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To estimate the prevalence of herbicide-resistant weeds, 87 wheat and barley farms were randomly surveyed in the Canterbury region of New Zealand. Over 600 weed seed samples from up to 10 mother plants per taxon depending on abundance, were collected immediately prior to harvest (two fields per farm). Some samples provided by agronomists were tested on an ad-hoc basis. Over 40,000 seedlings were grown to the 2-4 leaf stage in glasshouse conditions and sprayed with high priority herbicides for grasses from the three modes-of-action acetyl-CoA carboxylase (ACCase)-inhibitors haloxyfop, fenoxaprop, clodinafop, pinoxaden, clethodim, acetolactate synthase (ALS)-inhibitors iodosulfuron, pyroxsulam, nicosulfuron, and the 5-enolpyruvyl shikimate 3-phosphate synthase (EPSPS)-inhibitor glyphosate. The highest manufacturer recommended label rates were applied for the products registered for use in New Zealand, often higher than the discriminatory rates used in studies elsewhere. Published studies of resistance were rare in New Zealand but we found weeds survived herbicide applications on 42 of the 87 (48%) randomly surveyed farms, while susceptible reference populations died. Resistance was found for ALS-inhibitors on 35 farms (40%) and to ACCase-inhibitors on 20 (23%) farms. The number of farms with resistant weeds (denominator is 87 farms) are reported for ACCase-inhibitors, ALS-inhibitors, and glyphosate respectively as: Avena fatua (9%, 1%, 0% of farms), Bromus catharticus (0%, 2%, 0%), Lolium spp. (17%, 28%, 0%), Phalaris minor (1%, 6%, 0%), and Vulpia bromoides (0%, not tested, 0%). Not all farms had the weeds present, five had no obvious weeds prior to harvest. This survey revealed New Zealand's first documented cases of resistance in P. minor (fenoxaprop, clodinafop, iodosulfuron) and B. catharticus (pyroxsulam). Twelve of the 87 randomly sampled farms (14%) had ALS-inhibitor chlorsulfuron-resistant sow thistles, mostly Sonchus asper but also S. oleraceus. Resistance was confirmed in industry-supplied samples of the grasses Digitaria sanguinalis (nicosulfuron, two maize farms), P. minor (iodosulfuron, one farm), and Lolium spp. (cases included glyphosate, haloxyfop, pinoxaden, iodosulfuron, and pyroxsulam, 9 farms). Industry also supplied Stellaria media samples that were resistant to chlorsulfuron and flumetsulam (ALS-inhibitors) sourced from clover and ryegrass fields from the North and South Island., Competing Interests: The authors have declared that no competing interests exist.

A government research grant and industry co-funding allowed us to briefly provide free tests of suspected cases of herbicide resistance using traditional seedling-based assays. Information about the service was disseminated through the Foundation for Arable Research and other weed management professionals. Between 2019 and 2022 we received 248 seed samples collected by rural professionals from individual plants suspected of resistance. We successfully tested 195 samples from eight species from 57 farms. Rural professionals were good at recognising cases of resistance; 77% of these seed samples from 40 farms (70%) had resistance confirmed. We gained insights into contextually unique cases of resistance previously undescribed in the literature in New Zealand, including ryegrass resistance to glyphosate in a barley crop, Poa annua resistance to iodosulfuron in wheat, and Phalaris minor resistance to iodosulfuron in wheat. Resistance was typically confirmed 6–8 weeks after planting seed samples, though for species with seed after-ripening requirements to ensure consistent germination, results took as long as 6 months. The results facilitated conversations about long-term resistance avoidance strategies. Future costs of an independent resistance testing service would ideally be shared by the stakeholders benefitting from the crop protection product value chain, from herbicide producers to land managers. [ABSTRACT FROM AUTHOR]