Integrated weed management

Contrary to long-term belief and previous studies, researchers are no longer advocating that farmers sow their weediest paddock last and apply an additional knockdown herbicide.

Alternatively, using other methods, or ‘tools in the toolbox’ such as using integrated weed-management strategies, is more important, impacts greater on yield and helps reduce resistance pressure on herbicides.

One strategy to reduce the pressure on preemergent herbicides is to only use each mode of action twice in a four-year period. The accepted rule of thumb is that if you’re on to a good thing with herbicide, change it.

Crop competition is becoming increasingly effective with increased seeding rate, reduced row spacing, choosing varieties based on growth habit and sowing earlier proving to increase crop competitiveness.

Research presented by University of Adelaide researcher Christopher Preston at Bendigo GRDC Updates explained how time of sowing increased competitiveness and profitability.

“The financial benefit from sowing earlier outweighed the cost of the additional knockdown herbicide application if a paddock was sown later,” he said.

BCG research manager Claire Browne said many years of research results indicated that when wheat was sown earlier, it yielded better than a later sowing date.

“When sown earlier, the soil temperature is warmer, giving rise to more rapid growth and faster canopy closure, thus also providing better weed competition,” she said.

When using an earlier sowing time, growers must choose a variety that is suited to a longer growing season to avoid flowering in high-risk periods.

BCG time-of-sowing trials at Kalkee in 2016 showed both Trojan and Cosmick had a halftonne increase when sown earlier. Variety and time of sowing competitiveness of barley was trialed at Manangatang and Nhill in 2016 with results showing Hindmarsh and Urambie were poorer at competing with weeds compared with Compass, La Trobe and Rosalind.

Row spacing is another method that can be used to increase crop competitiveness.

Australian Herbicide Resistance Institute researcher Peter Newman presented work in Western Australia by DAFWA at the Merredin Research Station that looked at the effect of row spacing over 10 years.

Research showed narrow row spacing increased competitiveness of crops and lowered weed pressure. Use of these practices has shown a reduction in weed-seed set and importantly, an increase in profitability. Delaying sowing did not show a change in weed numbers, even when an adequate knockdown was applied.

The agricultural industry needs to continue to be proactive about integrated weed management and growers need to be aware of options they have in reducing pressure on pre-emergent herbicides.

“The financial benefit from sowing earlier outweighed the cost of the additional knockdown herbicide application if a paddock was sown later”

– Christopher Preston