aggarwal says these results clearly show herbicide resistance has started to limit the effectiveness of break crops as rotational tools and highlights the need for strategic use of these chemistries.

"Strategic chemical use might include rotational use of the Group D herbicide propyzamide with Group J and Group K herbicides in the break crop phase, as annual ryegrass from all high-break-crop-intensity paddocks showed a high level of susceptibility to propyzamide," he says.

"As a rule of thumb, herbicides from the same mode of action should be avoided for two consecutive years on the same land.

"Further, over half of the common sowthistle biotypes found resistant to Group B sulfonylureas and imidazolinone herbicides raises concerns for judicious use of imi-tolerant technology."

Ongoing research

Dr Aggarwal says agronomic tactics to increase crop competition over weeds, and practices such as wick wiping and clipping weeds mechanically at their reproductive stage just above the crop canopy, were promising in reducing broadleaf weeds' seed set in pulse crops.

"We investigated the planting densities of faba beans and found that increased seeding densities from the standard practice of 24 plants per square metre to 30 plants/m2 has the potential to reduce seed set of broadleaf weeds such as vetch by up to 45 per cent," he says.

A research trial established at Turretfield Research Centre in SA in 2019 tested the response of wild turnip to wick wiping with a glyphosate-MCPA mixture and the application of weed clipping at different growth stages.

"Early wick wiping treatments up to two weeks after wild turnip pod initiation resulted in up to 62 to 74 per cent reduced weed pod set compared to no wick wiping," Dr Aggarwal says.

"In comparison, later clipping treatments at three weeks after wild turnip pod initiation (green and squashy weed seeds) reduced pod set by up to 94 per cent compared to the no-clipping treatment."

Research has also been investigating best management practices with new and emerging herbicide-tolerant pulse technologies.

Research findings with the SARDI Group C-tolerant lentil germplasm has shown potential to control prickly lettuce and Group B-resistant common sowthistle.

"This will offer growers alternative herbicide options, particularly for controlling Group B-tolerant broadleaf weeds in pulse crops," Dr Aggarwal says.

While this technology is in the pipeline, he says there are practical steps all growers can take now to better manage herbicide inputs, he says.

"Being proactive in the approach to herbicide management through rotating different modes of action is essential and will help sustain the use of chemistries in the long term," he says.

"Above all, growers must follow the chemical labels and seek advice where required.

"The adoption of proven strategies such as crop topping and harvest weed seed collection measures, wherever possible, will help to prevent rapid spread of resistant weeds and their soil seedbank build-up."

The outcomes of this project were delivered via the GRDC-SARDI strategic research partnership.

This partnership has facilitated a range of projects which provide innovative research outcomes relevant to SA's cropping zones.

GRDC Research Code DAS00168BA

More information: Dr Navneet Aggarwal, 08 8841 2404, navneet.aggarwal@sa.gov.au