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Power of crop rotation to combat herbicide resistance

Glenn Milne had his first direct contact with herbicide-resistant weeds back in 1996 while working for Dupont on the Darling Downs in Queensland.

He investigated a field infested with sowthistle that had survived an application of chlorsulfuron (Group 2 [B]), and the paddock history revealed the problem. Eleven years of wheat-on-wheat and the annual application of chlorsulfuron (in this case, Glean) was the only weed control tactic used.

“When we tested the sowthistle population for resistance to Glean, the plants survived an application of 10 times the field rate,” he says. “I have never forgotten that it only takes one resistant plant to break an effective herbicide. When the selection pressure is high, and survivors are allowed to set seed, the resistant plants soon dominate, and herbicide options start to dwindle.”

After working for many years as a private agronomist, Glenn now owns and operates the 600 ha family farm on the Jimbour Plains north of Dalby, where he uses crop rotation as a powerful tool within the WeedSmart Big 6 strategy to manage weeds and combat herbicide resistance.

The WeedSmart Big 6 is an integrated weed management program that growers can apply across all cropping systems. A diverse approach that includes several chemical and non-chemical tactics places downward pressure on the weed seed bank and reduces herbicide resistance risk.

“Unfortunately, the herbicide resistance challenges we have faced on this farm have come from biosecurity breaches – through contaminated planting seed, contractor machinery movement and hay brought in from other areas,” he says. “Once resistance weeds establish, you have to take a nil-tolerance approach to weed seed set.”

Glenn’s farming system takes advantage of the variety of crops suited to the Darling Downs region to mix and rotate herbicides, reducing the selection pressure on each mode of action.

“Our dryland program comprises of about 80 or 90 per cent summer crops, mainly sorghum, mungbean and maize, and cotton occasionally,” he says. “We keep the rotation flexible and respond to seasonal conditions. Sorghum is a key crop for us because the sorghum stubble supports mycorrhiza in the soil, which benefits the following crop.”

Their winter crop options include wheat, barley, chickpea and faba bean. Glenn says their rotation has resulted in grass-free winter crops. The imi-tolerant wheat, Sunblade, is a valuable option when previously-applied imi herbicides have persisted in the soil.

“Herbicide resistance is a reality we must factor into our herbicide planning,” says Glenn. “Agronomists have a leading role in advising strategies that minimise the risk, and growers need to get on board. Using a range of herbicides, including residuals, is critical, and sometimes it pays to include some of the new, more expensive products to save money in the long run.”

In addition to his diverse herbicide program, Glenn uses crop competition, the double-knock and scouts for survivors to keep ahead of herbicide resistance.

“Our most difficult weeds are fleabane, tall fleabane and cow vine,” he says. “At the moment, I have three young lads who live nearby to help with chipping and removing any escapes from the paddock. In the future, I plan to add an optical sprayer and potentially employ extra labour to keep on top of the chipping – I really believe it is money well-spent.”

All the winter crops are sown with an air seeder on 38 cm row spacing at high seeding rates to maximise yield and shade out weeds. The summer crops are on the less-competitive 76 cm configuration with a twin-disc MaxEmerge precision seeder. Glenn is considering running two seeders to halve the row spacing for his summer crops.

“The double-knock is good practice, particularly for glyphosate and haloxyfop,” he says. “Agronomists need to stand firm on their recommendations and encourage growers to implement the second knock, either as a herbicide or non-herbicide tactic. We often chip or spot-spray and, if necessary, run a Kelly chain over problem areas.”

“One area we need to improve on is targeting small weeds, even if it means more frequent spraying,” says Glenn. “This is where optical sprayers and even robotics can make a big difference to overall spray efficacy.”

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