Preventing herbicide resistance in 'at risk' weeds

Herbicide resistance is an increasing threat facing both growers and agronomists across Australia's northern grain region. Already 10 cropping weeds have been confirmed as herbicide resistant in various parts of this region, and more have been identified at risk of developing resistance, particularly to glyphosate. Below are some strategies to assist in preventing the development of herbicide resistance in some of these weed species.

Barnyard and liverseed grasses

Seedlings of barnyard grass (top right) and urochloa (bottom left) - it is important to be able to identify weeds at seedling stage

Seedlings of barnyard grass (top right) and urochloa (bottom left). Photo from DAF's Crop weeds of northern Australia

Know your weed

There are two common barnyard grass species, which are distinguished by presence or absence of awns attached to the seed. These are known as barnyard grass (Echinochloa crus galli), and awnless barnyard grass (Echinochloa colona), which is the more common weed of cropping. Purple-red bands are sometimes seen on awnless barnyard grass leaves, particularly when the plant is suffering stress. Seedlings of liverseed grass (Urochloa panicoides), also known as Urochloa, are easily distinguished because of their broad, pale yellow-green leaves with hairs on the leaf margins and sheaths.

Newly shed seed of barnyard grass and liverseed grass exhibit strong dormancy, and thus most of this seed will not germinate until the following season. Barnyard grass emerges throughout late spring and during summer following significant rain, whereas liverseed grass will mostly emerge in one large flush in late spring. Emergence is mostly from seed near the soil surface. Local research showed that 30-70% of liverseed grass seeds emerged during the first two years after shedding, and very few seeds persisted in the soil after 4 years.

Hints for better management

For better control with herbicides:

  • Target small weeds (2-3 leaves) when using knockdown herbicides, particularly paraquat and Spray.Seed in fallows.
  • Use higher glyphosate rates for moisture stressed weeds.
  • Use double knockdown technique with glyphosate followed by paraquat or Spray.Seed for dense populations.
  • Be aware of potential antagonism with tank mixes of glyphosate and atrazine for populations mixed with grasses and broadleaf weeds. Apply sequential sprays or increase glyphosate rates.
  • Apply pre-plant atrazine as close as possible to the next rain for effective incorporation.
  • Add Dual Gold to atrazine to improve annual grass control, particularly for liverseed grass.
  • Incorporate mechanically pre-emergence applications of atrazine plus Dual Gold for maximum effectiveness.

Risk to Group C herbicides

A few populations of liverseed grass in southern Queensland have been confirmed resistant to atrazine (Group C) in paddocks where sorghum was grown as the pre-dominant crop with regular atrazine use for 10-15 years. As well, a population of barnyard grass has developed Group C resistance in northern New South Wales. Elsewhere, the summer grasses have a relatively low risk of developing resistance to Group C herbicides in rotations with winter and summer crops.

Strategies to prevent or minimise the risk for Group C resistance

  • Apply atrazine mixed with Dual Gold (Group K) in sorghum.
  • Cultivate between the crop rows.
  • Grow winter crops and focus on lowering the seed-bank with effective fallow control.
  • Include other summer crops such as mung bean, sunflower and cotton, and use other grass selective herbicides (Group A, B and D).

Risk to Group M herbicides

The summer grasses have a high risk of developing glyphosate resistance, particularly for growers practicing minimum or zero tillage. In a typical winter cereal/chickpea/sorghum rotation, these weeds are sprayed 16-17 times with glyphosate with minimal use of other herbicides apart from atrazine (Group C) over 5 years.

Several populations have been confirmed as resistant recently.

The overall strategy to reduce the risk for glyphosate resistance is to ensure that no survivors of glyphosate applications are allowed to set seed. Use the general tactics considered to decrease risk (see article 'Stopping herbicide resistance in Queensland' ) as well as specific tactics outlined below.

