New and old herbicides - the best integration to prolong their impact

Take home messages

• The efficacy of the herbicide mix; clethodim and butroxydim is significantly greater than either clethodim or butroxydim when applied stand-alone.
• The efficacy of the herbicide mixture; trifluralin and prosulfocarb is significantly greater than either trifluralin or prosulfocarb when applied stand-alone.
• There is little or negligible resistance to some mixtures of pre-emergent herbicides.
• A comprehensive test of old and new herbicides (stand-alone and mixtures) helps to find solutions for effective control of weeds and mitigation of resistance.

Background

In Australia herbicide resistance has increased in a number of major weed species such as annual ryegrass. Resistance has evolved to many foliar post-emergent herbicides and their efficacy has been largely compromised. Grain growers have responded to resistance by the adoption of soil-applied pre-emergence herbicides. Many grain growers have responded to the escalating herbicide resistance challenge by adopting harvest weed seed control in combination with strategic use of old chemistry. However, new cases of herbicide-resistant weeds continue to be reported, largely due to the overreliance on herbicides in current farming operations. New pre-emergence herbicides have also been developed and commercialised in Australia, but initial cases of field resistance have recently been reported. Herbicide mixtures have been shown to decrease the risk of resistance evolution but they have been rarely tested on weed seed samples collected from problematic paddocks and targeted or random geographical surveys.

From 2020 there are several new herbicides available to control annual ryegrass. This study aims to report the most up-to-date state of knowledge of herbicide resistance levels in the most damaging weeds infesting Australian grain crops. The study aims to reinforce the benefits of proactively testing for herbicide resistance in major weed species with special emphasis on the adoption of herbicide mixtures to mitigate the impact of herbicide resistance on farm profitability. The observed efficacy of weed control, the frequency of resistance to old and new herbicides and the future of herbicide resistance testing with a focus on herbicide mixtures are reported and discussed.

Methods

Weed seed sample collection

Seed samples of annual ryegrass were collected from cropped paddocks at different locations in Western Australia (WA) in 2018 and 2019. Approximately 140 populations of annual ryegrass were screened and tested for resistance. Within each farm, samples were collected from paddocks chosen according to the grower/consultant resistance perceptions. Weed seed was bulked to obtain one population per sampled paddock at the time of collection. Seeds were stored in dry conditions and prepared for herbicide testing. Herbicides were applied at the correct stage to the soil (PRE, pre-emergence herbicides) or to two-leaf seedlings (POST, post-emergence herbicides) at dosages indicated in Table 1. Plant survival was assessed after one month after treatment. Mean values of plant survival and resistance frequency are presented in this paper as percentages (%).

Data analyses

Populations were classified as susceptible with zero to 5% plant survival. Populations with resistant survivors were classified into two groups: those with ≥20% plant survival and those having <20% survival (this includes all plant survival between 6% and 19%). ANOVA was conducted on plant survival data expressed as percentage and means were separated with Tukey’s test. Resistance frequencies were analysed by chi-square analysis and means separated with heterogeneity tests.

Table 1. Herbicide products, formulations, Herbicide Resistance Action Committee (HRAC) classification, use (PRE, POST or mixtures) and dosages used to assess plant survival (%) in populations of annual ryegrass and wild radish collected in Western Australia in 2018 and 2019 from cropped paddocks.

**Data not shown in this paper but will be presented orally at the ‘2020 GRDC Grains Research Updates’.

Results and discussion

Overall, there was 15% plant survival to the POST herbicides tested, which indicates substantial herbicide resistance in a large proportion of samples, whereas the mean survival to PRE herbicides was only 2%, reflecting effective control of the many annual ryegrass field populations tested.

Post-emergence herbicides (POST)

As expected and reported in several random herbicide resistance surveys of annual ryegrass in WA, resistance frequency to diclofop-methyl was high, with >90% samples tested classified as resistant, with a mean plant survival of >40% observed in the samples tested (Figure 1). Approximately 60% of the tested samples were clethodim-resistant and the overall frequencies of resistance and developing resistance were significantly lower than diclofop-methyl (Table 2). Overall, the mean survival observed across all tested samples was 12% (Figure 1). With butroxydim, there was a similar frequency of developing resistance but significantly lower frequency of resistance samples (Table 2). Similarly, the mean survival observed was 6%, which was statistically lower than the plant survival in response to clethodim (Figure 1). The survival to the mixture of clethodim + butroxydim was the lowest (approximately 2%) but not significantly lower than survival to butroxydim. Conversely, the proportions of samples classified as developing resistance or resistant in response to the mixture clethodim + butroxydim were significantly lower than in response to stand-alone application of either herbicide (Table 2).

Figure 1. Mean plant survival (%) observed across 140 field populations of annual ryegrass collected in Western Australia in 2018 – 2019 and tested for herbicide resistance at the recommended label dose of three POST ACCase stand-alone herbicides and a binary mixture. Different letters indicate significantly different mean values separated by multiple comparisons by a post-hoc Tukey test (P < 0.05).

Table 2. Herbicide resistance frequencies of 140 populations of Lolium rigidum collected in Western Australia in 2018 - 2019 and tested for herbicide resistance to POST ACCase herbicides and their mixtures. Tested samples were divided into three categories according to the percentage survival observed at the recommended label dose. Herbicide “Resistance”was diagnosed with ≥ 20 % survival, “Developing” resistance with survival ranging between 6% - 19% and “Susceptible” samples with survival ≤5%. Within each column different letters indicate significantly different resistance frequencies (as proportions of samples resistant, developing or susceptible to each respective herbicide). Values were separated by multiple comparisons with a chi square heterogeneity test performed using the statistical software R with the command prop.test.

