Summer weed control vital for water use efficiency

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Controlling summer weeds is the largest single contributor to improving the water use efficiency of cropping systems across southern Australia

GRDC’s landmark National Water Use Efficiency (WUE) Initiative (2008–13) concluded that for every dollar invested in summer weed control there can be up to a $5 return in the form of additional yield due not only to improved stored soil water but also additional nitrogen and other system benefits.

In 2008 GRDC established the five-year, $17.6 million WUE Initiative to counter slowing productivity growth and challenge growers and researchers to lift WUE of grain-based production systems by 10 per cent.

System productivity

Key researchers leading the initiative, which involved 16 regional grower research teams across the 300 to 700 millimetre annual rainfall areas, were Dr John Kirkegaard (CSIRO) and Dr James Hunt, (previously CSIRO, now La Trobe University).

“The initiative focused on four themes: break crops and crop sequences; summer fallow management; managing in-crop water use; and managing constrained and variable soils,” Dr Hunt says. “Summer fallow management proved to be a major driver of WUE and yield improvement.

Image of crop trials at Tottenham, NSW
Controlling summer fallow weeds improved soil water, nitrogen and subsequent crop yields in NSW Department of Primary Industries trials at Tottenham, NSW. PHOTO: Colin McMaster, NSW DPI

KEY POINTS

  • Summer weed control has been demonstrated to boost winter crop yields by up to 60 per cent
  • Early control is best but late control is better than no control

“Using 20 replicated experiments on summer weed control we were able to demonstrate up to 60 per cent yield improvements in winter crop performance. This clearly shows the benefit of controlling summer weeds for improving yields of winter crops. In seasons with high growing-season rainfall yield improvement was driven by nitrogen availability, in seasons with low growing-season rainfall by water availability alone and in average seasons by both nitrogen and water availability.”

Dr Hunt encourages growers to thoroughly manage summer weeds at the three-to-five-leaf stage using herbicides at registered label rates, as generally herbicide efficacy is highest when the summer weeds are young and actively growing.

Complete weed control, spraying 10 days after a significant rain event, resulted in the greatest subsequent winter crop yield, but even controlling summer weeds three weeks after a rain event yielded more than the nil control treatment.

“Typically, the return on investment was approximately $3.50 for each dollar spent for the later treatments compared with approximately $5.60 for complete control.”

Control versus grazing

Growers commonly cite provision of forage for stock during the summer months as a reason not to control summer weeds. However, whole-farm simulation conducted by Dr Andrew Moore (CSIRO) as part of the WUE Initiative clearly demonstrated that this is a false economy. In the system where weeds were allowed to grow for forage, a small increase in meat and wool production and decrease in supplementary feeding was offset by a large decrease in winter crop yields. Weeds growing during the hot, dry summer months did not convert water efficiently to dry matter and grazing did not reduce the amount of water or nutrients that they used.

Dr Hunt says growers with mixed enterprises are better off controlling summer weeds and using the water and nitrogen they save to grow more grain and fodder, which they can carry over for summer feeding.

“A further benefit of summer weed control is destroying the green bridge that can harbour pests and diseases between seasons potentially reducing winter crop performance,” Dr Hunt says.

“Additionally, as we see rainfall patterns changing it is important to conserve any out-of-season rainfall and a major part of achieving this is through summer weed control.”

More information

Dr John Kirkegaard, CSIRO Agriculture & Food
0458 354 630
john.kirkegaard@csiro.au

Dr James Hunt, La Trobe University
0428 636 391
j.hunt@latrobe.edu.au