Crop Clinic: Focus on Weeds

Wild oats: avoid herbicides which have known resistance; spray topping advice, left.

Herbicide-resistant weeds land up north

Queensland Wheat Research Institute (QWRI) weeds agronomist Steve Walker warned more than 80 advisers attending a GRDC-supported Research Update seminar in Dalby that six broadleaf weeds have resistant populations, including the common brassicas —African turnip weed, Indian hedge mustard, turnip weed and charlock.

"Last year we came across the first resistance in wild oats, to Group A herbicides, and paradoxa grass has since joined the list.

"Both wild oats and paradoxa grass are resistant to Group A Fops herbicides on at least five farms in the North Star region in New South Wales," Dr Walker said.

"We believe both were subject to the same selection pressure for the development of resistance: 10 years' application of Fops herbicides on wild oats and in pulse crops."

Dr Walker said resistance has likely spread to other farms through contaminated wild oats carried by harvesters or through contaminated seed.

Weeds developing resistance are often resistant to all other chemicals in that chemical class.

Dr Walker urged advisers to check farm paddock history before recommending any herbicide, particularly Groups A and B, preferably not to use Group A herbicides at all, and to monitor paddocks closely for low levels of resistance.

"A management option approved for this season is selective spray topping with the chemical Mataven in wheat only," Dr Walker said. Three litres to the hectare of Mataven is the normal rate of application; selective spray topping uses 1.5-2.25 litres with the addition of Uptake.

"You hit the wild oats at early jointing. It is not a substitute for normal weed control of wild oats, but it stops seed set and can clean up paddocks after a normal spray program.

"A combination of Groups A and K makes a good tool for managing wild oats."

Dr Walker said any resistant weed management program required complete weed control throughout non-cropping stages of production and probably would include summer cropping with sorghum or mung beans.

Controlling wild oats could be a long process, given that on one farm the weeds research team had counted between 400 and 500 wild oat seeds per square metre of soil.

Glyphosate warning

Dr Walker said advisers and growers should take heed of a couple of situations in southern New South Walesand in Victoria where extensive use of glyphosate has resulted in the development of resistance in ryegrass.

The worry was that if farmers relied on glyphosate alone in zero-till situations — using it perhaps four times a year — they were applying a lot of selection pressure on different weed populations.

"The application rate is not as important in the development of resistance as the percentage of kill achieved; the higher the selection pressure you apply, the more you select for resistance."

Region North, South, West