15 June 1998

At least seven weed species have evolved resistance to at least one of three of the most commonly used herbicide groups in Queensland's wheat and sorghum industry, according to a University of Queensland study.

The three-year project funded by a $152,838 Grains Research and Development Corporation (GRDC) grant was led by Associate Professor Steve Adkins from the University's School of Land and Food and Dr Steve Walker from the Queensland Wheat Research Institute, with the assistance of University of Queensland scientific officer Don Wills.

The study documented the world's first cases of herbicide resistance among four weed species - turnip weed, climbing buckwheat, African turnip weed, and liverseed grass.

It has since led to a further three-year, $250,000 GRDC grant enabling the research team to monitor new farming strategies implemented in the light of the weed resistance findings and to examine possible wild oat herbicide resistance in the northern New South Wales wheat industry.

For the study, 17 weed species common to Queensland's north-east grain region were investigated for resistance to herbicides representing three chemical groups - clodinafop-propargyl (group A), chlorsulfron (group B) and atrazine (group C).

The research showed that resistance had developed after five years group A use, three to 10 years group B use and two to 15 years group C use.

Dr Adkins said the study found that even though the herbicides were highly effective, resistance was likely to develop when solely used for weed control.

He said the study's main recommendation was that farmers and agronomists adopt a more integrated approach to weed management, requiring a thorough understanding of resistant weed biology including the inheritance of resistance, seed longevity, dormancy and germination characteristics.

'At present, farmers experiencing herbicide resistance have tended to use alternative weed control methods which may be more expensive, less effective and less convenient,' Dr Adkins said.

'In addition, agronomists, who give advice in weed control need to change their approach from using simple recipes with herbicides for weed control to a more integrated approach to weed management.

'Integrated management means using the viable tools for weed control together, for example, herbicides in combination with tillage, seed-cleaning or a biological control agent.'

Mr Wills said weeds depleted the soil of nutrients and water, competing for available light with the desired crop. They could also clamber over a crop smothering it which interfered with harvesting or contaminated the end product, he said.

He said many farmers involved in the study had already started modifying their farming practices to enhance weed management.

'Some have turned portions of their land back into pasture so that sheep can keep the weeds down while the soil regenerates for future cropping. The old practice of continuous cropping on the same piece of land year-in, year-out is not sustainable anymore,' Mr Wills said.

'Initially, there was resistance from some farmers to modifying practices as it spelt less profit in the short-term. It was thought having a summer and a winter crop was being flexible enough.'

The study found:

- Two collections of turnip weed, three collections of Indian hedge mustard, five collections of common sowthistle, one collection of climbing buckwheat and one collection of African turnip weed were resistant to the recommended rate of group B. Resistance was not discovered in London rocket, wild turnip, spiny emix, New Zealand spinach and paradoxa grass;

- One collection of wild oats was resistant to the recommended rate of group A;

- Two collections of liverseed grass were resistant and three collections possibly resistant to the recommended rate of group C. Resistance was not discovered in bladder ketmia, mintweed, parthenium weed, green amaranth and barnyard grass.

The project has resulted in four technicians being trained in aspects of research into weed science, an informative newsletter, Herbicide Resistance Reporter, being distributed to more than 150 farmers and agronomists twice a year, and various lectures and seminars being delivered by the project team.

For the project, 109 collections of 11 species were sampled from farmers' paddocks in the Goondiwindi, Moree and Roma districts during winter months. Seed was collected from surviving plants sprayed with group A and B.

During summer months, 66 collections of six species were sampled in the eastern Darling Downs and central Queensland. Seeds were collected from surviving plants sprayed with group C. All seeds were grown in University greenhouses, treated with the appropriate herbicide and tested for resistance.

For more information, contact Dr Adkins (telephone 07 3365 2072) or Mr Wills (telephone 07 3365 2165).