Fallow strategies to prevent or minimise the risk for Group M resistance

  • Apply Flame (Group B) to a clean paddock, or mixed with a knockdown herbicide to a weedy paddock, in early summer fallows prior to wheat or chickpea.
  • Avoid using in paddocks known to have potentially dense grass populations. (Local research: flame gave very good residual control of barnyard grass for 2-3 months.)
  • Apply atrazine (Group C) in early spring fallow prior to sorghum. (Local research: pre-plant atrazine is most effective when applied as close as possible to the next rain, as this incorporates the herbicide into the grass germinating zone and minimises losses from volatilisation.)
  • Rotate with Spray.Seed or paraquat (Group L).
  • Double knockdown with glyphosate followed by Spray.Seed or paraquat on survivors anytime in fallow.
  • Double knock with glyphosate followed by full-disturbance cultivation at sowing.
  • Cultivate.

In-crop strategies to prevent or minimise the risk for Group M resistance

  • Ensure maximum control in sorghum, such as atrazine (Group C) ± Dual Gold (Group K). Local research: Atrazine 2.5L + Dual Gold 2 L incorporated at sowing consistently gave more than 95% season long control for the summer grasses.
  • Cultivate between wide-row crops, or apply paraquat with a shielded sprayer.
  • Use grass selective herbicides effectively in other summer crops, such as Verdict (Group A), Spinnaker (Group B) or trifluralin (Group D) in mung beans; and several group A herbicides (e.g. Verdict, pendimethalin, Stomp (Group D), or Dual Gold (Group K) in sunflowers).

Common sowthistle

Know your weed

Sow thistle seedling - it is important to be able to identify weeds at seedling stage

Sowthistle seedling. Photo from DAF's Crop weeds of northern Australia

Common sowthistle (Sonchus oleraceus) is currently the most prevalent weed of cropping in the northern region. The weed has increased in importance in the last few decades, particularly as farming systems have moved towards reduced and zero tillage.

The weed is not restricted to any season, as seed can germinate at any temperatures from 5oC to 35oC. Fresh seeds have no innate dormancy, enabling them to germinate immediately following dispersal onto moist soil or after the next rain. As such, emergence patterns are determined mostly by rainfall. Therefore, it is important to monitor for weed emergence after each rainfall event. The majority of seed emerges from the top 2 cm of soil, and no seed emerges from 5 cm or deeper, which is one of the reasons for its increased prevalence in reduced till systems.

The weed is a prolific seed producer with a single plant producing up to 25,000 viable seeds in a fallow, highlighting the importance for follow-up control measures to prevent major replenishment of the seed-bank. Although most seed moves only several metres from the parent plant, a small percentage may disperse greater distances. Thus, it is important to control plants prior to flowering along fence lines, adjacent land and roadsides to prevent new infestations.

Local research has shown that the majority of the seed on the soil surface does not persist for more than a year, but a small portion of seed buried at depths greater than 5 cm will persist for at least 3 years. Thus, an important part of this weed's management strategy is to ensure seed remain on or near the soil surface. Then, control all emerged seedlings for at least a year, which will result in major depletion of the seed-bank, provided there has been no seed replenishment.

Hints for better management

Since the weed emerges all year, rotation of crops is a less useful tactic than for weeds that grow predominantly in winter or summer, and management should focus on all phases of the rotation to keep weed numbers low.

Crop competition greatly impedes common sowthistle growth, particularly in winter cereals. This tactic is important, as it reduces seed production on survivors or escapes from herbicide application.

For better control with herbicides:

  • Spray small seedlings (2-5 leaves).
  • Use higher water volumes for atrazine, paraquat and Spray.Seed, such as 100 L.
  • Be aware of potential antagonism from glyphosate mixes with 2,4-D or metsulfuron (eg Ally). Other options are in Table 1.
  • Control late flushes in winter crops with selective herbicides or pre-harvest sprays instead of waiting for the first fallow spray after harvest.

