Practical strategies for problem weeds; glyphosate resistant barnyard, liverseed grass and sowthistle, group A resistant wild oats and feathertop Rhodes grass

Lawrence Price & Richard Daniel
Northern Grower Alliance

The issue

In recent years, grass control in the summer fallow has become an increasingly difficult and expensive component of many northern farming systems. This is attributable in part due to the heavy reliance on glyphosate. This has led to the selection of weed species that were naturally more glyphosate tolerant and/or selected for glyphosate resistant populations of weeds.

Although this paper will focus on herbicide control options of these weeds, it is clear we need to better understand and employ other weed management tactics to successfully and economically control these significant threats to cropping profitability.

Awnless barnyard grass (Echinochloa colona)

Awnless barnyard grass (ABYG) has been a major summer grass issue for many years. It is a difficult weed to manage for at least three key reasons:

1. Multiple emergence flushes (cohorts) each season;
2. Easily moisture stressed, leading to inconsistent knockdown control; and
3. Glyphosate resistant populations are becoming widespread.

Resistance levels

Prior to summer 2011/12, there were 21 cases of glyphosate resistant ABYG. Collaborative survey work was conducted by NSW DPI, DAF Qld and NGA in summer 2011/12 with a targeted follow-up in 2012/13. Agronomists from the Liverpool Plains to the Darling Downs and west to areas including Mungindi collected ABYG samples that were tested at the NSW DPI, Tamworth Agricultural Institute.

The key outcome was that the number of ‘confirmed’ glyphosate resistant ABYG populations had nearly trebled. Selected populations were also evaluated in a glyphosate rate response trial. This showed that some of these populations were still only suppressed when sprayed with 12.8 L glyphosate /ha. Additionally it has been found recently that the glyphosate ‘resistance’ expression is increased when conditions are warmer ie: glyphosate resistant populations are even ‘more resistant’ under hotter temperatures.

The days of solely relying on glyphosate for ABYG control are behind us.

Residual herbicides (fallow and in-crop)

There are a range of active ingredients registered in either summer crop (e.g. metolachlor - Dual Gold® and atrazine) or in fallow (e.g. imazapic - Flame®) that provide useful management of ABYG. The new fallow registration of isoxaflutole (e.g. Balance®) can provide useful suppression of ABYG and also has strong activity against other problem weed species. Few, if any, residual applications will provide complete control. However they are important tools that need to be considered to reduce the population size exposed to knockdown herbicides as well as to alternate the herbicide chemistry being employed. Use of residuals together with camera spray technology (for escapes) can be a very effective strategy in fallow.

Double knock control

This approach uses two different tactics applied sequentially, in this case two herbicides with different modes of action. In reduced tillage situations, it is frequently glyphosate first followed by a paraquat based spray as the second application or ‘knock’. Trials have shown that glyphosate followed by paraquat can give effective control even on glyphosate resistant ABYG but timing and stress are important. Ensure glyphosate rates are robust. Another strategy can be to use paraquat as both ‘knocks’, particularly for populations where glyphosate effectiveness has been poor.

Timing of paraquat application as the second “knock” for ABYG control has generally proven flexible. The most consistent control is obtained from a delay of around3 to5 days, which also can allow for lower rates of paraquat to be used. Longer intervals between knocks may be warranted when ABYG is still emerging at the first application timing, shorter intervals are generally required when weed size is larger or moisture stress conditions are expected. High levels of control can still be obtained with larger weeds but paraquat rates will need to be increased to 2.0 or 2.4 L/ha (250 gai/L formulations).

Knockdown control

A number of Group A herbicides (e.g. haloxyfop - Verdict® and clethodim - Select®) are effective on ABYG but are only registered in summer crops such as mung beans. It should be noted that Group A herbicides are generally more sensitive to weed moisture stress or size. Therefore application to large or mature weeds can result in poor efficacy.

