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 with 255 species having resistant biotypes. In New Zealand, since 1979, resistance was found in 13 taxa, with >25 herbicides in 8 chemical classes showing reduced effectiveness (i.e. HRAC groups A, B, C, D, F, G, H, N and O). Cases included weeds in turf, pastures, orchards, vineyards, forage and arable crops. Surprisingly little is known about the spatial extent or frequency of this problem in New Zealand. We estimate that 14,000 farms have land-use histories like those favouring herbicide resistance historically. Sampling simulations of ≥10% farms provided good estimates of resistance prevalence for most regions and crop types. Acceptable sampling rates varied with target population size, actual resistance prevalence, and detection certainty. Our simulations provide a sobering caution regarding our ability to delimit the problem cheaply or accurately. Detection rates lower than 75% always give imprecise prevalence estimates. Sampling and screening involved 7.4 h labour and 27 km of travel costing $759 NZD per farm. Sampling 10% of farms would cost >$1 million NZD if lower risk farms were excluded, or >$3 million in exhaustive surveys. Regional farm and industry breakdowns could guide cost-sharing arrangements for surveys.