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Herbicide resistant weeds in maize in New Zealand : A survey for herbicide resistant weeds in the Bay of Plenty and Waikato

Research on maize weeds in New Zealand is extensive, but few systematic efforts were made to catalogue weed species and determine their prevalence and abundance. Broadleaf weeds were early problems but became manageable when photosystem-II inhibitors and synthetic auxins were introduced in the 1950s. Annual grasses became the next problematic weeds, and newer herbicides were introduced in the 1970s. The first cases of resistance were reported in the 1980s, Chenopodium album L. and Persicaria maculosa Gray both evolved resistance atrazine after several years of selection. Perennial weeds Oxalis latifolia and Calystegia sepium ssp. roseata were identified as problem weeds. More herbicides were introduced. Atrazine-resistant C. album evolved additional resistance to dicamba in the eastern Waikato. The quarantine weed Abutilon theophrasti was unintentionally introduced to multiple maize farms. Most recently, the annual grass Digitaria sanguinalis evolved resistance to nicosulfuron in Bay of Plenty and Waikato maize. The most recent ‘weeds survey’ in maize is two decades old, in twenty Waikato, four Bay of Plenty and six Gisborne maize fields. Above-ground (spring seedlings) and below-ground (soil seedbank) weeds were recorded. Knowledge of current weed distribution and abundance is derived from multiple smaller studies, not a systematic study of maize weeds across a region. Published studies of herbicide resistant weeds across the world are documented on a database. Estimates of weed species relative risks of evolving resistance were done in previous studies using those data. A list comprising thirty-nine common weeds associated with maize was generated from literature. Weeds were ranked for their risk of evolving herbicide resistance with a scoring protocol that accounts for the specific herbicides used in New Zealand maize. Seven weed species were classified as having a high risk of developing herbicide resistance: Echinochloa crus-galli (L.) P.Beauv., Chenopodium album, Eleusine indica (L.) Gaertn., Xanthium strumarium L., Amaranthus powellii S.Watson, Solanum nigrum L. and Digitaria sanguinalis. Seventeen species were classed as moderate risk and 15 were low risk. Herbicide classes associated with more resistant species were classed as high risk, these included acetohydroxy acid synthase (AHAS) inhibitors and photosystem-II inhibitors. Synthetic auxin herbicides had a moderate risk but only two herbicides in this class (dicamba and clopyralid) are registered for maize in New Zealand. All other herbicide mode-of-action groups used in maize were low risk. When accounting for herbicide groups used in maize, E. crus-galli, E. indica, C. album, D. sanguinalis and A. powelli were the five highest risk weeds. Maize growers from two lists (Agribase and FAR) were called in random order for weed sampling. Thirty-six Waikato and sixteen Bay of Plenty maize fields were sampled prior to harvest, in late February-early March 2021. Weeds present were identified, and their percentage cover was estimated in two 133m transects. The first transect was in the ‘headland’, the area close to the field edge, and the second area in the centre of the maize field. Transect starting locations were randomized to minimize bias. Within each transect, six soil samples were taken at 33m intervals. Seeds were collected from individual plants throughout the field. As soon as possible, soil samples were taken back to the Ruakura glasshouse facility and germinated in trays. Seedlings were identified and enumerated, then soil was mixed; trays were assessed three times each. One-hundred and thirty-four plant species were observed in the fifty-two sampled sites in the field assessments and soil seedbanks. Digitaria sanguinalis (98%), Chenopodium album (85%) and Persicaria spp. (71%) were the most widespread weeds, with Setaria pumila, Sonchus oleraceus, Solanum nigrum, Erigeron spp. and Rumex obtusifolius seen in more than half of the farms. Digitaria sanguinalis, C. album, Persicaria spp., Cyperus rotundus, Echinochloa crus-galli had the highest percentage cover estimates, also common were Cynodon dactylon, Elytrigia repens, Paspalum distichum and Solanum nigrum. Winter and spring weeds Juncus bufonius, Poa annua and Stellaria media occurred frequently in soil seedbanks. A principal component analysis was done for weed percentage cover and weed seedbank densities for the fifty-two farms. For