Weed management solutions

AgResearch is a key provider of world-class science supporting pastoral weed management and decision-making at all scales from paddock to nation and across all stages of the invasion process.

Early detection crucial

New Zealand has a vast pool of at least 22,000 exotic plant species. It is likely that next century’s worst invasive weeds will emerge from this pool. Detecting these ‘sleeper’ weeds is crucial for avoiding potentially huge future impacts and management costs such as those for former sleeper weeds (e.g., wilding pines, gorse and broom). Early in an invasion, when a weed is geographically limited in its distribution, and spreading slowly, management costs are low and eradication is possible.

Pastoral sector weed research strategy 2024

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Internal biosecurity

Solution one: Sleeper weed hazard assessment

In collaboration with regional councils, Department of Conservation and the Ministry for Primary Industries, we are developing a tool for the identification of ‘sleeper’ weeds within New Zealand’s known pool of exotic plant species.

The tool utilizes a novel model for weediness that is based on three algorithmically determined parameters: the number of papers published, global occurrences, and climate match to New Zealand.

Solution two: Regional weed management scenario analysis

With Manaaki Whenua Landcare Research(external link), we have created a framework for socio-ecological scenario analysis to improve regional and national effectiveness of weed management.

We modelled the spread of Chilean needle grass under management on sheep and beef farms in Marlborough. Region-wide success (number of infested farms less than 1% of the potential maximum) depended on interactions between the probability of detection and the probability that biosecurity officers communicate to farmers about a neighbouring farm’s infestation. This research reveals the benefit of investment in internal biosecurity extension.

Together, these resources bolster our ability to tackle new weeds promptly, curbing their environmental and agricultural impacts

Graeme Bourdôt, AgResearch Principal Scientist

Herbicide resistance

Continuous use of herbicides with the same mode of action over several years targeting the same weed can lead to the development of herbicide-resistant biotypes of the weed.

Known cases of such ‘evolved’ resistance have been increasing steadily in New Zealand since the 1979 discovery of fathen (Chenopodium album) in maize crops with resistance to atrazine. By 2022, there were 22 cases of confirmed herbicide resistance. Globally, as of October 2023, 523 unique resistance cases were reported from 99 crops and 72 countries.

While the use of rotational cropping in New Zealand is very helpful in avoiding resistance evolution, there is no room for complacency as many of the different crops either use the same herbicides or herbicides from the same mode-of-action group.

Identifying farming practices that prevent resistance and maintain the effectiveness of herbicides is a priority for farmers, growers and the New Zealand economy.

Dr Chris Buddenhagen, AgResearch Senior Scientist

By involving the sector directly and sharing herbicide resistance findings, our research has dismantled the stigma around herbicide resistance for rural professionals. Management of herbicide resistance is crucial to sustain yields and the financial stability of primary production systems. Herbicide mixtures, herbicide rotations, cover crops, cultural and biological controls are key management options that also minimize herbicide use and environmental impacts.

Herbicide-resistant perennial ryegrass surviving in a wheat crop

Alternatives to herbicide

The discovery, development, and commercialisation of synthetic herbicides of varying chemical classes and biochemical modes of action have provided farmers with a suite of herbicide products with activity against many of the most damaging pasture weeds. The efficacy and selectivity of these herbicides has encouraged pastoral farmers to rely upon them for weed control. But this practice is increasingly becoming unsustainable due to the evolution of herbicide resistance in weeds, public health and environmental concerns over herbicide use, increasing industry and market requirements for residue-free farm produce, regulatory constraints on herbicide application practices, fewer new herbicides becoming available, withdrawal of herbicides from the market, and the global increase in organic agriculture.

Our research has shown that many arable cropping farms in New Zealand have weed populations that contain individuals resistant to commonly used herbicides. Their detection and appropriate management will be crucial to prevent the resistance evolving.

Solutions

A “Growers Leading Change” group has embraced the resistance challenge in the New Zealand arable sector, developing grower-led-solutions to ensure the continued efficacy of at-risk herbicides. A key practice change will be the rotation of herbicide modes of action to lessen the selection pressure for resistance. Less reliance on herbicides is also part of the solution and our research has identified the following options:

Cover crops - Field trials have shown the effectiveness of cover crops for managing weeds and reducing the weed soil seed bank in maize
Deferred summer grazing - This practice has enabled grasses and legumes to out compete California thistle in pasture
Bioherbicides - Naturally occuring plant pathogenic fungi have potential as bioherbicides as illustrated by giant buttercup treated with the fungus Sclerotinia sclerotiorum
Biocontrol - Exotic insects, such as the Green Thistle Beetle, could be effective biocontrol agents for Californian thistle and other pasture weeds
Mechanical control - Mowing Californian thistle in pasture, and grubbing nassella tussock in modified tussock grasslands have shown to be effective