Research by BCG and CSIRO has found that zero-tolerance of summer weeds pays off in most years, and that growers should cash-in on low glyphosate prices and the big rains received this summer by keeping paddocks weed free.
The research was conducted as part of GRDC’s national water-use efficiency initiative, which aims to lift water-use efficiency across the country by 10%. Modelling by CSIRO shows that capturing and better use of summer rain is one of the best ways of improving crop yields and water-use efficiency in the Wimmera Mallee. This has been backed up by BCG’s ongoing field research into summer moisture conservation. As far back as 2000, BCG found that controlling summer weeds gave the subsequent wheat crop a yield benefit of 1.3 t/ha or 60% compared to wheat sown where summer weeds grew unchecked. At Curyo in 2008, controlling summer weeds doubled wheat yields from 1.2 t/ha to 2.5 t/ha. Despite high glyphosate prices there was still a 500% return on investment.
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However, 2009 trial results from BCG’s water-use efficiency site at Hopetoun have demonstrated that as with all things in farming, the issue of controlling summer weeds is not clear-cut. As distinct from this summer, only 90mm fell at Hopetoun in the summer of 2008/2009, with the biggest single fall being 27mm. This was enough to trigger emergence of weeds which required spraying, but soil sampling revealed that only an additional 12 mm of water was stored by sowing in late April 2009. This did not translate into a measurable difference in yield of the barley crop, which yielded 3.4t/ha on sandy soil and 2.8t/ha on clay soil regardless of weeds or stubble retention. This outlined a ‘worst case’ scenario for growers who adopt a zero-tolerance summer weed policy in that their investment in weed control was not met with significant return in grain yield. |
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Fortunately, analysis of 120 years of rainfall data for the district has shown that such instances are rare. The percentage of years in which enough summer rain falls to bring up weeds but not be stored for subsequent crops varies from 29% on a heavy clay at Charlton to 2% on the sandy loams of the Mallee. Zero-tolerance of summer weeds pays off in the vast majority of years, particularly on lighter soil types with good fallow efficiencies. Given the large quantities of rain that much of the district has received this summer, growers should be confident that if paddocks are kept clean, water will be stored. This water will be vital for 2010 crops.
If money is short, growers should focus weed control efforts on better paddocks with soil types that have better fallow efficiencies, or where some water was already available for 2010 wheat crops prior to the summer rains such as pulse stubbles or fallows. Residual herbicides are useful but 2010 rotations need to be considered.
Broadleaf summer weeds and grasses are best controlled at the juvenile stage when they are actively growing and have not had a chance to deplete much soil water. Appropriate spray mixes and water rates should be used, and close attention paid to suitability of spray conditions, particularly Delta T.