Key points ABYG

• Glyphosate resistance is widespread in many ABYG populations. Tactics against this weed must change from glyphosate alone.
• Utilise residual herbicides wherever possible and aim to control ‘escapes’ with camera spray technology.
• Try to ensure a double knock of glyphosate followed by paraquat is used on at least one of the larger early summer ABYG germinations.
• Utilise Group A herbicides for in-crop control of ABYG and use crop agronomy to ensure strong crop competition.
• Cultivation can be very effective on this weed but multiple emergences are likely each summer season.

Feathertop Rhodes grass (Chloris virgata)

Feathertop Rhodes grass (FTR) started to become an important weed in southern Qld and northern NSW in around 2008. It is a small seeded species that germinates on, or close to, the soil surface. It has rapid early growth rates and easily becomes moisture stressed.

Some likely reasons for the difficulty of managing FTR include:

• It is a species with higher levels of natural tolerance/resistance to glyphosate and has been selected by glyphosate dominated fallow and roadside management strategies;
• It is frequently poorly controlled by paraquat alone or even a double knock of glyphosate followed by paraquat;
• QDAF research showed FTR is one of the first weed species to colonise bare areas and can germinate on smaller rainfall events than many other problem species; and
• Minimum/zero tillage practices are likely to have contributed to the threat posed by this weed as cultivation or seed burial (to depths of 2cm or deeper) can be effective management tools.

The three key FTR characteristics that can be exploited in managing the weed are listed below:

1. Seed viability does not appear to be improved by seed burial (in contrast to many other weed species);
2. Seed longevity is short (around 12 months). If effective control strategies can be used for a period of 12 to18 months, the seedbank of FTR can be rapidly run-down; and
3. New incursions of FTR are often in well-defined patches (in contrast to weeds such as common sowthistle). Aggressively treatment of these patches can prevent whole of paddock blow-outs.

Residual herbicides (fallow)

Evaluation of a wide range of residual herbicides has shown a number of promising candidates for FTR management. Currently the only registered herbicide product for residual control in fallow is Balance® (isoxaflutole). Additional product registrations for fallow use are being sought by the grains industry.

Residual herbicide (in-crop)

Utilising residual herbicides in-crop will allow the use of additional weed management approaches. In-crop use benefits from:

• Crop competition;
• Change in crop being grown and available herbicide options;
• Herbicide application often under more favourable conditions than in fallow or where a level of mechanical incorporation occurs; and
• ‘Increased disturbance’ planting may provide benefits for FTR management via weed seed burial or removal of early weed emergence.

Currently there are no registered herbicide products for residual control of FTR in-crop. Residual herbicide strategies for awnless barnyard grass control (e.g.  Dual® Gold, Flame®, trifluralin e.g.  TriflurX® and pendimethalin e.g.  Stomp®) applied in a range of summer crops have been noted to reduce the emergence of FTR.

FTR is predominantly a summer weed but the first cohort of emergence can occur during the winter crop phase. Screening of herbicides, currently registered for residual control of other weeds, in winter cereal or chickpea production has shown encouraging levels of activity. Residual herbicide strategies for the control of a range of both grass and broadleaf weeds (e.g. Balance®, Treflan®, Stomp®, Sakura® and terbuthylazine - Terbyne® Xtreme) applied in a range of winter crops have been noted to reduce the emergence of FTR. It should be noted that none of the listed products are registered for in-crop application for control of FTR.

Residual herbicides for FTR in non-crop situations

FTR frequently dominates in non-crop areas with a potential for re-infestation of adjacent crop areas. For non-crop areas, there is a registration for 7L/ha  Arsenal Xpress® (imazapyr 150g/L + glyphosate 150g/L).

Knockdown herbicides (in-crop)

The main registrations for knockdown of FTR are for the use of Group A (grass selective) herbicides in cotton, mungbeans and other broadleaf summer crops.

