Published on 29/07/2024
FAR senior field officer Ben Harvey says not finding glyphosate resistance was a surprise, and follow-up work planned for this season will probably include a fenceline spraying demonstration trial at Chertsey as a result.
“We know a lot of glyphosate is used on fencelines, and we also know using it over and over again on the same patch of ground is a high risk factor for developing resistance, so we want to include that in our focus going forward.
“We’re also investigating mechanical tine weeding as a crop management option, and we want to look at other tools like increasing crop competition to combat weeds, and novel spring crops to break the cycle of weed resistance.”
Ben’s hoping to do some trials on novel herbicides as well, but stresses that these are not going to save the day, even if they do become available here in New Zealand.
“In Australia, they have been some use, but the level of herbicide resistance there is worse than ours, and in some cases they’ve just shifted resistance from one MOA group to another.”
Outside the box
“We can’t rely on novel herbicides. Getting such products into NZ is problematic; the market here is very small and some of the chemical companies are baulking at the long delays in achieving registration, so we need to look outside the box for our own solutions.”
Integrated Weed Management (IWM) principles are a big part of this message, he says, and while the concept may sound complicated, lots of resources are available, many of which can be readily incorporated piece by piece without entailing an all or nothing approach.
“Even a small thing like shifting away from autumn sowing can help. For example, if all your crops are autumn sown, and you know you have a paddock where herbicide resistance is an issue, maybe shift that to a cover or break crop sown in autumn, followed by the maincrop in spring.”
This can have financial implications for growers, but the financial implications of not making changes to tackle known herbicide resistant weeds are worse, he says.
The last thing he’s advocating is growers doing away with herbicide MOA groups known to have developed resistance on their farms.
“We’re not saying don’t use them. We still need growers to be using robust herbicide programmes with MOA Groups 1 and 2, especially where resistance is known to be limited to pockets on certain paddocks.”
Use them properly
Rather it’s a case of making sure they’re used properly – applied at full recommended rates, in combination with other MOA groups as appropriate, via effective spray nozzles to achieve good plant coverage.
While the news was not encouraging from the five year herbicide resistance survey project, funded by MBIE, Ben says so far he hasn’t seen any infestations that are so bad growers can’t endeavour to get rid of them.
“Some of them might have bad patches, where they can see the weeds are resistant, and they will take that part of the crop off before harvest for balage, to stop the weeds from going to seed. It’s costing them money, obviously, but they’re still able to try and control those plants.
“What we want to do is convince growers who aren’t yet affected to take steps to stop resistance becoming a problem for them too.”
As well as not finding resistance to glyphosate, the surveys didn’t find any resistance in Vulpia hair grass, which he says was very good news.
One population of field pansy was found that might have been resistant to herbicide, but this could not be confirmed.
Some of the other problem weeds that people talk about, like wireweed and mallow, they’re a problem because of their physiology and the way they grow, not because they resist herbicides, he adds.
Five regions
The project surveyed farms in five arable regions from 2019 to 2023 – Selwyn district; South Canterbury; South Otago and Southland; Wairarapa, Manawatu and Hawke’s Bay; and Mid Canterbury.
Farms were randomly selected, with the aim of visiting 20-30% of all arable farms in each region.
Surveys were carried out in January, just before harvest, when weeds that had survived herbicide application were most likely to have produced viable seed.
On each farm, two suitable paddocks were sampled (usually a cereal crop). Growers also provided input if they suspected herbicide resistance was present on the property.
Seed was collected from any weeds that appeared to have survived normal herbicide application, with focus on grass species that had known resistance overseas.
Seeds were then sent to AgResearch to be grown out, and the resulting plants tested for resistance to a range of appropriate herbicides.
Almost 4000 individual plant-herbicide interactions were tested in total, with 63 different species of weed collected across 168 farms.
Thirty of the collected species were tested with 21 different herbicide active ingredients, across eight different MOA groups.
South Canterbury recorded the highest levels of herbicide resistance, with 71% of farms surveyed recording at least one instance of a herbicide resistant weed.
Regional differences
In comparison, in the eastern North Island, herbicide resistant weeds were only found on 11% percent of farms.
Of Mid Canterbury farms surveyed, 60% recorded herbicide resistance, in Selwyn district 54% and Southland 59%.
Regional differences are suspected to relate to differences in farming systems, as areas where crop rotation options are limited tend to have higher levels of resistance, says Ben.
Other factors include: whether or not pasture is commonly included in rotations, the prevalence of grass seed crops, particularly ryegrass, in the rotation, and whether or not spring crops, which can disrupt weed lifecycles, are commonly included in rotations.
Ryegrass, both annual and perennial, is the main offender, with resistance to Group 1 and 2 herbicides.
“Resistance seems to be everywhere, but especially in South Canterbury.”
Imported ryegrass seed lots have provided a pathway for introducing herbicide resistance onto NZ farms. In addition, rare individual plants can gain resistance-conferring mutations.
Ben says while it was not discovered during the surveys, reports from industry that resistance may be emerging in some other herbicide groups, most notably Group 15 pre-emergence herbicides, are concerning.
For more detail contact Ben Harvey ben.harvey@far.org.nz