Five years of surveys for herbicide resistant weeds

Prior to the herbicide resistance surveys, 14 weed species were reported to be herbicide resistant. The arable and viticultural industries have high agrochemical usage and are believed to be at a high risk for herbicide resistant weeds.

Arable farms from Canterbury, Otago, Southland, Hawkes Bay and Wairarapa; maize farms from Bay of Plenty, Waikato, Hawkes Bay and Wairarapa and vineyards from Marlborough, Canterbury, Gisborne and Hawkes Bay were surveyed for weeds from 2019-2023. Seed from weeds were collected and tested with a range of herbicides.

Widespread resistance was observed for weeds already known to be resistant: ryegrass in arable farms (29% of arable farms) to group 1 and 2 herbicides, ryegrass in vineyards (40%) to glyphosate and fathen in maize (31%) to atrazine. Wild oats were observed to be resistant to group 1 and 2 herbicides in arable farms. Willow weed, poa and chickweed were resistant to group 2 herbicides in Southland arable farms. Summer grass appears to be an emerging resistant weed in Waikato maize.

Several weeds were observed to be resistant for the first time in arable farms in New Zealand: sow thistles to group 2 herbicides, prairie grass to pyroxsulam and lesser canary grass to group 1 and 2 herbicides. Arable farms in the North Island had fewer resistant weed species and a lower proportion of farms with any resistant weeds (15%), than those in the South Island (49%), and vineyards in the North Island also had a lower proportion of resistant vineyards (10%), than those in the South Island (50%). Herbicide resistance was more prevalent in Waikato maize than those of other North Island regions.

After five years of surveys, the number of resistant weed species recorded is now 21.