Risk to Group B herbicides

In the last two decades, numerous populations of common sowthistle in southern Queensland have developed resistance to the Group B herbicides, such as chlorsulfuron (e.g. Glean) and metsulfuron (e.g. Ally). The risk for more weeds developing Group B resistance is high in winter crop rotations, where Group B herbicides are used repeatedly without mixing with other effective herbicide groups.

Strategies to prevent or minimise the risk for Group B resistance

  • Rotate to Group I herbicides, such as Tordon 242, MCPA, 2,4-D, Tordon 75D, and Starane, and/or apply mixes of Group B and I herbicides. (Local research: Ally + MCPA gave 100% control of small seedlings.)
  • Avoid using Group B herbicides in fallows if applying Group B herbicides in the preceding or following crop.
  • Minimise seed production on survivors using crop competition.

Risk to Group M herbicides

Common sowthistle has a moderate risk of developing glyphosate resistance, particularly in cropping systems that are zero-tilled and reliant on glyphosate alone for fallow weed control. Typically, glyphosate is applied more than 20 times over 5 years with many of these targeting sowthistle.

The overall strategy to reduce the risk for glyphosate resistance is to ensure that no survivors of glyphosate applications are allowed to set seed. Use the general tactics considered to decrease the risk (see article 'Stopping herbicide resistance in Queensland' ) as well as specific tactics outlined below.

Fallow strategies to prevent or minimise the risk for Group M resistance

  • Mix glyphosate with Starane, Grazon DS, or Cadence (Group I) (see Table 1).
  • Rotate with Spray.Seed or paraquat (Group L).
  • Double knockdown with glyphosate followed by Spray.Seed or paraquat on survivors anytime in fallow.
  • Double knock with glyphosate followed by full cut cultivation at sowing. (Local research: cultivation was much more effective on common sowthistle less than 10 cm diameter.)
  • Apply atrazine (Group C) in late winter or spring fallow prior to sorghum after treatment with a knockdown herbicide. (Local research: atrazine at 3.6 L/ha controlled common sowthistle for several months.)
  • Use tillage, grazing, chipping, or detector sprayer with alternative mode of action herbicide to control and stop survivors setting seed.
Table 1. Control of sowthistle in winter fallow on the Darling Downs
TreatmentsMode of Action (MOA)Rate (product/ha)Weed sizeControl (%)
Glyphosate 450 g/LM0.8 L3-leaf95
Glyphosate 450 g/LM0.8 L7-leaf91
Glyphosate 450 g/LM1.6 L3-leaf98
Glyphosate 450 g/LM1.6 L7-leaf98
Spray.SeedL1.6 L3-leaf98
Spray.SeedL2.4 L7-leaf99
Glyphosate 450 g/L + StaraneM+I0.8 L + 1 L3-leaf95
Glyphosate 450 g/L + CadenceM+I0.6 L + 115 g3-leaf97
Glyphosate 450 g/L + Grazon DSM+I1.2 L + 0.4 L7-leaf100
Glyphosate 450 g/L + Atrazine 500 g/LM+C0.8 L + 3.6 L3-leaf97
Glyphosate 450 g/L followed by Spray.SeedM+L0.8 L & 2.4 LDouble-knockdown100

Glyphosate treatments were applied in 60 L/ha and Spray.Seed in 100 L/ha. For the double knockdown treatment, glyphosate was applied to weeds with 3 leaves, and Spray.Seed was applied 1 week later when untreated weeds had 7 leaves.

In-crop strategies to prevent or minimise the risk for Group M resistance

  • In-crop weed control is important in keeping weed numbers low using a combination of selective herbicides and crop competition. This reduces the selection pressure for glyphosate resistance in the following fallows.
  • Cultivate between wide-row crops, or apply paraquat with a shielded sprayer.

