Weed responses to high break crop intensity systems

Take home messages

• Controlling Group A and Group B herbicide resistant annual ryegrass and Group B resistant common sowthistle in break crops is becoming increasingly challenging.
• Resistance in ryegrass to pre-emergent herbicides such as Boxer Gold® and Sakura® has been confirmed in high break crop intensity (HBCI) systems. Therefore, very careful use of these chemistries is required, particularly in the break crop phase, with integration of alternatives such as propyzamide and other non-chemical tactics.
• Common sowthistle from paddocks with high imidazolinone (IMI)-use history had high incidence of IMI resistance in HBCI rotations. However, IMI resistance levels in paddocks where only sulfonylureas (SU) have been used, raise concern for resistance development to IMI in other broadleaf weeds, particularly where there is high use of Group B herbicide tolerant break crops.
• The Group B tolerant faba bean; Bendoc provides a new opportunity for managing broadleaf weeds, although care is required to avoid overusing the Group B tolerant varieties.
• Crop competition has the potential to reduce seed set of broadleaf weeds in some break crops.

Background

A number of improved herbicide tolerant break crop options are available such as triazine tolerant (TT) canola, IMI tolerant (Clearfield®) canola, and lentils e.g. PBA Hurricane XT. Their relatively high market prices, improved agronomic and disease characteristics and harvest efficiency have resulted in an expansion of the area sown to pulses and canola in South Australia (SA). The additional benefits of pulses in a cropping rotation include a break in cereal disease cycles, disease management, nitrogen fixation (Wani et al. 1994; Angus et al. 2015, Evans et al. 1989; Crews and Peoples, 2004; Fan et al. 2006, Hauggaard-Nielsen et al. 2009), and the use of diverse herbicide chemistry compared with that used in the cereal phase, particularly for grass weeds. This uptake has largely occurred in the Mid North (MN), Yorke Peninsula (YP) and Lower Eyre Peninsula (LEP), where the total area under break crops is higher than the national average (Figure 1).

Ryegrass control in break crops relied heavily on Group A chemistry (fops and dims). This has contributed to increased herbicide resistance, in particular to the dim chemistry, making ryegrass control challenging. Consequently, herbicides with different modes-of-action (Groups D, J and K) have been adopted to manage dim-resistant ryegrass in HBCI rotations. Careful management of Groups D, J and K herbicides is required to minimise selection for resistance to any single mode of action. The introduction of TT canola, IMI tolerant canola and lentils has also improved broadleaf weed control options such as triazines and IMI herbicides. However, they have resulted in a decreased frequency of other weed control tactics in these break crops (Storrie, 2014). Over-reliance on triazines and IMI herbicides in herbicide tolerant break crops for improved broadleaf weed management could result in shifts in the weed spectrum and increase the incidence of herbicide resistance. This has occurred in Canada with the evolution of sulfonylureas (SU) and IMI resistant wild mustard (Warwick et al. 2005), as a result of high intensity use of acetolactate synthase (ALS) inhibiting herbicides. This resistance in broadleaf weeds presents a big challenge for IMI tolerant lentils (Menalled, 2010).

A more sustainable approach is to include herbicide tolerant break crops as part of an overall weed management strategy, rather than focusing on short term weed control in individual crops and seasons. Introducing strategies such as crop-topping to reduce seed set can significantly reduce weed seed banks (Preston, 2014). The adoption of effective diverse strategies will therefore aid in reducing the risk of resistance developing in HBCI rotations.

Figure 1. Distribution of break crops across different regions of SA. Source: PIRSA Crop estimates

Paddock survey

A focus paddock survey was initiated in 2017 to increase understanding of potential changes in the weed flora and herbicide resistance, in response to low or high use of IMI in HBCI rotations (at least two break crops in the past five years) across different regions of SA. A total of 45 focus paddocks with HBCI rotations were selected (MN-16, YP-11, LEP-8, Upper Eyre Peninsula (UEP)-2, South East (SE)-4, SA Mallee-4). The selected paddocks had IMI tolerant break crops such as PBA Hurricane XT lentil (at least twice in the past five years) or Clearfield® canola (twice in the past five to six years) sown as a dominant break crop. In addition to these, paddocks with two non-IMI break crops (conventional lentil, conventional canola/TT canola, field pea, chickpea, faba bean, lupin) were included. Seeds from ryegrass and two dominant broadleaf weed species were collected prior to harvest in 2017. They were screened for resistance in outdoor pot trials conducted between autumn and spring in 2018 by Plant Science Consulting (Tables 1 and 2).

