Managing weeds after high rainfall

AFTER high rainfall hit the Western Australian grain belt in recent weeks, growers have been warned to manage weeds carefully in order to capitalise on crop potential this growing season.

Alex Paull
Managing weeds after high rainfall

Soil water probes and paddock inspections by the WA Department of Agriculture and Food have revealed soil water profiles in many areas are as wet as they were during winter last year.

Given the wet soil and warm temperatures, weeds are expected to be a problem throughout the grain belt.

DAFWA senior development officer Jeremy Lemon said the ‘soil water bucket’ was full in some areas, and sub-soil moisture would help set grain growers up for a good start to the growing season – provided weeds were well managed.

Lemon said different weed control strategies will be required for different regions.

“In low to medium rainfall areas, summer weeds will reduce soil water and mineralised nitrogen and reduce crop yield potential, if not controlled promptly,” Lemon said.

“However, growers in higher rainfall areas with soils subject to waterlogging should consider leaving weeds to dry the soil, as large weeds can dry soil as effectively as a summer crop.

“These growers could leave paddocks until a minimum of four weeds prior to seeding, before applying herbicide.”

Lemon warned that delayed control would mean weeds would be larger and harder to control, and may support soil pests, such as root lesion nematodes.

“The risk of winter waterlogging and the uncertainty of both autumn and winter rainfall make decisions about delaying weed control in high rainfall areas difficult,” Lemon said.

Soil water simulations and research measurements show that sandy and loam soils will lose about 20-30 millimetres of moisture during a fallow period prior to sowing, if average rainfall prevails and less, if there is further rainfall before the break of the season. 

Lemon said subsoil constraints would also influence soil water availability to the crop.

“Soil water deeper than 30 centimetres is largely protected from evaporation loss. Subsoil constraints, however, will prevent the effective use of this water, such as salinity, soil acidity and compaction,” he said.

Lemon said it was important to monitor paddocks for weeds and the risk of crop pests and diseases.

“It will all depend on what happens between now and the break of the season,” he said.

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