Stop moisture and nitrogen loss: manage summer weeds

Photo of fallow trial site

The Tottenham fallow trial site where the summer fallow management research was conducted.

Photo of fallow

Fallow at the Tottenham trial site.

PHOTOS: Colin McMaster

Trials highlight the value of controlling weeds to prevent losses of nitrogen and soil moisture in summer fallows.

Every dollar invested in controlling summer weed growth in fallows returned up to $7.20 per hectare of benefit in a trial in canola paddocks near Forbes, New South Wales, in 2012.

As a profit driver, summer weed control was found to be more important than stubble management because clean, weed-free fallows increased both moisture and nutrient availability. 

“Summer weed control is a key driver for profitable cropping systems in central NSW,” says Colin McMaster, a research and development agronomist from the NSW Department of Primary Industries (DPI). 

“The timing of that control is also important. The greater the delay in introducing control, the greater the economic loss,” he says. 

Maximising profit

The Forbes trial was part of a series of summer fallow management trials from 2010–12, funded by the GRDC through the Central West Farming Systems ‘Rain & Grain’ project.

These trials evaluated summer weed-control treatments and the impact of weed growth during a summer fallow period on stored moisture, nutrient retention and overall grain yield.

“The trials highlighted the importance of controlling summer weeds to conserve nitrogen as well as moisture,” Mr McMaster says. “Water and nitrogen are clearly important for grain yield, as they influence the size and number of grains, as well as the protein content of grains such as wheat.”

More water and nitrogen means more tillers and more grains per head. 

Research in 2012 showed that an increase in nitrogen through the control of summer weeds is vital to capture the extra benefits of any additional summer water. Likewise, a high water supply is required to capture the benefits of additional nitrogen.

This additional nitrogen, Mr McMaster explains, is likely due to an increase in mineralisation arising from a wetter soil surface where weeds are controlled, as well as less nitrogen being used by weeds.

“For every millimetre of moisture lost through summer weed growth in the trial, mineral nitrogen levels were reduced by 0.56 kilograms per hectare,” he says.

“This means that if an extra 75mm of moisture was conserved by controlling summer weeds, there would also be 42kg/ha of extra available nitrogen for the following winter crop.

“Summer weeds should be controlled when small and actively growing, as this prevents weeds from draining stored soil moisture and nitrogen when they grow. It also lowers the amount of herbicide needed and generally increases the effect of the herbicide.”

Testing treatment methods

Four weed-control treatments were tested in the trials in Forbes over the three years. These were:

  • nil spray – no summer spray, only a knockdown applied before sowing;
  • ‘miss first spray’ – when the first initial spray for the fallow period is not applied;
  • full spray – a ‘zero tolerance to weeds’ approach, with herbicide applied about 10 days after a significant rainfall event (20mm or more); and
  • delayed spray – when herbicides are applied about 24 days after a significant rainfall event.

The full and delayed spray treatments conserved the most stored moisture for subsequent winter crops.

In the 2011 trial, nitrogen losses increased as the delayed, missed or nil spray controls were used, compared with the full spray treatment. When summer weeds were controlled with the full spray treatment, the soil had the highest level of mineral nitrogen. 

“A zero tolerance to summer weeds, using the full spray method, increased the level of mineral nitrogen by 69kg/ha and 45kg/ha in 2011 and 2012, respectively,” Mr McMaster says.

Maximising profitability

Previous research in central NSW has shown that good summer rainfall and summer fallow management can enhance sowing opportunities in some seasons and can lead to a 50 per cent increase in yield potential in certain crops.

“For every dollar invested in fallow herbicides, the ‘miss first spray’ treatment returned $1.90/ha, the delayed spray treatment returned $3.90/ha and the full spray treatment returned $7.20/ha,” Mr McMaster says.

Nitrogen fertiliser coupled with good summer weed control (leading to increased stored moisture) showed higher grain yields compared with when fertiliser was applied to weedy fallows with low stored moisture.

The highest returns in the trials were when full weed control was achieved. This was due to higher residual plant-available water and nitrogen contributing to increased grain yields.

Canola yields were highest under the full or delayed spray treatments.

“Where full fallow weed control is used, the return on investment can be up to 720 per cent,” Mr McMaster says. 

Negative returns occurred when no summer weed control was carried out. 

These trials show how managing land in the months or years before sowing can be more important for increasing water use efficiency than any in-crop management. 

“While many factors can influence how much plant-available water is stored in a fallow period, good summer weed management consistently has the greatest impact,” Mr McMaster says.

More information:



Keen interest in ‘farms of the future’


Advice at hand for fallow grass-weed control

GRDC Project Code CWF00013

Region South, North