Ryegrass and common sowthistle were the dominant weed species encountered in the HBCI focus paddocks (Tables 1 and 2). Ryegrass resistance to SU and Dens (Axial®) was detected in almost all paddocks (Table 1). A high incidence of ryegrass resistance to IMI was observed in both high IMI-history paddocks (56% of samples) and no-IMI history paddocks (63% of samples). In addition, a total of 46% of ryegrass populations were found to be resistant, and 21% had started developing resistance to clethodim in HBCI paddocks (Table 1). This has started to limit the effectiveness of break crops as rotational tools. Resistance to the Group J and Group K herbicides Boxer Gold® and Sakura®, albeit at low levels, was confirmed in ryegrass and is a concern for HBCI rotations (Table 1). One quarter of the ryegrass populations exhibited resistance to Boxer Gold® (≥20% survivors). Half of the ryegrass biotypes resistant to Boxer Gold® originated from HBCI paddocks on LEP where canola was the dominant break crop. Biotypes with ≥ 20% survival to Sakura® were not detected, although <20% survival in pot trials (developing resistance) was confirmed in one third of ryegrass populations, predominately from LEP. The confirmed increase in resistance to pre-emergent herbicides highlights the need for careful and strategic use of these chemistries in HBCI rotations, particularly in the break crop phase. This might include rotational use of the Group D herbicide propyzamide with Group J and Group K herbicides in the break crop phase, as ryegrass from all HBCI paddocks showed a high level of susceptibility to propyzamide.

Table 1. Herbicide resistance in annual ryegrass collected from high break crop intensity paddocks in SA. Resistance (where ≥ 20% survival was confirmed in pot tests) and developing resistance (where < 20% survival was confirmed in pot tests) is presented.

*numbers (percent)

In common sowthistle, more than half of the biotypes exhibited resistance to SU and IMI herbicides (Table 2). The majority of SU resistant populations exhibited cross-resistance to imazapic (93% of samples) and Intervix (imazamox + imazapyr) (73% of samples). Only one population was confirmed to be susceptible to both SU and IMI. All common sowthistle populations from paddocks with high IMI-use history were resistant to imazapic, and 69% were resistant to Intervix. It is concerning that 50% of populations from paddocks with non-IMI history were resistant to both IMI herbicides. The target site of all Group B herbicides is the enzyme ALS. Multiple target-site mutations in the ALS gene have been confirmed in resistant weeds, conferring resistance across chemical families of ALS-inhibiting herbicides (Tranel and Wright 2002). ALS target site cross-resistance to SU and IMI was observed in common sowthistle and is of concern for the sustainability of HBCI rotations dominated by IMI tolerant PBA Hurricane XT lentil and Clearfield® canola varieties. One population of common sowthistle from the MN was also found to be resistant to the Group I herbicide 2,4-D and exhibited weak cross-resistance to IMI.

The herbicide resistance screening results identified that IMI herbicides were still effective in controlling other broadleaf weed species such as bedstraw, bifora, marshmallow, wild radish, wild turnip, Indian hedge mustard and medics (Table 2). However, an increased reliance on herbicide tolerant break crops, including IMI-tolerant pulse and canola varieties, without incorporating alternative weed control strategies, could result in weed species shifts and increased resistance in existing species. To address these issues, a project with joint investment of GRDC and SARDI has been initiated to develop effective management strategies, including integrated weed management practices for grasses and broadleaf weeds, to maintain the sustainability of HBCI systems.

Table 2. Percent resistant broadleaf weeds (≥ 20% survivors) observed in high break crop intensity paddocks of SA, samples taken in 2017.

* Figures in parenthesis are the number of samples tested

Managing dim-resistant ryegrass in lentils

Two research trials were conducted during 2017 at the Hart field site (MN) and Maitland (YP) investigating the pre-emergent herbicide Ultro¤ (@ 1700 g/ha with active carbetamide, Group E) which is currently in development. Ultro¤ was compared to propyzamide (1000g active/ha), Sakura® (118g product/ha) and Boxer Gold® (2500g product/ha) in controlling dim-resistant ryegrass in lentils. PBA Hurricane XT lentils were sown on 31 May 2017 at Hart and on 6 June 2017 at Maitland. At Hart, dim-resistant annual ryegrass seed was broadcast at 160 seeds/m2 ahead of seeding and incorporated prior to herbicide application. The Maitland site had a background population of dim-resistant annual ryegrass. The effectiveness of pre-emergent herbicides was investigated at both sites. Herbicide strategies including propyzamide resulted in the lowest ryegrass spikes and seed set at both locations. Ultro¤ provided a 98% reduction in ryegrass seed set over unweeded control at both sites, and was similar to propyzamide, Sakura® and Boxer Gold® (Table 3). In addition, Ultro¤ did not impact on the yield of lentils compared to propyzamide, Sakura® and Boxer Gold®. The pending registration of this new mode of action herbicide (Group E) is expected in 2020 and is likely to be an important tool, along with Group D propyzamide, in reducing selection pressure for existing Group J and Group K pre-emergent and dim chemistry post emergent herbicides for ryegrass control in break crops.

¤Unregistered herbicide was included for experimental purposes only. It has been submitted to the APVMA for registration.

Table 3. Ryegrass seed heads and seed set at maturity, and lentil yield at Hart and Maitland, in 2017. Numbers with different letters are significantly different means (P<0.05).

*Unregistered herbicides were included for experimental purposes only. The results within this document do not constitute a recommendation for that particular use by the author or author’s organisation.

