Words and images supplied by the Foundation for Arable Research
It has been suspected for some time and now the numbers prove it; herbicide resistant weeds are widespread on arable farms.
The confirmation comes after five years of herbicide resistant weed surveys on arable farms, each year in a different region.
The Foundation for Arable Research has just published an Arable Extra summarising the results.
FAR senior field officer Ben Harvey says the surveys show that resistance to commonly used herbicides is widespread, with very high levels in some regions.
The most common resistant weed is ryegrass (Lolium species). Regions often have their own difficult-to-control weeds.
In all regions, the only resistance detected was to Mode-of Action groups 1 and 2. Group 1 herbicides are used to control grass weeds in a range of broadleaf and cereal crops. Group 2 herbicides have activity on a wide range of grass and broadleaf weeds and are also used widely in cereals. Both classes of herbicides are used post-emergence on the weeds. Worldwide, resistance to Group 2 herbicides is the most widespread form of herbicide resistance.
No resistance to glyphosate (Group 9), the world’s most popular herbicide, was recorded on arable farms, although it has been detected outside of the random surveys.
South Canterbury recorded the highest levels of herbicide resistance, with 71 percent of farms surveyed recording at least one instance of a herbicide resistant weed. In comparison, in the eastern North Island, herbicide resistant weeds were only found on 11 percent of farms. Of Mid Canterbury farms surveyed, 60 percent recorded herbicide resistance, in the Selwyn District 54 percent and Southland 59 percent.
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, Ben says.
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 life cycles, 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 New Zealand farms. In addition, rare individual plants can gain resistance-conferring mutations.
Wild oats are another weed growers need to keep on top of, particularly in Canterbury, with rogueing required to eliminate plants resistant to Group 1 herbicides.
Southland has two particularly problematic weeds, chickweed which is resistant to chlorsulfuron (Glean®) and Rayless chamomile, resistant to Group 2 herbicides.
In North Island maize growing areas, fathen is resistant to atrazine.
While arable growers in Canterbury, Southland and Otago are more likely to be affected by herbicide resistance, it is worth noting that all of the regions surveyed had some degree of resistance, Ben says.
“Growers should always be vigilant and follow up on any patches of weeds that appear to have survived herbicide application.” Assistance is available through FAR and agrichemical company representatives.
More funding is being sought to further investigate integrated weed management to support growers to find ways to combat herbicide resistant weeds on their farms.