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Soil residual herbicides have come full circle in the north

Thursday, 22nd May

Renewed interest in using soil residual herbicides to control fallow weeds in the northern grains region is supported with research and development programs.

NSW DPI extension officer and weeds technical specialist, Tony Cook, said using soil residual chemistry following weed seed set prevention tactics has many advantages.

Mr Cook said there is active research and commercial interest in registering more soil residual products to reduce the number of weeds germinating during the fallow.

“The main advantage is that there is a broad range of soil residual modes of action that allows more effective herbicide rotation,” he said.

“There is a significant level of resistance with post-emergent chemistry, making it more important to try other modes of action before these chemicals lose their potency.”

There are seven commonly used mode of action groups with soil residual herbicide activity currently available. These groups are B, C, D, H, I, J and K and within these groups there are products that target different and essential weed biological processes, effectively increasing the options for herbicide rotation.

“An over-reliance on post-emergent and knock-down herbicides for fallow weed control has put these effective products under great pressure,” said Mr Cook.

“A new strategy is to use glyphosate or paraquat as a knock-down soon after harvest, followed by a soil residual spray.

“This can control several weed germinations and reduce the need for glyphosate sprays during the fallow,” he said.

OLYMPUS DIGITAL CAMERAUpper Horton grain grower, Chris Bowman (left), and NSW DPI weeds technical specialist, Tony Cook, inspect a clean crop of barley. Mr Bowman has implemented many integrated weed management principles on his farm, including grazing, crop competition, strategic tillage and effective use of soil residual and post-emergence herbicides.

While the opportunities certainly exist and a growing bank of trial data is supporting the use of soil residual herbicides as part of an integrated weed control program, Mr Cook emphasises that expectations must be realistic and more experience with soil residual chemistry is required in the northern growing region.

“There can be no skimping or short cuts and these products come with some important management considerations that must be addressed for them to be effective,” he said.

“Incorporation in the soil, uniform application, timely rainfall and stubble management all need to be right for these products to reliably control weeds,” said Mr Cook.

“This is not a ‘set and forget’ option and risks such as residual chemical affecting the following crop must be taken into account.”

Mr Cook recognises that 100 per cent control with soil residual herbicides is rare.

“Under certain field conditions these products can breakdown faster than expected and a flush of weeds might germinate late in the fallow, or the effectiveness may be patchy across a paddock.

“A follow-up with a post-emergent herbicide is usually required,” he said. “Using a detector sprayer to apply the post-emergent product will significantly reduce costs and the amount of post-emergent applied to achieve 100 per cent control and thereby help preserve the pre-emergent chemistry.”

Each product has a plant-back period on the label that must be observed and if the fallow period has been dry it may be necessary to test for residual herbicide levels before planting sensitive crops.

Mr Cook said that organic matter, soil clay content, temperature and rainfall all impact on the breakdown of the chemical in the soil.

“One way to check for residual chemical is to hand-sow indicator plants, such as conventional canola (to test for Group B residues), at different sites across the paddock and to water them in,” he said.

“Do this two to four weeks before the planned sowing date and monitor the indicator plants to see if they germinate normally.”

Other indicator plants are used to test for residues of other MOA groups.

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