AgResearch has found that 20 of 28 tested seed lines had some resistance to Group 1 or Group 2 herbicides. Nine of them had high resistance, but none was resistant to glyphosate.
Group 1 herbicides block the formation of lipids in roots and growing points (typically grass herbicides).
Group 2 herbicides block the normal function of the enzyme which is critical in amino acid (protein) synthesis.
The research was carried out to understand the risks of imported ryegrass seed lots providing a pathway for introducing herbicide resistance to New Zealand farms.
A similar pattern is expected when local ryegrass seed lines are trialled this year.
AgResearch scientist Chris Buddenhagen said herbicide resistance was so widespread that it was safer for farmers to assume that resistance was present.
"If you spray any ryegrass paddock you will get some resistance. My conclusion is to expect it. Even in the 10 lines where we didn’t detect it, I expect there are low rates of resistance," the weed ecologist told growers at the Foundation for Arable Research’s Crops 2022 field day near Ashburton recently.
Evidence from Australia showed resistance could become a much bigger issue, but farming systems such as crop and herbicide rotations were helping to prevent this in New Zealand.
As part of the research, seedlings were sprayed with three different herbicides at the highest rate, with the survivors collected and taken to a glasshouse. These were then resprayed to confirm they had survived the herbicide rather than emerging afterwards.
Herbicides can kill susceptible weeds, but may fail to kill individuals that happen to have gained resistance-conferring mutations. These survivors can then go on to reproduce, particularly if the same mode-of-action herbicides are used repeatedly.
Mr Buddenhagen said the results reflected the global community, as New Zealand imports grass seed from at least 23 countries for multiplying them to northern hemisphere markets.
The resistant lines were mainly diploid turf grasses from the United States and Europe. No tetraploids tested were resistant.
Mr Buddenhagen said he did not know what the threshold for an acceptable level of resistance would be.
"A rate of 1:100 is too much, but maybe 1:10,000 is acceptable, as that has been found in natural unexposed populations. But that is still a lot of plants."
Resistance rates ranged from 1:92 seeds to 1:69,000 seeds. The nine varieties with the highest resistance ranged from 1:92 to 1:8000.
There is no requirement for seed lines to be tested for resistance on arrival in New Zealand.
A seed company representative told growers that seed lots arrived just before being distributed to them for sowing, so there was no time for testing. This was only done at the end of the season.
To manage resistant grass weeds, farmers are being advised to rotate herbicides and use a combination of chemicals. Ryegrass seed remains viable in the ground for only 18 months.
Mr Buddenhagen said Group 1 and 2 were the main culprits.
"Glyphosate is recording no resistance and Group 15 [inhibitors of cell growth and division] chemistry is looking good for pre-emergence, providing options to avoid high-risk chemistries to manage resistance coming on to farm through imported seed lots, but also to minimise the potential emergence of resistance in home-grown seed crops over time."
Growers should read the labels and pay attention to the chemical groups they were using, he said.
