Kill weeds early to save moisture from summers rainy days

Author: Sharon Watt | Date: 05 Nov 2014

Fallow weed management

Early control of weeds over the coming summer will place southern region grain growers in an advantageous position ahead of next year’s cropping programs.

Research funded by the Grains Research and Development Corporation (GRDC) shows that early control of fallow weeds can lead to significant increases in the levels of stored plant-available water and increased availability of nutrients (especially nitrogen), benefiting crop yield and grain quality.

ICAN agronomic consultant John Cameron says weeds that are not killed or only partially killed will rapidly remove significant amounts summer moisture.

“Early summer weed control is critical,” Mr Cameron said. “Even low populations of weeds can rob the soil of valuable water.”

Mr Cameron, who co-authored the GRDC’s Summer Fallow Weed Management manual, said soil water saved by killing fallow weeds was often stored deep in the profile and its value is often far higher than that of in-crop rainfall.

Deep-stored water is often used by the crop later in a dry season to maintain grain number during the critical period of stem elongation to anthesis. As a result, in the right soil, deep stored soil water can have a water use efficiency of up to 60 kilograms of grain/millimetre of stored water.

Mr Cameron said the amount of nitrogen tied up in dead and dying weeds was another important reason for taking out weeds as soon as possible.

“In many environments, the contribution of the extra nitrogen made available to the next crop from control of summer weeds is just as (and sometimes more) important than the contribution from extra water stored,” he said.

An adequate supply of nitrogen is critical to capture the benefits of high levels of stored soil water and reciprocally, a high water supply is required to capture the benefits of nitrogen fertiliser.

Trials in southern New South Wales have illustrated that for every millimetre of moisture lost via summer weed growth, a further 0.64 kgN/ha was made unavailable to the following crop.

Mr Cameron said uncontrolled weeds not only reduced soil moisture and nitrogen, but could also lead to equipment blockages that impeded efficient sowing operations.

“Maintaining a weed-free fallow generates other benefits as well – it helps growers to sow on-time and can also minimise many disease and insect issues that stem from a ‘green bridge’ being allowed to survive over summer.”

With higher levels of stored soil water, many growers can consider sowing a proportion of their crop earlier than the traditional sowing window if conditions suit.

This ability to sow a crop at the desired time into moisture can deliver yield and risk management benefits.

Research in NSW showed the extra soil moisture saved by controlling summer fallow weeds almost doubled the probability of being able to sow during the month of April.

“Having extra stored soil water improves the chance of successful crop establishment as it reduces the risk of failure associated with crops sown on limited soil water,” Mr Cameron said.

In several recent seasons in the southern and western grains regions, drier growing season conditions have been offset by higher than average fallow rainfall.

In these situations, managing summer rainfall by controlling weeds and maintaining stubble cover has been a key management strategy to optimise profit, according to Mr Cameron.

The new Summer Fallow Weed Management reference manual for grain growers and advisers in the southern and western grains regions draws heavily on the data produced by the much acclaimed GRDC-funded Water Use Efficiency Project.

The manual provides essential information on the value of stored water, the effects of summer weeds on nitrogen, other weed effects on following crops, use of herbicides in the fallow and effective spray application. It also includes grower and researcher case studies and research data.

The Summer Fallow Weed Management manual is available for viewing and downloading via the GRDC website at or hard copies can be ordered through the GRDC’s Ground Cover Direct, free phone 1800 11 00 44 or email

To assist growers with pre-harvest herbicide application to control weeds in winter crops, and post-harvest control of summer weeds, the GRDC has also produced an updated Pre-Harvest Herbicide Use Fact Sheet which is now available at

Further information on effective weed control is available at the GRDC’s new Integrated Weed Management Hub,, which has been developed specifically to help Australian grains industry advisers and growers find the latest weed research and management advice quickly and efficiently, as well as the WeedSmart website,


Media Interviews

John Cameron, ICAN

02 9482 4930 or 0427 209709


Sharon Watt, Porter Novelli

0409 675100

Caption: A crop in which summer fallow weeds were managed (left) and where no fallow weed management occurred (right).  Photo: Col McMaster, NSW DPI.

GRDC Project Code ICN00009, ICN00013, ICN00012

Region South, North