Herbicide double knock

Glyphosate followed by paraquat is generally an effective strategy for ABYG management. However the same approach is rarely effective for FTR management. In contrast, a small number of Group A herbicides can be effective against FTR but need to be managed within a number of constraints. These constraints are listed below:

• Although they can provide high levels of efficacy on fresh and seedling FTR, they need to be followed by a paraquat double knock to get consistent levels of control.
• Group A herbicides have a high risk for resistance selection, again requiring follow up with paraquat.
• Many Group A herbicides have plantback restrictions to cereal crops.
• Group A herbicides generally have narrower application windows (in terms of FTR growth stage) for effective control compared to herbicides such as glyphosate on other weeds (ie Group A herbicides will generally give unsatisfactory results on flowering and/or moisture stressed FTR).
• Group A herbicide products vary in their effectiveness on FTR.

A permit (PER12941) is valid until 31/8/2019, in Queensland only, for the control of FTR in summer fallow situations prior to planting mungbeans. The permit is for the application of haloxyfop 520 g ai/L formulations (e.g.  Verdict®) at 150-300mL/ha followed by paraquat (250 gai/L) at a minimum of 1.6 L/ha, within 7 to14 days after the first application. In addition there has been a recent registration of Shogun® (propaquizafop) for FTR management in fallow, used alone or followed by a paraquat double knock, depending on FTR growth stage.

Key points

• Glyphosate used alone or double knock glyphosate followed by paraquat is generally unsatisfactory in terms of FTR control achieved;
• Utilise residual chemistry wherever possible and prepare a plan to control ‘escapes’ (e.g.  camera spray technology); and
• Utilise aggressive patch management for new FTR incursions (including manual weeding and chipping) and preferably follow up with residual herbicides over previous patches where weeds may have seeded.
• Cultivation is often the most effective and economic salvage tool for mature FTR plants.
• Consider (infrequent) strategic cultivations for seed burial (repeated tillage may simply return seeds to the soil surface).
• Burning appears a useful tool where FTR blow outs have occurred in patches or even in larger areas to reduce seed viability.

Other tactics to consider

Liverseed grass (Urochloa panicoides)

Liverseed grass is another widespread weed in the northern grains region. Unlike ABYG, liverseed grass is generally noted for a single main germination (flush) each season.

Residual herbicides (fallow)

The only product currently registered for residual control in fallow is Flame® (imazapic). Evaluation of a wide range of residual herbicides has generally shown inconsistent residual control of liverseed grass (particularly compared to ABYG and FTR.

Residual herbicide (in-crop)

There are a number of residual herbicide options registered for in-crop use e.g. Dual® Gold, TriflurX®, Stomp® and imazamox -. Raptor®). A good strategy for paddocks with high seed burdens of liverseed grass seed is to grow crops that allow the use of these residual herbicides. Use of these herbicides in registered winter crops can also assist in liverseed grass management.

Herbicide Double knock

A double knock of glyphosate followed by paraquat is generally an effective option with paraquat followed by paraquat also an option to consider. The paraquat followed by paraquat approach is likely to be more successful particularly on moisture stressed populations.

Knockdown control

A number of Group A herbicides (e.g.  Verdict® and Select®) are effective on liverseed grass but are only registered in summer crops such as mung beans. It should be noted that Group A herbicides are generally more sensitive to weed moisture stress or size. Application to large or mature weeds can result in poor efficacy.

There are no currently registered in-crop knockdown herbicide options for liverseed grass.

Wild oats (Avena sterilis & A. fatua)

Group A resistant wild oats are a consequence of repeated use of herbicides from the same herbicide in winter crops in the northern cropping region. We have few alternatives to Group A herbicides for in-crop wild oat control in cereals and no post emergent alternatives in pulse crops.

Residual herbicides (in-crop)

Pre-emergent herbicides such as Avadex® (triallate) or Treflan® (trifluralin) can provide between 40 to 80% reduction in wild oat numbers in zero tillage farming systems with minimal incorporation. Other herbicides such as Sakura® (for cereal crops) or Terbyne Extreme® (broadleaf crops) offer suppression of wild oats and could be useful when combined with crop competition.  This can be effective in cereal crops, however chickpeas generally provide minimal competition and as such are reliant on post emergent options.

