Herbicide-resistant ryegrass in a Canterbury arable rotation

How it arose. Repeated use of the same mode of action applies uniform selection. A small number of plants with a target-site mutation or enhanced metabolism survive, set seed, and because annual ryegrass is wind-pollinated and largely outcrossing, resistance genes spread through pollen as well as through seed. The seedbank is long-lived, so each survivor contributes to a problem that persists for several seasons. Continuous cereal and grass seed cropping with few broadleaf breaks means the same grass-active chemistry was used year after year.

Why A-to-B is not rotation. Group A (ACCase inhibitors such as diclofop and haloxyfop) and Group B (ALS inhibitors such as chlorsulfuron) are different chemical families, but alternating between them is not a mode-of-action rotation in any meaningful sense, because the resistance risk is managed by target site diversity, not brand diversity. Worse, both groups are single-gene, high-risk chemistries, and the population has already demonstrated it can survive both. Using a Group B product after Group A simply selects the same survivors twice.

Practical implication. There is currently no effective selective post-emergence grass herbicide available in wheat on this farm. Control must therefore rest on non-chemical tactics and on herbicides with different modes of action, such as Group D pendimethalin, which act on germinating seedlings rather than established plants and give partial control only.

The 40 plants/m² in three paddocks is already above the threshold where seed set must be prevented, not merely suppressed. Every plant allowed to set seed adds years to the problem.

Principles. Drive down the seedbank, prevent new seed set, diversify modes of action, and stop seed spreading to the five clean paddocks.

Plan.

1. Harvest weed seed control. Chaff deck: $55,000 capital, $40/ha operating. On a 380 ha programme the capital amortises at about $11,000/yr, and operating is $15,200/yr, around $69/ha all up. It intercepts seed at harvest and is the single most important tool once plants are already in the crop.
2. One-off mouldboard plough on the worst ~60 ha at $210/ha = $12,600. This buries seed below emergence depth; it also brings up old seed, so it is a reset, not a cure.
3. Full spring fallow on the worst 40 ha: forgone gross margin ≈ $64,000. Multiple germinations followed by knockdown deplete the seedbank substantially.
4. Break crop on ~60 ha: faba beans at $650/ha versus wheat at roughly $1,600/ha, an opportunity cost of about $57,000, but it opens different herbicide groups and a different sowing window.
5. Crop competition and pre-emergence: higher seed rate and narrow rows (~$60/ha) plus Group D at $95/ha across the affected area, about $20,000 in year one.

Total year-one and year-two outlay is roughly $170,000–$180,000, close to the overdraft headroom, against an ongoing cost of resistance of about $67,500/yr in yield alone on 200 ha, plus contamination downgrades.

What is given up. Cash margin in the short term, one season of wheat on the fallow, and some flexibility in the rotation. In exchange, the plan protects the five clean paddocks and the long-term value of the ryegrass seed enterprise, which is the real exposure.

Sequencing matters. Chaff deck and crop competition first, because they are cheap per hectare and protect the clean paddocks immediately; ploughing and fallow only where density justifies it.

It should change the plan very little, and treating it as the solution would be a mistake.

Why not wait. The seedbank will keep growing for four years if nothing is done, and the cost of resistance is ongoing, roughly $67,500/yr in wheat yield alone plus contamination risk to the seed crop. Waiting also risks spread into the five clean paddocks, which is the larger long-term loss. The new chemistry would arrive onto a much bigger problem.

Why not build the plan around it. Any new mode of action is likely to be expensive per hectare, initially limited in the crops it can be used in, and subject to exactly the same selection pressure that produced this problem. A single-gene target site will select resistant individuals quickly if used repeatedly. Regulatory approval also carries timing risk; four years is an expectation, not a contract.

How to use it. Reserve the new chemistry as a one-off rescue option on the worst paddocks, used in mixture or sequence with a different mode of action, and never as the backbone of the programme. Keep the non-chemical base - harvest weed seed control, competition, fallow, break crops - permanently in place, because that base is what protects every chemistry the farm uses.

Test before relying on it. Before any registration, run a resistance screening on seed collected from the three affected paddocks. If the population has enhanced metabolism, cross-resistance to the new group is possible and the rescue option may be worth less than expected.

The plan should therefore be built to work without the new product, and be flexible enough to exploit it if and when it arrives. Any plan that depends on a future chemistry is a plan with a single point of failure.