Costs and risks associated with surveying the extent of herbicide resistance in New Zealand

Herbicide resistance has repeatedly developed under intensive herbicidal weed management regimes globally (255 species have herbicide resistant biotypes). In New Zealand, since 1979, resistance has been found in 13 taxa, with >25 herbicides in 8 chemical classes showing reduced effectiveness (i.e., groups A, B, C, D, F, G, H, N and O). Cases documented included weeds in turfs, pastures, orchards, vineyards, forage and arable crops. Surprisingly little is known about the spatial extent or frequency of this problem in New Zealand. We assessed that at least 14,000 farms have land-use histories like those favouring herbicide resistance historically. Simulations show that sampling ≥10% of these farms should provide good estimates of resistance prevalence for most regions and crop types. Acceptable sampling rates varied with the size of the population under consideration, actual resistance prevalence, and the certainty of detection. Our simulations provide a sobering caution regarding our ability to delimit the problem cheaply or accurately. If detection rates are lower than 75% prevalence estimates are always imprecise. Per farm, sampling and screening costs were estimated at 8.9 hours labour, 270 km of travel $1087 (>$1.5 million NZD overall at 10% sampling). Regional and farm sector breakdowns provide an indication of the risk distribution and could guide cost-sharing arrangements for surveys.