Western Australian agronomist Grant Thompson has a clear message for southern region grain growers when it comes to wild radish – aim for early control when weeds are small and learn from the mistakes made interstate. WA growers are battling radish that is becoming resistant to multiple herbicide groups.
Target: wild radish
- Best control is achieved if spraying early at two-leaf stage. Do not wait for five leaves.
- If radish density is high, a two-spray strategy can improve efficacy and grain yield.
- Some radish plants can shield others from contact with spray so a high water rate is required for best plant coverage.
- Preserve new Group H herbicides but in the process do not abuse or overuse other groups, particularly Group C.
The problem is a combination of factors – overuse of cheap Group I and Group B chemistry, exposure to Group F and Group C herbicides used in cereal crops, expensive broadleaf herbicides used below label rates, poor application conditions and water volumes that are too low for effective coverage.
Cross-pollination between radish plants is ensuring that multiple resistance genes are accumulating, or ‘stacking’, in paddocks.
Mr Thompson, from Crop Circle Consulting, says some paddocks with highly resistant populations may have four or five radish germinations per year.
“The seed has a long dormancy and in summer it can flower within three weeks of germination,” he says.
“Our trial work has focused on finding alternative sequential spraying options to avoid two sprays of Group H pyrasulfotole chemistry, such as new herbicides Precept® and Velocity®, for radish control in cereals.
Wild radish resistance is an emerging problem in Victoria’s Mallee region, but recent research by BCG is finding control of the weed is effective with a two-spray approach.
PHOTO: Simon Craig, BCG
“We are trying to avoid speeding up the onset of Group H resistance.” Mr Thompson’s recommendations for radish control include:
- spray early for most effective control – WA trial work has shown that spraying radish at the two-leaf stage is far more effective than trying to tackle it at a later maturity (this is supported by BCG research in the Wimmera–Mallee);
- two sprays are best for resistant populations – for radish with multiple-herbicide-group resistance a two-spray strategy involving an early spray at two-leaf stage followed by a second spray at the five-leaf spray is most effective; WA trials found leaving out the early spray puts extra pressure on success of the second spray for control;
- canopy penetration – high-density radish populations (such as 200 plants per square metre) can cause shading of radish by other plants; delaying spraying to five-leaf stage in big canopies will not achieve good coverage; and
- cultural options – consider methods such as tickling paddocks for good germination, chaff carts, and seed capture at harvest by creating windrows behind the header and burning; however, this selects for radish biotypes that drop pods before harvest.
“The take-home message is when you go in early at the two-leaf stage with the registered options that are available to you, it’s pretty easy to kill small radish,” Mr Thompson says.
Target two-leaf spray for control
While wild radish resistance has not yet attracted the same focus as grass weed resistance in the Wimmera–Mallee, some growers are finding the weed harder to control.
BCG, in Birchip, Victoria, conducted a GRDC-funded fast-track project last year to survey the distribution of wild radish resistance and gain a clearer picture of the extent of the problem.
BCG research manager Simon Craig says the survey showed the breakdown of some chemicals in controlling wild radish had started.
“With the chemicals we have been using, it is almost starting to ring alarm bells in terms of how WA growers noticed resistance building five to 10 years ago,” he says.
The project involved testing 20 commercial paddocks thought to have resistant radish. Sites were selected and 12 different chemicals were used and assessed for plant reduction, efficacy and seedbanks, then seeds were sent for testing.
Label rates of Precept® 300 (one litre per hectare) and Lexone® (200 grams/ha) gave 96 per cent control, while Flight® EC (720 millilitres/ha) gave 93 per cent control.
Growers need to be aware of the potential for crop damage with use of metribuzin, particularly on light, sandy soils. Velocity® (670mL/ha) and Jaguar® (1L/ha) were also very good; however, crop shading contributed to slightly lower percentage control (approximately 80 per cent).
“With the amines and esters, the results were mixed: some gave poor control and some were excellent, depending on the paddock and its history. The paddocks were chosen by agronomists because of their known history as being difficult to control.”
BCG’s Emerging Weeds project with the GRDC found best radish control is achieved at the two to three-leaf stage, compared with spraying at four to five leaves or GS30.
Velocity® (670mL/ha) achieved total control at all three timings while Flight® (720mL/ha) and Jaguar® (500mL/ha) controlled all radish plants at the two to three-leaf spray.
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