Turnip and mustard weeds

Know your weed

urnip weed seedling - it is important to be able to identify weeds at seedling stage

Turnip weed seedling. Photo from DAF's Crop weeds of northern Australia

The main weeds of this group in southern Queensland are turnip weed (Rapistrum rugosum), African turnip weed (Sisymbrium thellungii), and Indian hedge mustard (Sisymbrium orientale). There are also some minor infestations of wild turnip (Brassica tournefortii), wild radish (Raphanus raphanistrum), hedge mustard (Sisymbrium officinale) and London rocket (Sisymbrium irio). See 'Crop weeds of northern Australia - identification at seedling and mature stages' by Wilson, Hawton and Duff for more details.

Turnip weed seed persists longer in the soil than sowthistle and most grass weeds. Local research has shown that approximately 10% of turnip weed seed remained in the soil seed-bank after 4 years.

Hints for better management

As the seed has long persistence, crop rotation is not an effective tactic to run down the seed-bank. Turnip weed is very susceptible to competition from winter cereals.

For better control with herbicides, it is important to identify your 'turnip' weeds correctly as they can differ in herbicide susceptibility.

Risk to Group B herbicides

Several populations of turnip weed, African turnip weed, and Indian hedge mustard have been identified as resistant to Group B herbicides, such as Glean and Ally, throughout southern Queensland. The areas of Group B resistant broadleaf weeds are likely to increase further, particularly in the western cropping region, where the rotations are predominantly winter crops.

Strategies to prevent or minimise the risk for Group B resistance

  • Rotate to Group I herbicides, such as MCPA, dicamba and 2,4-D, and/or apply mixes of Group B and I herbicides.
  • Avoid using Group B herbicides in fallows if applying Group B herbicides in the preceding or following crop.
  • Minimise seed production on survivors using crop competition.

Wild oat

Know your weed

Wild oat seedling - it is important to be able to identify weeds at seedling stage

Wild oat seedling. Photo from DAF's Crop weeds of northern Australia

Wild oat, also known as black oat, is one of the most common weeds of winter crops and fallows across southern Queensland. Populations are generally a mix of 2 species, Avena ludoviciana and Avena fatua, but the differences between these have little impact on management strategies. Wild oat is a highly competitive weed with low numbers able to cause substantial crop yield losses.

Seedling emergence is controlled by short-term seed dormancy, temperature (10°C to 26°C) and soil moisture. The largest flush is generally in late autumn/early winter provided there is adequate soil moisture, but weeds will emerge periodically throughout the rest of winter and into spring. Seed production of a wild oat plant in wheat crops is estimated to be around 50-225 seeds, depending on weed density.

Most wild oat seeds in the surface soil do not persist for long periods. Local research has shown that about three quarters of the seed-bank in top 10cm of soil is lost after 1 year, and 98% after 2 years without any seed replenishment. The main mechanism of persistence is the deposition of new seeds as the result of reproduction from surviving weeds. Therefore, it is important to prevent seed production on escapes and survivors.

Hints for better management

The main aim of wild oat management should be to stop seed set.

This is achieved by using a combination of:

  • Crop rotation between winter and summer crops, or with long fallows in marginal rainfall areas.
  • Effective use of pre-emergence and early post-emergence herbicides, followed by selective spray topping if necessary.
  • Maximum crop competitiveness using high quality seed, optimum fertiliser, sowing depth, crop population and row spacing.

For weedy paddocks, an additional tactic is to use a strategic stubble burn followed by a late sowing. Local research measured that burning stubble in autumn destroyed approximately one third of the wild oat seeds on the soil surface. Late sowing allows for maximum weed emergence. These seedlings can then be effectively controlled with cultivation or a knockdown herbicide prior to crop sowing. Approximately 40% of wild oat seeds in the seed-bank germinate following the first major rain in late autumn/early winter, with another 10-30% germinating later in the season.

For better control with herbicides:

  • Spray seedlings with 1-2 tillers for translocated herbicides, as smaller wild oats may regrow.
  • Weed size is more critical for Group B and L herbicides than for Group A herbicides.
  • Be aware of potential antagonism from glyphosate mixes with atrazine (Group C) and 2,4-D (Group I) herbicides (see Table 1).
  • Mataven® is more effective at higher temperatures.

