WithTheGrain: Herbicide residue trials enter second year
Author: Rachael Oxborrow | Date: 11 Apr 2018
With conservation of soil moisture now a key aspect of modern grain growing practices, farmers are placing more emphasis on controlling summer weeds including volunteer crops using effective herbicides.
There has been anecdotal evidence of herbicide residues persisting in sandy soils through summer spraying regimes and thereby affecting crop biomass. Research initiated as part of a GRDC investment to investigate this issue is entering its second year.
South Australian Research and Development Institute (SARDI*) senior researcher Courtney Peirce began working in this area with an extensive glasshouse trial, noting much of the herbicide residue work to date had been done with heavy textured soils.
“Farmers are sometimes spraying multiple times per season with glyphosate or 2,4-D and there is a concern that this increased frequency of applications in a short time frame are leading to residues remaining in the soil which might be a problem for crop growth during and after emergence in some seasons,” she says.
As part of the first-year glasshouse trial, soil from Karoonda, South Australia, and Ouyen, Victoria, was taken as intact cores and then sprayed with either glyphosate, 2,4-D ester or a combination of both at two different rates before being watered to different levels over a four-week period, simulating Mallee summer rainfalls of decile 1, 5 and 9.
Initial results demonstrate the complexity of factors which influence the rate of degradation of herbicides in soils. Aside from soil properties such as pH and organic matter, climatic conditions particularly temperature and soil moisture are major factors in herbicide persistence.
Of the 144 different treatment combinations, there was no clear correlation between herbicide treatments (type or rate) and soil types. When assessed as crop dry weight after eight weeks growth in the glass house, three treatments resulted in reduced biomass by as much as 36 per cent, whilst 12 treatments inexplicably resulted in up to a 40 per cent increase in biomass.
Dr Peirce says the second year of trials will focus more on field evaluation with accompanying glasshouse trials using the same soils. Three sites have been established at Cooke Plains and Lameroo in South Australia and at Ouyen in Victoria and wheat, barley, lentil and lupin.
“There’s been some previous work that has shown a negative effect of glyphosate soil residues on lupin due to an interaction with soil phosphorous availability. Given this information and that lupin is a common sandy soil crop option, we’ve included it as part of our field trials,” Dr Peirce says.
“Our field trials aim to create a gradient of glyphosate, 2,4-D and combinations of both and were sprayed at the end of January and will be sprayed again just prior to sowing to give a site-specific picture of time of spraying and crop safety. The worst-case scenario results should come from the second spray application immediately prior to seeding.
"If there’s going to be a negative biomass affect we expect to see it in these treatments. If we do get some rain, which there hasn’t been much of this season to date, you would expect there to be a bit more breakdown of herbicide in sandy soils compared to heavier soils.
“So far, this season is panning out to be a worst case scenario with herbicide residues since there hasn’t been much rain and therefore limited opportunity for herbicides to break down."
The breakdown of most herbicides in soils is primarily dependent on microbial degradation and chemical hydrolysis, which are both driven by soil moisture. Many products have a minimum rainfall requirement before the commencement of the plant back period. For example, when applied to dry soils, 2,4-D amine requires at least 15 millimetres of rainfall prior to the commencement of the plant back period which can range from one day for wheat up to 14 days for canola.
The current project concludes at the end of the 2018 season and is expected to aid in developing the understanding around the short-term persistence of some herbicides in lighter soils.
“At the very least we should be able to demonstrate whether there will be any significant growth effect from the high rates of summer spray applications,” Dr Peirce says.
GRDC Crop Protection Officer – South Aaron Long says understanding the impacts persistent herbicides are having on southern region cropping systems is an area where the GRDC is focusing investment.
“It is important to remember that most plant back periods require moist soil before the countdown begins to safely plant sensitive crops,” he says.
“It is sometimes best to think about the total time of wet soil that a herbicide has been exposed to, as many microbes that are involved in the breakdown of herbicides require moisture to facilitate their process.
“Microbial decomposition is not the only process involved in herbicide breakdown as volatilisation, leaching, photochemical decomposition, adsorption to organic matter or clay or uptake by plants are other ways herbicides can leave our soils.”
With investment from GRDC, Independent Consultants Australian Network (ICAN) senior consultant Mark Congreve has developed a series of workshops on the soil behaviour of pre-emergent and post emergent herbicides as well as a useful publication Soil behaviour of pre-emergent herbicides in Australian farming systems – A reference manual for agronomic advisers used in these workshops, which is available from the GRDC website.
He says the various factors for herbicide breakdown made predicting their persistence difficult, sometimes causing frustration when it comes to the following season’s planting opportunity.
“Unfortunately, there are not just one or two simple factors that influence the length of herbicide persistence,” he says.
“The main factors include soil properties, climatic conditions in particular temperature and moisture, the chemistry of the herbicide and how it is degraded, for example by microbes or chemical hydrolysis and, in most situations, soil organic matter and pH.”
*SARDI is a division of Primary Industries and Regions SA
GRDC research codes: DAS00162-B, ICN00016
Dr Courtney Peirce,
SARDI senior research officer,
0419 817 325,
08 8429 0636,
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