Herbicides and crop competition for managing vetch and medics in a Group B tolerant faba bean

A trial was established at Turretfield Research Centre in 2017, focusing on control of vetch and medic in the Group B tolerant faba bean variety Bendoc. This trial was sown on 19 June 2017 in factorial randomised block design and included all combinations of three herbicide management strategies (T₁: simazine (1100g product/ha) post-sowing pre-emergent (PSPE), T₂: simazine (1100g product/ha) PSPE + Intervix¤¤ (750ml product/ha) at 5-6 crop node stage, and T₃: no herbicides) and three faba bean densities (12, 24 and 36 plants/m²). Vetch seeds were broadcast prior to sowing @ 50 seeds/m² to contribute to the existing background medic weed population. The main effects of herbicide management strategies and faba bean densities are summarised in Table 4, and the interactions between these two factors were non-significant.

The Group B tolerant faba bean Bendoc provided an opportunity to selectively use post-emergent Intervix¤¤ at the 5-6 node stage. Application of simazine (PSPE) + Intervix¤¤ (post-emergent) resulted in a reduction in vetch seed and medic pod density by 97% and 100%, respectively, over grower practice (simazine 1100 PSPE) (Table 4). Improved broadleaf weed control with simazine (PSPE) + Intervix¤¤ (post-emergent) also resulted in the highest faba bean yield. These results indicate the availability of Bendoc will benefit in selectively controlling vetch, medic and potentially other broadleaf weeds with post-emergence applied IMI in a strategy to minimise selection for resistance.

¤¤Unregistered herbicides were included for experimental purposes only. Please adhere to the rates listed on the labels/permit. Currently there is a permit (PER14726) to use imazamox (700g/kg product @ 45g/ha or 350g/L product @ 90mL/ha at 3-6 node stage) in Group B tolerant faba bean (e.g.Bendoc)

Crop competition has been shown to complement other weed control strategies. An increase in break crop plant density can lead to early ground cover, thereby reducing the competitiveness and seed production of weeds (Lemerle et al. 2006). The implementation of such a strategy can be especially effective in crops such as faba bean that have low plant densities and slow initial growth. In the present study, increasing faba bean density from 12 to 36 plants/m2 resulted in improved crop competition with weeds due to a significant increase in crop biomass and plant height (Table 4). The increased crop competitiveness significantly reduced vetch pods and seed set, and medic pod set, andsubsequently led to increased crop yield (Table 4). A faba bean density of 36 plants/m² resulted in 42% and 44% reduction in vetch seed and medic pod set, respectively, as compared to standard grower practice of 24 plants/m². There was no disease incidence and lodging with an increase in faba bean density at the trial site in 2017. This practice requires further investigations across different seasons and locations for these aspects, alongside yield and harvestability.

Table 4. Vetch and medic management in Group B tolerant faba bean (Bendoc) at Turretfield Research Station in 2017. Numbers with different letters are significantly different means (P<0.05).

*Unregistered herbicides were included for experimental purposes only. The results within this document do not constitute a recommendation for that particular use by the author or author’s organisation. Currently there is a permit (PER14726) to use imazamox (700g/kg product @ 45g/ha or 350g/kg product @ 90g/ha at 3-6 node stage) in Group B tolerant faba bean (Bendoc)

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and support of GRDC and SARDI, and the authors would like to thank them for their continued support. The authors are highly thankful to Christopher Preston, Gurjeet Gill, Rick Llewellyn, Ben Fleet and Sam Kleemann for their scientific input. The selection of focus paddocks would not have been possible without the help of Andrew Parkinson, Chris Davey, Craig Davis, Jeff Braun, Michael Brougham, Peter Cousins, Peter Hooper, Sam Homes, Sam Trengove and respective growers. The assistance received from Ruwan Lenorage in herbicide resistance screening work is highly acknowledged. Authors also thank Ashley Pilkington for making available for the current research studies the herbicide Ultro, currently in development.

References

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Fan, F., Zhang, F., Song, Y., Sun, J., Bao, X., Guo, T., Li, L., 2006. Nitrogen fixation of faba bean (Vicia faba L.) interacting with a non-legume in two contrasting intercropping systems. Plant and Soil 283, 275-286.

Lemerle, D., Verbeek B., Diffey, S., 2006. Influences of field pea (Pisum sativum) density on grain yield and competitiveness with annual ryegrass (Lolium rigidum) in south-eastern Australia. Australian Journal of Experimental Agriculture 46, 1465–1472.

Menalled, F., 2010. Integrated Weed Management in Lentils. Montana State University MontGuide. http://ipm.montana.edu/cropweeds/documents_cropweeds/extension/Integrated%20Weed%20Management%20in%20Lentils%20-%20mt201009AG.pdf

Warwick, S.I., Sauder, C., and Beckie, H.J., 2005. Resistance in Canadian biotypes of wild mustard (Sinapis arvensis) to acetolactate synthase inhibiting herbicides. Weed Science, 53, 631-639.

Contact details

Navneet Aggarwal
SARDI, Clare Research Centre
155 Main North Road, Clare 5453
0490380944
Navneet.Aggarwal@sa.gov.au