CONTROLLING summer weeds will more than pay for itself in 71-98 per cent of years in the Mallee and Wimmera, according to Birchip Cropping Group research.
CSIRO research scientist James Hunt said conserving summer rain was one of the most effective ways of lifting crop yields.
Yields have increased about one tonne a hectare on average in the past 120 years when growers have taken a "zero tolerance" approach to summer weeds in paddocks with full stubble retention.
Speaking at last month's BCG members' trial results review day, Dr Hunt said better yields were the result of better water capture and water use.
He said the Grains Research and Development Corporation national water-use efficiency project aimed to improve water-use efficiency across the regions by 10 per cent.
But Dr Hunt said computer modelling using the CSIRO's Agricultural Production Systems Simulator and paddock tests suggested it was possible to do better in the Mallee, increasing that efficiency by 30-40 per cent.
"In the long term, the Mallee taking a zero tolerance approach to summer weeds pays off - and handsomely," he said.
"But responses are very much soil type-dependent."
Dr Hunt said APSIM simulation, looking at rainfall records from 1889 to 2008, found an investment in summer weed control would have caused a measurable return in crop yield in about 98 per cent of years on lighter soils with "better fallow efficiencies", such as the sandy loams found at Swan Hill, Hopetoun and Walpeup.
The worst performers were clay loam and clay soils at Hopetoun and Charlton, where summer weed control would have paid off in 70-74 per cent of years.
Controlling summer weeds and retaining stubble increased the amount of plant-available water at sowing at both the saline clay flat and sandy dune by 10mm.
"The best we can expect that to turn into in terms of grain yield is 264kg/ha," he said.
The experiment will be continued for another three years.
Dr Hunt said he expected differences between the treatments to become more apparent in this year's growing season.
