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Unappreciated border biosecurity risks related to herbicide-resistant biotypes in imported seeds

Surveys from 2023 revealed that more than 40% of New Zealand vineyards and arable farms contain herbicide-resistant plants. Resistant ryegrass (Lolium spp.) are particularly common.


While multiplication of ryegrass provided NZ $80 million in export revenues in 2021, farmers suspected imported cultivar seed-lines contained herbicide resistant plants. Although ryegrass is a valued crop, herbicide resistant plants become a problem in the next rotation of land, be that of ryegrass or another crop, such as wheat.


The goals of this project were to gain an understanding of the risks of herbicide resistant seeds being imported in seed for sowing.  This knowledge could lead to new border agency biosecurity or voluntary seed industry mitigation measures to cut risks.

Specific objectives are to:

  • Document rates of herbicide resistance, or absence of herbicide resistance, in imported ryegrass seed lots.
  • Document rates of herbicide resistance, or absence of herbicide resistance, in contaminants of imported seed lots.


The research team planted out seed from 56 distinct lots (28 per year) from 52 varieties, into 1.5 by 5 meter plots over two years, with four replicates per seed-lot and herbicide (336 randomized plots per year). Plots were treated with haloxyfop (Group 1), iodosulfuron (Group 2) and glyphosate (Group 9) at the highest recommended rate. Each herbicide was applied to 30-80 thousand seedlings at the three-leaf stage, estimated survival rates were generally less than 2%. Seed contaminants were also screened for herbicide resistance.


Eight weeks after treatment, up to 20 survivors were collected per variety and herbicide, grown in the glasshouse and tillered for respraying to confirm resistance to the herbicide treatments – because some plants could have been late germinators or escaped exposure.


Out of the 56 seed-lots (11 were from NZ, 45 were imported),  resistance was confirmed for 44 lots (80%), including some breeder’s lines. There was no significant difference between seed lots sourced from New Zealand, or overseas. However, it appeared that Italian ryegrass was more likely to be resistant than perennial ryegrass. Estimated resistance rates were between 1:20 and 1:70000, with seed lots classed as resistant to either haloxyfop (9 lots), iodsulfuron (10 lots) or both (25 lots), but none survived glyphosate. Researchers speculate resistance-conferring variants may occur normally in ryegrass at rates of 1:7000 or rarer, but 29 seed lots had higher than expected rates.


Some imported ryegrass lines contained annual poa (Poa annua) contaminants that survived iodosulfuron as well. Blackgrass (Alopecurus myosuroides) that contained contaminants of linseed and ryegrass seed lots were also shown to be resistant to iodosulfuron.


In the final phase of this project, ending in 2024, researchers are growing other weed contaminants to produce seed for resistance testing.


Contact Project Leader Chris Buddenhagen: [email protected]


Publications of relevance to this project:

Buddenhagen, C. E., Hackell, D., Henderson, H. V., & Wynne‐Jones, B. (2022). Factors impacting the detection of weed seed contaminants in seed lots. Pest Management Science, 79(2), 881-890.

Buddenhagen, C. E., Rubenstein, J. M., Hampton, J. G., & Rolston, M. P. (2021). The phytosanitary risks posed by seeds for sowing trade networks. PLOS ONE, 16(11), e0259912.

Rubenstein, J. M., Hulme, P. E., Buddenhagen, C. E., Rolston, M. P., & Hampton, J. G. (2021). Weed seed contamination in imported seed lots entering New Zealand. PLOS ONE, 16(8), 1–13.