Growers are advised to ensure that spray applications remain effective as we move into in-season weed and pest control.
BCG researcher Claire Browne explained that stubble loads need to be considered when beginning in-season spraying.
“Where growers have inter-row sown, stubble is a great trellis for crops, however it provides an opportunity for sprays to bind, thus reducing efficiency,” Ms Browne highlighted.
This is because sprays bind to organic matter, of which stubble can be a significant component.
While the effect of stubble to interfere with pre-emergent sprays is well understood, stubble can still provide a barrier with in-crop applications.
“Sprays bind at different rates depending on the characteristics of the spray, but it can result in the spray not reaching its target or an uneven application over an area,” Ms Browne said.
“Reduced herbicide effectiveness and an increase in the number of weeds that potentially escape can be the negative result experienced,” explained Ms Browne.
Knowing the binding strength of the chemicals growers are using can assist in determining if stubble is likely to have a huge effect or not.
The soil/water adsorption coefficient (Koc) is the calculation that determines this, which is highly influenced by the level of organic matter.
Some examples of herbicide Koc values include Brodal® between 1622-2369 making it slightly mobile, Verdict™ at 75 making it mobile to moderately mobile, and Velocity® at 302 making it moderately mobile. The Koc value of other herbicides can be found in the GRDC pre-emergent herbicides fact sheet.
Ms Browne explained that to increase herbicide efficacy growers can increase their water rates, use nozzles to increase spray coverage, reduce the height of the spray boom and reduce travel speeds.
“By reducing travel speed there is less bounce and improved application evenness,”
“Water sensitive paper is a great way of measuring the effectiveness of the coverage from their spray application,” Ms Browne emphasised.
Growers can count droplets and measure the area of application using water sensitive paper. All growers need to do is place the water sensitive paper within the paddock and then spray over the top, a measurement can then be calculated.
Alternatively, SnapCard is an app produced with funding from GRDC, CSIRO and University of Western Australia. By taking a photo of the water sensitive paper through SnapCard it will calculate the area of application for you.
With these techniques growers can determine if they are meeting the targets for efficient spraying.
At the 2016 Hart Field Site stubble field day, Trent Potter from Yeruga Crop Research explained that for winter spraying a good coverage target is 12-15% area and 40 drops per square centimetre.
This will need to increase for fungicide spraying to at least 15% area coverage and 60 drops per square centimetre for efficient treatment.
There is extensive research and information about how to handle large stubble loads on the GRDC-funded ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmanian’ project website at https://thestubbleproject.wordpress.com/
For more information about the Stubble Initiative please call BCG on 03 5492 2787
This article was published in the Stock and Land, 28 June.
