Rain across areas of New South Wales and Queensland has grain growers rejoicing about improving soil moisture profiles as they prepare for winter crop planting, but with the wet weather comes an increasing weed threat.
Integrated weed management expert Mark Congreve from ICAN said conditions in many regions had been ideal for autumn weed germination.
“Research by the Queensland Department of Agriculture and Fisheries (DAF) has shown that a rainfall event of 20mm to 50mm, keeping the soil surface wet for a few days, combined with slightly milder autumn (or spring) temperatures are the preferred conditions to promote a major flush of weeds such as sowthistle and fleabane,” he said.
Mr Congreve is advising growers to be on the lookout for these weeds and take immediate action while they are still small, before they pass the rosette stage, as small weeds are far easier to control than larger growth stages.
“Significant levels of resistance to glyphosate are being detected in these species, so growers will probably need to do something different than a single glyphosate spray,” he said.
For sowthistle, he recommends growers use a double knock approach of glyphosate followed in seven to 10 days by a paraquat application. He said this approach was generally still effective, especially where glyphosate resistance was not present, or only at ‘weak’ levels. However, if resistance levels were high, or there was fleabane present, it was unlikely to be effective.
For fleabane, Mr Congreve said best results were generally achieved usually a tank mix of glyphosate plus a Group I (2,4-D or Lontrel or picloram (Tordon) based product, again followed by a paraquat double knock in seven to 10 days. However, he warned this combination could be problematic around sensitive crops, especially cotton that has not yet been defoliated.
“In situations where there are sensitive crops prohibiting the use of Group I herbicides, or there is a mixture of sowthistle and fleabane emerging together, growers may want to consider using a Group G herbicide to mix with glyphosate and then follow up with the paraquat double knock,” he said.
“Flumioxazin based herbicides (e.g. Valor) will assist in control of sowthistle, or consider Sharpen to be added to the glyphosate for both sowthistle and fleabane.
“The key to obtaining effective results from these Group G herbicides is to target small rosette size weeds with the first pass of the double knock program, while soil moisture is good and plants are rapidly growing.”
A widespread rainfall event often triggers extensive fallow spraying and this can lead to spray drift onto sensitive crops. To assist growers improve spray practices the Grains Research and Development Corporation (GRDC) and the Cotton Research Development Corporation (CRDC) have developed a new Tips and Tactics GrowNote: Reducing Herbicide Damage.
This GrowNote explains the difference between volatility and spray drift; how environmental conditions and spray application setup influence herbicide movement; and provides some alternative strategies to reliance on Group I herbicides.
“Correct choice of herbicide strategy, coupled with timely application to small weeds under suitable spraying conditions is the key to success” said Mr Congreve.
“Delaying the start of fallow weed control, with the hope of picking up further germinations will probably lead to large areas needing to be covered in a very short application window, which can often lead to problems of poor efficacy or the risk off-target spray movement if good application conditions are not available in that tight window”.