Knockdown control

Group A (Dim, Den & Fop) herbicides, have in the past given highly effective wild oat control, however resistance is now widespread in the northern cropping region. Testing for herbicide resistance may reveal alternative Group A herbicides that are still effective, however we now have wild oat populations  that are resistant to all Group A herbicides. When using Group A - Dim chemistry it is suggested to always add ammonium sulphate.

In cereal crops, Group B products (Hussar®, Atlantis® & Crusader®) can be used targeting small wild oats or flamprop-m-methyl (Oat Master®) which can provide useful salvage weed control. In chickpeas there are no knockdown alternatives to the Group A.

Rotation to summer crop

The most effective strategy to get on top of wild oats is to rotate to summer crop and use alternative herbicides or cultivation to control the wild oats in the winter fallow. Multiple seasons of summer crop will be required to get a high population under control.

Key points

• Consider rotation to summer crop in paddocks with high wild oat populations.
• Use residual herbicides to help reduce weed numbers.
• Use Group B herbicides or Flamprop-m-methyl in wheat.
• Save the group A products for chickpeas.

Common sowthistle (Sonchus oleraceus)

Common sowthistle is one of the most widespread broadleaf weeds in the northern cropping region with seed very readily dispersed by wind. It was once considered a winter weed but has become an issue all year round.

The two biggest challenges with common sowthistle are management of glyphosate resistant populations and the successful fallow control of large weeds. NGA have been evaluating options for residual, knockdown or double knock approaches during the last 18 months.

Residual herbicides (fallow or in-crop)

Products currently registered for residual control in fallow include Balance® and Terbyne Xtreme®. Balance, Terbyne Xtreme and simazine are all registered for in-crop residual control. NGA have been involved in screening other herbicides for residual control of common sowthistle with encouraging results from members of other herbicide mode of action groups.

Herbicide double knock

Using a double knock of glyphosate followed by paraquat can be an effective tool on small common sowthistle (e.g.  4-8 leaf). However the level of control can be variable, on larger weed (rosette) and at more advanced growth stages. Evaluation of alternative first knock candidates indicates that Group I products followed by paraquat are providing improved levels of efficacy compared to glyphosate followed paraquat on larger rosettes. Many of these options are used for fleabane control and often provide a level of residual activity for sowthistle.

Basta® (200g/L glufosinate ammonium) followed by paraquat appears promising but is likely to be restricted to optical spray situations due to the high cost. Screening is also underway for alternative second knock herbicides to paraquat.

Knockdown control

There are limited knockdown (single knock) options for common sowthistle. Sharpen® (saflufenacil) is registered for use in mixture with glyphosate. This can be an effective tool but needs to be applied on very small weeds for consistent control. Basta alone is also registered and maybe an option in ‘non-broadacre’ situations. Trial work in 2016/17 has shown promising knockdown activity from optical sprayer rates of Basta on larger common sowthistle.

Key points

• Commercially unacceptable efficacy from glyphosate or glyphosate followed by paraquat is becoming more widespread.
• The industry must develop ‘non-glyphosate’ based management strategies.
• Residual chemistry clearly has a benefit with encouraging levels of activity from a range of herbicide mode of action groups.
• Double knocks of Group I herbicides followed by paraquat appear promising and may be suitable where grass weeds are of less concern.
• Basta may become an option, primarily through optical sprayer setups, but more work is needed to ensure robustness across a range of environmental conditions.

Conclusions

Profitability is of course still paramount. The suggestion with these problem weeds is to focus on individual paddocks and adjust rotations to crops that most suit your environmental conditions but also enable the use of effective residual herbicides in the previous fallow or even in crop. Particularly for FTR, the seed bank appears only short lived and two years of effective management can ensure that paddocks return to full flexibility of rotational choice.

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.

Contact details

Lawrence Price
Northern Grower Alliance
PO Box 78 Harlaxton Qld, 4350
Ph: 0448106178
Email: Lawrie.price@nga.org.au

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