Strategies to mitigate and manage herbicide resistance for key herbicides and herbicide challenges when dry sowing
Strategies to mitigate and manage herbicide resistance for key herbicides and herbicide challenges when dry sowing
Author: Chris Preston, Jenna Malone (University of Adelaide), Navneet Aggarwal (SARDI), Craig Prior (Elders) | Date: 05 Feb 2025
Take home messages
- Modern pre-emergent herbicides make dry sowing a practical option.
- More persistent and less-soluble pre-emergent herbicides are the best choices for dry sowing.
- Rotating pre-emergent herbicides is essential to manage resistance to these herbicides.
Dry sowing is a practical option with current pre-emergent herbicides
With autumn rainfall being uncertain in recent years and large crop programs to sow, many have been looking at dry sowing as a means of getting the crop in on time. With the very late start to the season in South Australia in 2024, a considerable proportion of the crop was sown dry. Perhaps the most important lesson learnt about dry sowing from 2024 is that it can be done with the pre-emergent herbicides we currently have. In some cases, herbicides had been applied six weeks prior to the first rainfall and were still present and able to control annual ryegrass when rain arrived.
There are several factors to consider when choosing the right pre-emergent herbicide for dry sowing. Firstly, it is important to understand where moisture is in the soil profile. If the soil is dry to sowing depth, different herbicide decisions should be made compared with if only the first centimetre of soil is dry. If the soil is dry to depth, the main considerations are to choose pre-emergent herbicides that are less mobile in the soil, and which have longer persistence. In dry soil, the first rainfall will move herbicides further through the soil profile than if there is moisture in the soil. This can increase the amount of herbicide that reaches the crop seed resulting in crop damage. In addition, highly soluble herbicides can be moved below the root zone of the weeds, leading to poor control.
Achieving effective weed control
A second consideration in dry sowing is that the pre-emergent herbicide will be required to control the whole population of weeds, as there will be no knockdown herbicide used. This will put considerable pressure on the pre-emergent herbicide, and it should be expected there will be a few escapes. If pre-emergent herbicides with lower solubility are chosen, weeds may escape the herbicide on the shoulder of the furrow. There may be a need to follow up the pre-emergent herbicide with an early post-emergent herbicide to control ryegrass that escapes the pre-emergent herbicide.
In 2024, due to the low rainfall after sowing, a pyroxasulfone-based herbicide applied pre-emergent followed by prosulfocarb + s-metolachlor (Boxer Gold®) early post-emergent often provided better results than a prosulfocarb herbicide pre-emergent followed by aclonifen + pyroxasulfone + diflufenican (Mateno® Complete) early post-emergent. The low rainfall through July meant that activation of aclonifen + pyroxasulfone + diflufenican (Mateno® Complete) was delayed and some ryegrass that emerged after sowing was not controlled. The relatively low rainfall for the rest of the growing season meant there was less late ryegrass emerging after the post-emergent prosulfocarb + s-metolachlor (Boxer Gold®) had decayed. As the season stayed dry, some growers who had used a less effective pre-emergent herbicide choice planning to use aclonifen + pyroxasulfone + diflufenican (Mateno® Complete) early post-emergent, chose to not apply the aclonifen + pyroxasulfone + diflufenican (Mateno® Complete) and had more ryegrass in crops as a consequence.
While pre-emergent herbicides generally lasted well in the soil, some herbicides, such as prosulfocarb and prosulfocarb + s-metolachlor (Boxer Gold®) that have less persistence, did not control ryegrass as well as the more persistent herbicides. The small amount of loss of herbicide while sitting in the soil has a bigger effect on herbicides with less persistence than those with more persistence. Therefore, in dry sowing situations, herbicides with longer persistence should be used prior to sowing.
Dry sowing practice
Attention to seeding depth and best practice was another lesson learnt from dry sowing in 2024. With dry sowing, there will be an increased risk of crop damage, as there is no moisture in the soil to slow herbicide movement on the first rainfall events. This was particularly evident for bixlozone (Overwatch®) on wheat in 2024, where there was more damage than observed in previous years.
Damage was more likely on lighter soil types where herbicides are more mobile. However, damage was also seen where the crop was sown too shallow, as well as in situations where herbicide-treated soil was moved into the furrow. These problems highlight the need for additional attention to detail when sowing dry to ensure the crop is not excessively damaged by the herbicide.
Rainfall patterns can affect weed control following dry sowing
While most herbicides retained their efficacy with dry sowing in 2024, the amount and timing of rainfall influenced the effectiveness of each of the herbicides for weed control. This is illustrated in a dry-sowing trial conducted at Redbanks in South Australia in 2024 (Table 1).
Table 1: Herbicide products, active ingredients and rates used in the dry sowing trial at Redbanks, South Australia.
