Winter crops need a good summer weeds kill

Photo of Grass Patch grower Ron Longbottom

Grass Patch grower Ron Longbottom will be getting the boomspray out this harvest to get on top of problem summer weeds and help optimise the yield potential of next year’s crops.

PHOTO: Evan Collis

Controlling weeds during the next few months will help to set up 2015 crops for higher potential yields and lower the long-term weed seed bank.

Early weeds control, often starting during harvest, has been shown to significantly increase the amount of stored plant-available water (PAW) from any summer rain as well as nutrient availability – especially nitrogen – for subsequent winter crops.

Disease and insect pressure is also lowered by removing the weed/volunteer ‘green bridge’. In addition, crops can be planted earlier and on more marginal rainfall events with less risk of failure.

Effective summer fallow weed management provides options for using longer-season crop varieties and allows more of the main season crop to be planted on time – leading to potentially higher average yields across the farm as a whole.

Soil water that is saved by killing summer weeds is also often stored deep in the profile, where winter crops can access it late in a dry season to maintain grain numbers during the critical period of stem elongation to anthesis.

Tips and tools for managing common summer weeds in the western and southern regions are contained in a new GRDC Summer Fallow Weed Management manual.

The experiences of Grass Patch grain growers Ron and Kerrie Longbottom are featured in the manual.

Their main summer weeds are Afghan melon (Citrullus lanatus), prickly paddymelon (Cucumis myriocarpus), volunteer crops and, increasingly, small-flowered mallow (Malva parviflora).

In the early 1990s, Ron noticed that areas of crop where summer weeds had not been controlled (due to spray misses) always yielded less than areas where summer weeds were controlled. This kick-started a more rigorous summer-weed control program on his property.

He says he now gets his boomsprays out if he sees any weeds in the crop at harvest and occasionally he uses a summer cultivation to stimulate weed germination and control.

“Tramlines are used to guide the spraying and – with reasonable weather – the whole farm can be sprayed in seven to 10 days,” he says.

“Spraying starts anywhere from 2am to 4am and continues until about 7am, when the air temperature reaches 28°C. Dust becomes a big problem as the temperature increases and if there is too much dust, the weed control along the wheel tracks is poor.”

ICAN Rural principal and GRDC Summer Fallow Weed Management manual contributor John Cameron says growers spraying at night should be wary of surface temperature inversions because these can dramatically increase drift potential.

He recommends staying alert for the development of inversion conditions – around the time of sunrise is common – and not spraying at any time when a temperature inversion occurs.

Trials run by the Department of Agriculture and Food, Western Australia (DAFWA), on the Longbottoms’ property have shown soil moisture and nitrogen levels can be significantly higher where summer weeds are controlled early.

In some years, up to 17 kilograms per hectare more mineral nitrogen in the surface soil layers and 40 millimetres of extra soil moisture have been recorded at seeding following early summer weed control. This has equated to an extra 0.8 tonnes per hectare in crop yields and a 0.5 per cent improvement in grain protein.

Ron says having extra soil moisture in autumn provides him with a wider sowing window and he closely monitors soil water, using capacitance probes, as the season progresses to determine his nitrogen strategies for the year.

More information:

John Cameron, ICAN Rural
02 9482 4930

The Summer Fallow Weed Management manual is available at:

Weather essentials for pesticide application is available at:

A fact sheet on surface temperature inversions and spraying is available at:


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GRDC Project Code ICN00012

Region West