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Rethinking weed control

The increasing incidence of herbicide resistance means the days of relying solely on herbicides for effective knock-down of weeds are largely over, according to Northern Grower Alliance CEO Richard Daniel.

Mr Daniel spoke about the management of four key weeds that have taken hold in the north at the Grains Research and Development Corporation (GRDC) Grower Updates in Queensland and Northern NSW last year.

He said the summer grass weeds feathertop Rhodes grass, barnyard grass, and windmill grass, together with flaxleaf fleabane, were all showing signs of resistance to glyphosate, or had levels of natural tolerance.

“What we have done is pushed our system through the overuse of glyphosate and we are now unfortunately starting to see the results that were forecast back 15 or 20 years ago,” Mr Daniel said.

To rectify the situation it is vital that non-herbicide tactics are incorporated into weed management systems.

“We are at a stage where there are fewer and fewer new chemistry options, so what we have to do is safeguard the products we have got as much as we possibly can,” he said.

“The only way we are going to be able to do this is by introducing other tactics and techniques to assist weed management and focus on reducing the seed bank replenishment for these key weeds.”

Mr Daniel said strategic cultivation was one of the tools that growers were going to be forced to seriously consider.

“Most of the weeds we are having problems with now are shallow germinating weeds, which need light to germinate. For these species a very light cultivation can reduce weed emergence by up to 80 or 90 per cent,” he said.

“Strategies where a cultivation is used and then followed by a residual herbicide can also potentially remove up to two weed generations.

“It is really a case of working out where the least intrusive use of tillage can be implemented to take a bit of pressure off the herbicides.”

Windrow burning is another technique which is gaining momentum where the majority of weed seeds are collected in narrow windrows and then controlled by burning.

This is a technique that can maintain the benefits of stubble management over the majority of the paddock but dramatically reduce the seed bank replenishment.

Mr Daniel said in all cases management required “moving the battle front”. For example, flaxleaf fleabane was an extremely difficult weed to control in the summer fallow, particularly when plants were mature and toughened off.

However, it has proved to be quite easily controlled by a range of residual herbicides in the winter crop or in the winter fallow leading into the summer crop.

“Instead of bashing our heads against a very tough target in the middle of summer, particularly with something like fleabane, you move the battle to a part of the life cycle where the weed is much more exposed or much more susceptible to control.”

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