Herbicide product | Active ingredients | Application rate (L/ha or kg/ha) |
---|---|---|
Avadex® Xtra | Triallate 500g/L | 2 |
Boxer Gold® | Prosulfocarb 800g/L S-Metolachlor 120g/L | 2.5 pre-emergent 3 early post-emergent |
Luximax® | Cinmethylin 750g/L | 0.5 |
Mateno® Complete | Pyroxasulfone 100g/L Aclonifen 400g/L Diflufenican 66g/L | 1 |
Overwatch® | Bixlozone 400g/L | 1.25 |
Sakura® | Pyroxasulfone 850g/kg | 0.118 |
Sakura® Flow | Pyroxasulfone 480g/L | 0.21 |
TriflurX® | Trifluralin 480g/L | 2 |
Voraxor® | Saflufenacil 250g/L Trifludimoxazin 125g/L | 0.2 |
This trial was sown dry on 27 May and the first rainfall events were 6mm from 30 May to 1 June, followed by 11mm from 12 June to 15 June, and 10mm on 20 June and 21 June. These low sporadic rainfall events left the surface dry for long periods of time, leading to less control of annual ryegrass than normal (Table 2). The addition of saflufenacil + trifludimoxazin (Voraxor®) to the pre-emergent grass herbicides did not improve control of annual ryegrass in this trial. The addition of a broadleaf pre-emergent herbicide could be useful to control broadleaf weeds, as well as grass weeds, in dry sowing situations.
Table 2: Annual ryegrass control by pre-emergent and early post-emergent herbicides in a dry sown wheat crop at Redbanks, South Australia
Herbicide treatment | Annual ryegrass 28 DATa | Annual ryegrass 90 DATa |
---|---|---|
Plants/m2 | ||
Nil | 493 a | 327 a |
Sakura® Flow | 274 ab | 118 b |
Mateno® Complete | 250 ab | 119 b |
Overwatch® | 153 bc | 29 b |
Luximax® | 267 ab | 178 b |
Boxer Gold® followed by Mateno® Complete | 136 bc | 95 b |
TriflurX® followed by Mateno® Complete | 203 ab | 76 b |
Nil followed by Mateno® Complete | 206 ab | 84 b |
Overwatch® followed by Mateno® Complete | 41 c | 26 b |
TriflurX® followed by Boxer Gold® | 300 ab | 109 b |
Sakura® Flow + Voraxor® | 395 ab | 121 b |
Overwatch® + Voraxor® | 100 bc | 112 b |
Means in each column with different letters are significantly different (P <0.0001).
aDays after application of the early post-emergent herbicides on 16 June 2024.
The early post-emergent herbicides were applied on 16 June and were followed by 10mm of rain on 20 June and 21 June, 8mm on 26 June and 27 June and 12mm on 29 June. This was sufficient to activate the early post-emergent herbicides in this trial. Bixlozone (Overwatch®) followed by aclonifen + pyroxasulfone + diflufenican (Mateno® Complete) provided the best annual ryegrass control.
The continuing dry conditions meant that annual ryegrass numbers declined as the season continued, resulting in lower populations at 90 days after the early post-emergent application (Table 2). A similar level of control was provided by many of the herbicide choices at this time.
Managing resistance to pre-emergent herbicides
As pre-emergent herbicides are now the main tool for managing annual resistance in cropping systems, it is important that resistance to these herbicides is managed. We established trials to compare strategies for the management of resistance to pre-emergent herbicides. To ensure herbicide-resistant annual ryegrass was present, Group 15 resistant seed was sown into the trials. A set of potential resistance management strategies involving Group 15 and other pre-emergent herbicides were employed over three years (Table 3).
Table 3: Herbicides used for each resistance management strategy over three successive crops in a trial of resistance management strategies conducted at Roseworthy, South Australia from 2021 to 2023.
Crop Strategy | 2021: Wheat | 2022: Faba bean | 2023: Wheat |
---|---|---|---|
Nil | Untreated | Untreated | Untreated |
Rotate Group 15 | Sakura® | Avadex® Xtra | Sakura® |
Mix | Boxer Gold® | Boxer Gold® | Boxer Gold® |
Mix and rotate | Sakura® + Avadex® Xtra | Boxer Gold® + Avadex® Xtra | Sakura® + Avadex® Xtra |
Rotate other Groups | Luximax® | Overwatch® | Luximax ® |
The mix and rotate and rotate strategies resulted in lower annual ryegrass populations and less seed production (Table 4). This occurred despite the presence of resistance to some of the herbicides used. This indicates that pre-emergent herbicides should be rotated across the cropping program. Better annual ryegrass control also resulted in significantly increased crop yields in the trial (Table 4).
Table 4: Annual ryegrass populations and grain yield in Year 3 of the trial testing resistance management strategies at Roseworthy.
Strategy | Ryegrass density 5 WAS (plants/m) | Ryegrass seed heads (spikes/m) | Grain yield (t/ha) |
---|---|---|---|
Nil | 688 a | 2440 a | 1.98 c |
Rotate Group 15 | 56 c | 96 c | 3.34 a |
Mix | 155 b | 190 b | 2.93 b |
Mix and rotate | 11 d | 16 d | 3.36 a |
Rotate other Groups | 12 d | 26 d | 3.57 a |
Means in each column with different letters are significantly different (P <0.0001).
Acknowledgements
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The trial at Redbanks was funded by the South Australian Drought Resilience Adoption and Innovation Hub funded by the Department of Agriculture, Fisheries and Forestry through the Future Drought Fund.
Contact details
Christopher Preston
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au
GRDC Project Code: UOW2007-007RTX,