the percentage cover PCA C. album, C. rotundus, D. sanguinalis and Persicaria spp. were important taxa, and for the seedbank density PCA P. annua, J. bufonius, D. sanguinalis and S. media were important drivers of farm-farm compositional differences. Perennial weeds are a major problem in the eastern Bay of Plenty. Weeds with a history of evolved resistance C. album, P. maculosa and D. sanguinalis are common in both regions. Earlier studies showed that Chenopodium album L. evolved atrazine and dicamba resistance in Waikato farms, Persicaria maculosa L. in Waikato and Digitaria sanguinalis (L.) Scop. in the Bay of Plenty and Waikato farms. Apart from atrazine resistant C. album - presumed omnipresent in New Zealand maize - the prevalence of resistant weeds was unknown in maize. This study was the first systematic survey to estimate the prevalence of herbicide resistant weeds in Bay of Plenty and Waikato maize. Of the fifty-two farms visited, thirty-two, thirty-one and twenty-nine had C. album, D. sanguinalis and Persicaria spp. (P. lapathifolia, P. maculosa) seed collected and tested for herbicide resistance, respectively. Persicaria spp. seedlings did not survive any herbicide treatments, of atrazine (1500 g.ai.ha-1), dicamba (600 g.ai.ha-1), nicosulfuron (60 g.ai.ha-1) and mesotrione (96 g.ai.ha-1). Chenopodium album seedlings were treated with the same rates of the same herbicides as Persicaria spp., and no C. album samples survived dicamba, nicosulfuron or mesotrione, but samples from twenty-two farms survived atrazine with no visible damage. Farms with atrazine-resistant C. album populations had C. album higher seedbank densities than farms with susceptible populations. Digitaria sanguinalis seedlings were treated with nicosulfuron (60 g.ai.ha-1). Ten farms had D. sanguinalis samples that survived, but three of those had low rates of survival. Even nicosulfuron resistant D. sanguinalis were stunted by the herbicide. A dose-response experiment to confirm and determine the level of resistance was set up, where plants from twelve farms were treated with 0, 15, 30, 60, 120, 240 g.ai.ha-1 nicosulfuron. Most resistant samples survived 60 g.ai.ha-1, but two survived up to 240 g.ai.ha-1. None of the six farms near the the first nicosulfuron resistant D. sanguinalis case had resistance, except for sites managed by the same contractor as the original site. Farms with nicosulfuron-resistant D. sanguinalis populations had significantly higher seedbank densities and percentage cover for D. sanguinalis compared to farms with susceptible populations. A total of twenty-four farms (46%) had a resistant weed, five of those had both nicosulfuron resistant D. sanguinalis and atrazine resistant C. album. Resistant C. album was detected across both regions, but was only in one eastern Bay of Plenty farm; resistant D. sanguinalis was sporadically distributed, but only detected in Waikato randomly sampled farms. Results from the weed risk assessment show that more than half of species are at moderate-high risk of evolving herbicide resistance, but perennial weeds are less likely to evolve resistance. Commonly used photosystem-II inhibitors and AHAS inhibitors are high risk. The risk assessment may help maize growers avoid selecting for herbicide resistant weeds. The most observed weeds have either already evolved herbicide resistance (D. sanguinalis, C. album, P. maculosa) or were at high-risk of doing so (E. crus-galli). Perennial weeds are common in the eastern Bay of Plenty, especially C. rotundus. These weeds, and C. dactylon, E. crus-galli, E. repens, P. distichum and S. nigrum are abundant in fields. The most common weeds were tested for herbicide resistance, but only atrazine resistant C. album and nicosulfuron resistant D. sanguinalis were observed. Atrazine resistant C. album was common, in twenty-two farms (69% of tested farms, 42% of visited farms). No multiple-resistant C. album were detected. Nicosulfuron resistant D. sanguinalis was less common, in seven farms (23% of tested farms, 13% of visited farms). A similar proportion of resistant farms in Bay of Plenty-Waikato maize (46%) to Canterbury cereals (48%) was observed. Arable cropping is a repetitive environment that is highly selective for herbicide resistant, and tolerant, weeds. To prevent weeds from evolving herbicide resistance, integrated weed management practices are need. Chemical and non-chemical weed control strategies can reduce weed pressure and mitigate resistance evolution.
Type of thesis
The University of Waikato
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