Identification of the first glyphosate-resistant capeweed (Arctotheca calendula) population.

BACKGROUND: Glyphosate is routinely used in Australia to control the Arctotheca species Arctotheca calendula (L.) Levyns (referred hereinafter as capeweed). This study identifies the first global case of field-evolved glyphosate-resistant capeweed, collected from the grainbelt of Western Australia. RESULTS: In 2020, a capeweed biotype that was collected from Borden in the southern Western Australian grainbelt was confirmed to be glyphosate-resistant (referred hereinafter as Spence population). When compared to the pooled mortality of six field-collected, glyphosate susceptible capeweed populations (S1, S2, S3, S4, S5 and S6), the Spence population was found > 11-fold more resistant to glyphosate than the pooled results of the susceptible populations (S1-S6) at the lethal dose of 50% (LD50 ) level. The growth of the Spence population was also less affected, requiring > 13-fold more glyphosate to reduce growth than the pooled susceptible populations at the growth reduction of 50% (GR50 ) level. Sequencing of the plastidic 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene indicated no known single gene mutation imparting glyphosate resistance. This study, however, did not investigate any other known mechanisms that impart glyphosate resistance. When screened at the field-applied rate, this Spence population was also found to survive an inhibitor of acetolactate synthase (ALS) (metosulam) and an inhibitor of phytoene desaturase (diflufenican). CONCLUSIONS: This is the first confirmation of glyphosate resistance evolution in a capeweed population globally. With capeweed resistance already confirmed to photosystem-I inhibiting herbicides (paraquat and diquat), this study emphasizes the importance of using integrated measures that do not depend only on the use of non-selective herbicides for controlling herbicide resistance-prone capeweed populations. © 2021 Society of Chemical Industry.

Pyroxasulfone efficacy for annual ryegrass control is affected by wheat residue height, amount and orientation.

BACKGROUND: Pre-emergent herbicides play an important role in conservation agriculture, however, crop residues on the soil surface in these systems can intercept a considerable amount of herbicide during application. Cutting crops relatively high at harvest has some advantages, such as allowing faster harvest, and this also means that there is less horizontal residue on the soil surface. This field study tested the impact of standing wheat residue height and amount of horizontal residue on the interception, leaching and weed-control efficacy of the pre-emergent herbicide pyroxasulfone in the 2015 and 2016 growing seasons. RESULTS: Spray coverage of pyroxasulfone declined from 14.6% to 7.5% with increasing amounts (0 to 4 t ha-1 ) of horizontal wheat residue. Horizontal wheat residue at 1 t ha-1 had 10.3% spray coverage (more herbicide interception) compared with 15.4% for the equivalent amount of standing residue. Greater amounts of horizontal residue also significantly reduced the efficacy of pyroxasulfone in controlling ryegrass in the field and decreased pyroxasulfone concentrations in the soil. Rainfall after herbicide application increased herbicide efficacy for all residue amounts. Generally, cutting standing residue higher resulted a relatively small decrease in spray coverage at the soil surface and weed control efficacy, and this was significant only between nil stubble and 0.3 m cut height. CONCLUSION: Cutting residue relatively high, leaving less on the surface, improves spray coverage and herbicide efficacy compared with having more horizontal residue. This research may assist farmers and advisors to maximize the efficacy of pre-emergent herbicide in no-tillage systems. © 2019 Society of Chemical Industry.

Rainfall affects leaching of pre-emergent herbicide from wheat residue into the soil.

No-tillage with stubble retention is a widely used cropping system for its conservation and yield benefits. The no-tillage farming system in southern Australia relies heavily on herbicides for weed management, but heavy crop residues may have a negative impact on the activity of pre-emergent herbicides applied. Any herbicide intercepted by the crop residue may not reach the soil surface without timely rainfall and may dissipate due to volatilisation, photo-degradation and/or microbial activity. Two experiments were carried out to investigate the interception of prosulfocarb, pyroxasulfone, and trifluralin herbicides by wheat residue and retention following simulated rainfall. For the first experiment, there were four simulated rainfall amounts (0, 5, 10, and 20 mm), three intensities (5, 10, and 20 mm h-1) and five application times (immediately after spraying herbicide, 6 h, 1, 7, and 14 days after spraying). In the second experiment, 20 mm of rainfall was applied at 10 mm h-1 in either 4 × 5 mm rainfall events over two days, 2 × 10 mm rainfall events over one day, or a single 20 mm rainfall event, with a no-rainfall control treatment. Bioassays were used to assess the herbicide activity/availability in the soil and remaining on the residue, using cucumber (Cucumis sativus L.) and Italian ryegrass (Lolium multiflorum Lam.) as indicator plants. At higher rainfall amounts, most of the herbicide leached from the stubble into the soil soon after application; more so with rain in one event rather than multiple events. However, the intensity of rainfall had no effect. Pyroxasulfone leached easily from the residue to the soil to potentially offer good weed control, prosulfocarb had an intermediary leaching effect, while only a small amount of trifluralin leached from stubble after rain. Therefore, in no-tillage situations with large amounts of crop residue present on the soil surface, herbicides that leach easily from the residue should be considered, like pyroxasulfone.

Effect of crop residues on interception and activity of prosulfocarb, pyroxasulfone, and trifluralin.

Crop residue retention on the soil surface in no-tillage system can intercept pre-emergent herbicides and reduce their efficacy. Three experiments were conducted to investigate the effect of crop residue amount (0, 1, 2 and 4 t ha-1), moisture (wet versus dry), type (wheat, barley, canola, chickpea and lupin) and age (fresh or aged for one year) on the interception and subsequent leaching of prosulfocarb, pyroxasulfone, and trifluralin from the residue into soil. Bioassays, using cucumber and annual ryegrass as indicator plants, were used to assess herbicide activity/availability in the soil and on the residue. Herbicide interception increased considerably as residue quantity increased from 2 to 4 t ha-1. After simulated rainfall, which washed herbicide into the soil, complete control of ryegrass occurred for trifluralin with 0 t ha-1 residue, for prosulfocarb with 0 and 1 t ha-1 residue, and for pyroxasulfone with all residue rates. Therefore, with rain or irrigation, pyroxasulfone was the herbicide least affected by high residue loads. Less chemical leached from the crop residue into the soil after rainfall, when prosulfocarb and trifluralin were applied to wet residue compared with dry residue, but the initial moisture condition had no effect on the leaching of pyroxasulfone from residue. If practically possible, farmers should minimise spraying prosulfocarb and trifluralin onto wet crop residue. Barley and wheat residues intercepted more herbicide than an equivalent mass of canola, chickpea or lupin residue, which was largely due to the increased ground cover with cereal residues. The effect of residue age on herbicide interception and leaching was relatively small and variable. Overall, more herbicide reached the soil when sprayed on one-year old residue than new residue, which was largely due to reduced ground cover with aged residue. A strong positive linear relationship existed between ground cover percentage and growth of bioassay species (r2 = 0.75). This means that there was little difference in the ability of residue to adsorb and retain herbicide between crop residue types and ages, such that farmers can simply use the ground cover of the crop residue to assess interception.