Agriculture Victoria’s Steven Field said a range of issues could lead to herbicide failure, in addition to the most obvious cause — resistance.
“This year in particular, farmers want to make the most of the valuable chemicals that they have,” he said.
“Under-dosing, inadequate coverage, hard water and temperature can all impact a herbicide’s efficacy.”
At recent workshops co-ordinated by Agriculture Victoria, herbicide specialist Peter Boutsalis from Plant Science Consulting, explained how grain growers can maximise the efficacy of a number of herbicides.
“Weeds can survive for many reasons, with one being herbicide resistance,” Dr Boutsalis said.
“For all herbicides, it’s important to target young weeds at the correct growth stage.
“It’s also important to not under-dose with the chemical, and for weeds to not be stressed due to frost, heat or moisture at spraying.
“With glyphosate, warm temperatures can reduce the efficacy, as this particular herbicide is more active under cooler conditions. However, night spraying is not advised, as it increases the risk of spray drift due to inversions.”
Dr Boutsalis said water quality was important for glyphosate.
A water volume of 50 to 80 litres/ha is ideal and coarser nozzles also advisable.
“Having concentrated droplets improves uptake from leaf surfaces,” Dr Boutsalis said.
The tank mixing order is also important.
Dr Boutsalis advised growers to add water to the tank first.
“The tank should first be filled with water to about 70 per cent capacity,” he said.
“Water conditioners should be added before any herbicides.
“Next, add granular herbicides, and after agitation, water soluble herbicides, then EC liquid herbicides.
“Finally, add adjuvants and wetters.”
In contrast to glyphosate, ‘dim’ herbicides such as clethodim are more active with warmer temperatures, and frosts within two days of application can reduce their efficacy.
Other factors such as stubble load, nozzle choice and sprayer speed can also have an effect on herbicide efficacy.
Mr Field reminded farmers they are legally required to keep records of their use of agricultural chemicals, including herbicides.
“Not only is record keeping something that farmers are obliged to do, it also helps them to manage issues such as herbicide failures, as they have a history of what chemicals have been sprayed on what paddocks,” he said.
The herbicide workshops were supported by the Wimmera Catchment Management Authority.
Farmers with concerns about herbicide resistance should have weed seeds or plants tested.
Information is available from: plantscienceconsulting.com.au
For more information about herbicides, visit: grdc.com.au/resources-and-publications/resources/herbicide-behaviour
8°C