Risk to Group A herbicides

Many wild oat populations have been identified as resistant to Group A herbicides in southern Queensland. The risk for further development of Group A resistance is low in areas with rotations consisting of a mix of summer and winter crops, as growers can reduce the wild oat seed-bank during winter fallows. However, the risk is higher in areas with predominant winter cropping where there is a high reliance on Group A herbicides.

Strategies to prevent or minimise the risk for Group A resistance

  • Include summer crops and control wild oats effectively in fallow with knockdown herbicides or tillage.
  • Grow barley for greater crop competition.
  • Rotate to Avadex BW (Group E) followed by selective spray-topping with Mataven L (Group) in wheat.
  • Use selective spray-topping with Mataven L (Group K) following a Group A application in wheat.
  • Rotate to Mataven L (Group) as early post-emergence, but only in combination with a competitive bread wheat crop and with low wild oat numbers.
  • Rotate to Atlantis or Hussar (Group B) in wheat.

Selective spray-topping with Mataven L should be applied when the weed begins elongation but prior to head emergence. This tactic can control seed production (up to 90%) on survivors of earlier applications of Group A or E herbicides, and/or on late emerging weeds. However, be aware that several wild oat populations were confirmed recently as resistant to Mataven L (Group) in NSW. Also, check wheat cultivar sensitivity as herbicide tolerance differs between cultivars.

Risk to Group M herbicides

The risk of wild oat becoming resistant to Group M herbicides ranges from low to high, depending on crop and herbicide rotation. The risk is greater in rotations that have predominantly summer crops, using minimum or zero tilled systems that rely mostly on glyphosate for fallow weed control.

The overall strategy to reduce the risk for glyphosate resistance is to ensure that no survivors of glyphosate applications are allowed to set seed. Use the general tactics considered to decrease the risk (see article 'Stopping herbicide resistance in Queensland') as well as specific tactics outlined below.

Fallow strategies to prevent or minimise the risk for Group M resistance

  • Double knockdown with glyphosate followed by Spray.Seed or paraquat on survivors anytime in fallow (see Table 1).
  • Double knock with glyphosate followed by full cut cultivation at sowing.
  • Rotate with Spray.Seed or paraquat (Group L) for main flush and ensure good spray coverage.
  • Rotate with atrazine (Group C) in late winter or early spring fallow prior to sorghum, but following treatment with a knockdown herbicide. (Local research: atrazine at 3.6 L/ha controlled wild oat for several months.)
  • Cultivate the main flush(s) in fallow.

In-crop strategies to prevent or minimise the risk for Group M resistance

  • In-crop weed control is important in keeping weed numbers low using a combination of selective herbicides (Group A, B, E) and crop competition. This reduces the selection pressure for glyphosate resistance in the following fallows.
Table 1. Control of wild oat in winter fallow on the Darling Downs
TreatmentsMode of action (MOA)Rate (product/ha)Time of applicationControl (%)
Glyphosate 450 g/LM0.8 LPre-tillering87
Glyphosate 450 g/LM0.8 L1-2 tillers98
Glyphosate 450 g/LM1.6 LPre-tillering97
Glyphosate 450 g/LM1.6 L1-2 tillers100
Spray.SeedL1.6 LPre-tillering100
Spray.SeedL2.4 L1-2 tillers100
Glyphosate 450 g/L + Atrazine 500 g/LM+C0.8 L + 3.6 LPre-tillering89
Glyphosate 450 g/L + SurpassM+I0.8 L + 1.2 LPre-tillering85
Glyphosate 450 g/L & Spray.SeedM & L0.8 L & 2.4 LDouble-knockdown100

Glyphosate treatments were applied in 60 L/ha and Spray.Seed in 100 L/ha. For the double knockdown treatment, glyphosate was applied to pre-tillering weeds, and Spray.Seed was applied 1 week later when untreated weeds had 1-2 tillers.

Last updated 23 April 2015