Herbicide Resistance: A Growing Challenge

Herbicide resistance is becoming an increasing challenge across New Zealand, affecting pasture, cropping, vineyards, forestry, and lifestyle blocks alike. It occurs when a weed population adapts to survive a herbicide that once provided reliable control - often after the same mode of action has been used repeatedly over time.

This isn’t a product failure or a sudden change in formulation. It’s a predictable biological response driven by natural selection. Repeated use of the same herbicide (including widely used actives such as glyphosate) gradually removes susceptible weeds, allowing naturally resistant survivors to dominate. The result is reduced efficacy, spray failures, and higher long-term control costs if not addressed early.

How herbicide resistance develops (and why it can happen fast)

In any weed population, a tiny number of plants may naturally carry genetic traits that make them less sensitive (or resistant) to a herbicide. When that herbicide is applied, susceptible plants die, and the resistant ones survive and seed - shifting the population each season.

Why resistance can catch you off guard

Resistance doesn’t always creep in slowly. You can have years of “business as usual” control, then suddenly the same spray program fails hard. If you rely on one product year after year, resistance is not a question of if - it’s when.

What increases the risk?

• Some weed species and herbicides are more prone to resistance than others.
• To truly rotate, you need products with a different mode of action (not just a different brand name).
• Some resistance is controlled by a single gene (can appear quickly); other cases involve multiple genes (slower to build).

What’s happening in New Zealand?

Globally, glyphosate resistance has been increasing since it was first documented in 1996, and dozens of weed species now show resistance.

In New Zealand, the first confirmed case of glyphosate resistance was identified in annual ryegrass in a Marlborough vineyard, through research led by the Foundation for Arable Research (FAR) with Ministry for Primary Industries (MPI) funding.

Warning signs to watch for

• A particular weed species survives a routine treatment that has worked for years (same location, same product).
• You see significantly reduced efficacy in the same spot with the same program - and especially if it happens twice.

Important: Not every spray miss is resistance. Weather, timing, coverage, water quality, plant stress, and rate all matter - but consistent reduced performance should trigger investigation.

Practical management strategies

1) Prevention (best option)

• Rotate herbicides with different modes of action over time.
• Avoid repeating the same mode of action year after year in the same area.
• Use integrated weed management where possible (competitive pasture/crop, grazing management, cultivation where appropriate, etc.).

2) If resistance is confirmed

• Switch to alternative modes of action that target both susceptible and resistant plants.
• For ryegrass, a 2–3 year program with alternatives can help break the cycle, as ryegrass seed typically doesn’t persist longer than about three years in soil.

Why “just replace glyphosate” isn’t simple

Finding true glyphosate substitutes is difficult because few products match its combination of low cost, high safety, non-selective control, and no residual soil effect. Alternatives often involve trade-offs in one or more of these areas.

Bottom line

Herbicide resistance is manageable - but it rewards people who plan ahead. Rotate modes of action, avoid repeating the same program indefinitely, and investigate repeated “survivors” early before they become the new normal.

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