Last year, as part of a field survey looking at herbicide resistance in Canterbury, several arable farmers were told that they had glyphosate resistant ryegrass on their farms. In April this year, after further testing, they were told that was not the case. Good news for the farmers concerned, but what led to the initial diagnosis of resistance, and why was it withdrawn?
Dr Andrew Pitman, FAR’s General Manager of Research, says such turnarounds are not uncommon in research, where initial findings are questioned, retested several times over, and then either accepted or rejected. He says what was different in this case, was that the implications of the initial findings were so serious, that it was better to treat them as accurate, than wait for more information.
“The initial diagnosis of glyphosate resistance was made after one round of testing of plants grown from seed collected from 48 farms in the Selwyn District. This initial round of testing also showed resistance to group A and/or group B herbicides on the properties concerned. At this stage, we notified the farmers that glyphosate resistant ryegrass had been found on their farms and set about discussing options for resistance management with them. But as we were aware of the potential impacts of this initial diagnosis, that was not the end of the scientific process.
“Three further lots of testing were carried out by our collaborators to confirm the initial results. The results of these three follow-up tests eventually allowed us to reject our initial diagnosis of glyphosate resistant ryegrass. The follow-up tests were designed to investigate different aspects and causes of herbicide resistance and looked at things like dose response and air temperature at time of treatment.”
He says that after the fourth round of tests, the hypothesis of glyphosate resistance was rejected. However, the group A and B resistance diagnosis remains in place.
“Retesting for resistance to herbicide group A and/or B confirmed the initial diagnoses were correct; in fact, a dose response test showed that some plants were able to survive three to six-times the recommended label rate of group A herbicides.”
What next?
As part of the MBIE funded research project, FAR in conjunction with AgResearch are continuing the weeds surveys across the country. In Year 1 (2018/2019) of the project the survey was in Selwyn District, covering the area between the Rakaia and Waimakariri River. In Year 2 (2019/2020), the survey was in South Canterbury, covering the area between the Rangitata and Waitaki Rivers. The seed from South Canterbury will be tested in the coming months. Sampling in Southland and Waikato is being discussed for the 2020/2021 season.
FAR will also be investing in developing integrated weed management strategies to ensure arable farmers are well placed to manage herbicide resistance in the future.
This work was funded by the five-year MBIE project Managing herbicide resistance. The project is led by AgResearch and FAR is a co-funder. Seed collection and resistance management discussions were carried out by FAR, while resistance testing was carried out by AgResearch.
For further information about this project contact Matilda Gunnarsson or Phil Rolston.