Take the test to be clear on herbicide resistance
With rapidly increasing herbicide resistances, it becomes more important to understand which herbicides will still work for you.
University of Adelaide researcher Dr Chris Preston says once growers have experienced resistance to a certain herbicide, there is huge value in testing weeds to find out what herbicides will work.
“While there are some rules about what types of cross-resistances there might be between herbicides, there are lots of variations; cross-resistance isn’t absolute,” he said. “These variations can sometimes help growers make a decision about what product they might use.”
For example, in a GRDC-funded Fast Track trial in the Griffith, NSW, region, a large number of seed samples were collected and sent in for testing. While all the samples tested positive for Logran® resistance – which had been expected – surprisingly, two thirds of the growers’ samples showed no resistance to Hussar®.
“So they suddenly had a product they could use, even though the technical rules would say you’re likely to get cross resistance. You’d never know that unless you tested,” Dr Preston said.
In planning a herbicide resistance test, Dr Preston recommends considering a few key questions.
He says the main objective should not necessarily be to test the herbicide that failed because experience has already suggested that. In his view, the key question should be: ‘What are the chemicals I want to use in the next few years and are they working’.
The second key consideration Dr Preston recommends is where the sample is collected from.
“It’s really important that you take your samples from the areas that you’re concerned about,” he said. “Testing facilities often see samples randomly collected from across the paddock. This can include areas where herbicides have been effective, and as a result the test could show the sample as being susceptible to a herbicide where the area of concern is actually resistant.”
If there are multiple areas in a paddock, Dr Preston recommends testing them separately because resistances can vary across a paddock and so the combined result may not reflect the separate patches of resistance.
In terms of cost-effectiveness of resistance testing, Dr Preston says if growers have a paddock of concern and want to know what herbicide options are available, then testing three or four options can be helpful.
“To test for 3-4 herbicides, that will cost $350 to $400 which is a lot cheaper than spraying the paddock,” he said. “Testing multiple herbicides can also have a cost benefit if the test shows it is possible to use a cheaper herbicide rather than going to a more expensive option.”
Dr Preston also recommends growers talk to their agronomist to decide which herbicides to test.
Dr Christopher Preston, 08 8313 7237, email@example.com
For more tips on testing for resistance, visit www.weedsmart.org.au/10-point-plan/