Pre-emergence herbicides (PRE)

The mean survival to trifluralin was significantly greater than all other PRE herbicide treatments tested and ranged widely from 0% up to approximately 90% in a few samples (Figure 2). Similarly, trifluralin resistance was found in > 50% of the samples (Table 3). Survival to prosulfocarb was significantly lower than trifluralin, but greater than all other PRE treatments except for cinmethylin (Figure 2). There was a similar frequency of developing resistance to prosulfocarb and trifluralin, which was significantly higher than all other tested herbicides (Table 3). The observed mean plant survival to all other herbicide treatments was similarly low, including stand-alone and binary mixtures (Figure 2). There were no highly resistant samples to several PRE herbicides except for 3% to prosulfocarb and 17% to trifluralin (Table 3). The frequency of developing resistance was similar in all other treatments except for the mixtures trifluralin + triallate and pyroxasulfone + triallate (Table 3). There were no samples found to be resistant to these two highly effective mixtures: trifluralin + triallate and pyroxasulfone + triallate (Table 3).

Figure 2. Mean plant survival (%) observed across 140 field populations of annual ryegrass collected in Western Australia in 2018 – 2019 and tested for herbicide resistance at the recommended label dose for four PRE stand-alone herbicides versus six binary mixtures of the same herbicides versus two new herbicides. Different letters indicate significantly different mean values separated by multiple comparisons by a post-hoc Tukey test (P < 0.05).

Table 3. Herbicide resistance frequencies of 140 populations of Lolium rigidum collected in Western Australia in 2018 - 2019 and tested for herbicide resistance to PRE herbicides and their mixtures. Tested samples were divided into three categories according to the percentage survival observed at the recommended label dose. Herbicide “Resistance”was diagnosed with ≥ 20 % survival, “Developing” resistance with survival ranging between 6% - 19% and “Susceptible” samples with survival ≤5%. Within each column different letters indicate significantly different resistance frequencies (as proportions of samples resistant, developing or susceptible to each respective herbicide). Values were separated by multiple comparisons with a c2 heterogeneity test performed using the statistical software R with the command prop.test.

**Field samples with survival >6% to cinmethylin re under investigation and data is not reported.

Conclusion

Annual ryegrass can evolve multiple resistance to D, J and K herbicides. However, consistently, herbicide mixtures of PRE herbicides (Group D, J and K) were highly effective in controlling resistant ryegrass. No resistance was found to some herbicide mixtures of PRE herbicides. Herbicide mixtures should be tested and then adopted to reduce population size and risk of herbicide resistance.

In Australia proactive herbicide resistance testing has been considered by growers and consultants to closely monitor the rate of herbicide resistance evolution. As there are some field populations of annual ryegrass reported to be multiple-resistant to trifluralin, prosulfocarb, triallate and pyroxasulfone (Busi et al. 2012, Busi and Powles, 2013, 2016, Brunton et al. 2018), it is recommended to closely monitor resistance on a proportion of the problematic paddocks on farm.

Some herbicide mixtures remain fully effective and appear immune to resistance, and therefore, effective herbicide mixtures of ‘old’ and newly commercialised herbicides need to be identified by screening large numbers of field populations of annual ryegrass. Results immediately conveyed to growers and consultants could allow that herbicide mixture to be widely adopted as an effective solution to mitigate weed resistance.

It is finally emphasized that adoption of harvest weed seed control and other measures of integrated weed management should continue, in order to achieve diversity of selection pressures on weeds, keep weed numbers low and complement effective control achieved with knock-down, PRE and POST herbicides.

Acknowledgments

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 author would like to thank them for their continued support.

Thank you also for all the support received from the GRDC Western Region Team (Jo Wheeler, Elizabeth Von Perger and Curtis Liebeck) and the collaborators from the ConsultAg Team (Geoff Fosbery, Brad Joyce, Garren Knell, Trent Butcher, Ben Whisson).

A special mention to all growers and advisers involved in this research including Trevor Syme, Dustyn Fry, Geoff Fisher, Gary Lang, Matt Panizza, Ryan Pearce, Derek Young, Scott Young, Russel Burgess and Kit Leake.

The Australian Herbicide Resistance Initiative is a GRDC investment initiative.

References

Busi R, Gaines TA, Walsh MJ and Powles SB, Understanding the potential for resistance evolution to the new herbicide pyroxasulfone: field selection at high doses versus recurrent selection at low doses. Weed Res; 52(6): 489-499 (2012).

Busi R and Powles SB, Cross‐resistance to prosulfocarb and triallate in pyroxasulfone‐resistant Lolium rigidum. Pest Manag Sci; 69(12): 1379-1384 (2013)

Busi R and Powles SB, Cross‐resistance to prosulfocarb+ S‐metolachlor and pyroxasulfone selected by either herbicide in Lolium rigidum. Pest Manag Sci; 72(9): 1664-1672 (2016).

Brunton DJ, Boutsalis P, Gill G and Preston C, Resistance to Multiple PRE Herbicides in a Field-evolved Rigid Ryegrass (Lolium rigidum) Population. Weed Sci: 1-5 (2018).

Contact: Dr Roberto Busi, AHRI email: roberto.busi@uwa.edu.au

Contact details

Dr Roberto Busi
35 Stirling Hwy, Crawley, 6009 WA
08 6488 1423, 0415 185 553
roberto.busi@uwa.edu.au
@robbert115