Paraquat ain’t paraquat: improving the use of paraquat for cost-effective glyphosate and paraquat resistance management on-farm

Overall Objective

The aim of the study is to conduct novel research to guide WA grain growers to control weeds by choosing the most cost-effective use pattern of knock-down herbicides.

In 2022 for the first time in WA paraquat resistance and glyphosate resistance were identified in annual ryegrass.  Glyphosate resistance is perceived to increase in WA field situations.  Thus, an increasing proportion of WA growers have “switched” to paraquat and relying heavily on paraquat to control ryegrass in double knock-down treatments prior to crop sowing, chemical fallow or along fence lines.  Previous field research has shown that different paraquat formulations and adjuvants can results in a wide range of weed control efficacies.

The objective of this proposal is to systematically to measure the efficacy of different paraquat formulations, quantify interaction with pre-emergent herbicides and fine-tune the use of adjuvants or wetter to deliver highly effective knock-down treatments and drastically mitigate the emerging issue of dual glyphosate and paraquat resistant ryegrass.

Project Synopsis

This study quantified the kill rate (%), the suppression of biomass (% untreated control) and the reduction of seed set (% untreated control) of two annual ryegrass populations multiple resistant to glyphosate and paraquat and two populations resistant to glyphosate only.  One population susceptible to all herbicide treatments tested was used as control.  A total of 28 herbicide treatments including stand-alone, mixture and double-knock treatments were evaluated to provide insight for the cost-effective control of ryegrass and long-term management of multiple herbicide resistance.

The overall analysis of results suggests that herbicide mixtures and integration of different herbicide modes of action are effective to control glyphosate and paraquat resistance.  Under controlled conditions it was observed that herbicide mixtures applied to smaller resistant ryegrass plants can be more effective than double knock strategies (see Table 5).

In the presence of only glyphosate resistance, as expected, the majority of paraquat-based treatments have been observed to be significantly more effective than glyphosate-based treatments.  Under controlled conditions it was observed that mixtures of glyphosate and Terrad’or were not effective to control glyphosate resistance (Tables 6-8).  In the field it was also observed that glyphosate, Terrad’or ® and Boxer Gold® or Mateno Complete (treatments #3, 4, 5, 28) did not deliver a significantly greater control of glyphosate-resistant ryegrass (Table 15).  Mixtures of glyphosate with Overwatch® appeared to deliver significantly greater control of glyphosate resistance (Table 15).

The results obtained with ryegrass with dual resistance to glyphosate and paraquat were more complex.  In general treatments including glyphosate (and a range of other residual herbicides across different modes of action) did not deliver a significantly greater weed control.  Conversely, treatments with paraquat in combinations with other herbicide modes of action and herbicides with residual activity were significantly more effective (Tables 9-14).

There were difference between the two populations with dual resistance to glyphosate and paraquat but in general the overall resistance (compounded ability to survive, grow and set seed) to glyphosate and paraquat was dramatically reduced when paraquat was tank-mixed with, Mateno Complete (pyroxasulfone, diflufenican and aclonfen), Overwatch (bixlozone), Sakura (pyroxasulfone) + Terrad’or (tiafenacil), Ruslter (propyzamide) or co-formulated with high dosage of amitrole (ie 125g/L paraquat + 250g/L amitrole).  Two sequential treatments of paraquat (“double knock” treatments) appeared more effective than glyphosate followed by paraquat.  It appears that treatments with paraquat mixed with other modes of action (ie applied to 2-leaf stage ryegrass plants) are more effective than double knock strategies which generally delay treatments with a paraquat applied to resistant plants at a more advance growth stage (ie ryegrass plants at a 4-6 leaf stage).  This was confirmed also in the field trials with the population 90-23 with dual resistance to glyphosate and paraquat.  The same field trial revealed relatively low control of ryegrass for most treatments.  The data from this trial indicate that double knock strategies are delivering greater control of ryegrass (Table 16).  The was effective treatment (#27) was based on glufosinate (+ Terrad’or in the 1^st knock) followed by paraquat mixed with Sakura® + Voraxor®.  This treatment was also very effective under controlled conditions with plants grown in pots (See Tables 9-11 and Table 16).

The findings of this study provided useful insight for herbicide stewardship and herbicide resistance management.  Field trials in sites or regions where dual paraquat and glyphosate resistance is detected will continue in 2024 for a final year to validate 2022 and 2023 field results.  A very big effort was made to extend these results in 2023 and 2024 with the aim to raise awareness on the emerging issue of glyphosate and paraquat resistance selected on farm in marginal areas such as fencelines and firebreaks. The research findings were widely shared to industry via social media, presented and discussed with industry stakeholders at > 20 different events including 2024 Bendigo GRDC Updates and 2024 Perth GRDC Updates, Salmon Gums, Lake Grace, Yuna, Mukinbudin, Darkan regional GRDC Updates and two field walks organized with MADFIG and Stirling to Coast Groups.