Managing glyphosate-resistant summer grasses

Spray rig applying herbicide.

Summer grasses are among the weed species with the highest potential to develop resistance to glyphosate. To date, there are three populations of barnyard grass and two populations of liverseed grass resistant to glyphosate in Queensland and New South Wales.

There are a number of reasons for this:

  • reliance on glyphosate for the majority of grass control
  • limited options for fallow control other than glyphosate
  • limited post-emergent options for control within the crop
  • high seed production.

The suspected cases of resistant populations of barnyard and liverseed grass increase each year. There are also several populations of liverseed grass that are resistant to atrazine (group C).

Feathertop Rhodes grass is another weed becoming more common under farming systems that predominantly use glyphosate.

Opportunity weed management

The herbicide mode of action group (MOA) must be considered when controlling weeds. Using one MOA continuously will ultimately lead to herbicide resistance. Therefore, becoming familiar with which MOA group a herbicide comes from will enable more effective long-term weed control.

Different crops allow for different weed management tactics. These can be either by different MOA´s or different non-herbicide options. This creates ´opportunities´ to implement an Integrated Weed Management (IWM) system using a range of herbicide and non-herbicide options.

The term ´opportunity cropping´ is used for crop rotations that aren´t ´locked in´ and depend on availability of moisture. This idea can also be applied to IWM, so no single weed control tactic is overused.

Rotating herbicide MOA´s

There are a number of different herbicide MOA´s suitable for managing summer grasses (see Table 1). The number and type of MOA´s differ with each crop. For example, cotton enables up to 5 MOA groups to be used on grasses, whereas sorghum allows only two.

Crop selection does not need to be determined by the weeds present in the field (although in some cases this may be useful), however when rotating crops it is an opportunity to rotate herbicide MOA groups as well.

Important points for herbicides

  • Paraquat and paraquat + diquat are best applied with higher water volumes. It is essential to get good coverage on weeds. These are most effective on small grasses.
  • Barnyard grass control can be variable with atrazine, prometryn and fluometuron.
  • Imazapic , imazamox and imazethapyr have long plant backs to a range of crops (see label).
  • Atrazine has a long plant back to cotton (see label).

Using non-herbicide options

The pressure on herbicides to provide the majority of weed control can be reduced by utilising non-herbicide options where possible. This can be done in a number of ways including altering planting times, encouraging crop competition, cultivation, chipping and spot spraying. These can also be used in conjunction with herbicides to enable them to be more effective. Possible non-herbicide options and how they can be used are listed in Table 2.

Strategic IWM

  • know the key weed species in each field
  • know the history of herbicide use in each field. If glyphosate has been predominately used, it´s time to change
  • know what herbicides are effective on key weeds and aim for optimum control
  • rotate between herbicide groups
  • use a selection of non-herbicide control options where possible
  • ensure survivors do not seed and replenish the soil seed bank
  • avoid introduction or spread of weeds by contaminated seed, grain, hay or machinery
  • manage weeds in surrounding non-crop areas
  • review the control achieved, and adjust future management strategies accordingly.
Table 1: Different herbicide MOA´s suitable for managing summer grasses (BG is barnyard grass; LG is liverseed grass; FTR is feathertop Rhodes grass) 

Tactic

MOA

Active

Product

Fallow

Cotton

Sorghum

Sunflowers

Soybeans

Maize

Mungbeans

Pre-plant / Fallow knockdown herbicides

L

Paraquat

 

BG LG

           

L

Paraquat+Diquat

Spray.Seed

BG LG

           

L+Q

Paraquat+Amitrole

Alliance

BG

           

Pre-emergent herbicides

B

Imazapic

Flame

BG LG

           

B

Imazethapyr

Spinnaker

       

BG

 

BG

C

Atrazine

     

BG

   

BG

 

C

Diuron

   

BG

         

C

Prometryn

   

BG

         

C

Fluometuron

   

BG LG

         

C

Fluometuron+prometryn

Convoy

 

BG LG

         

D

Trifluralin

   

BG LG

 

BG LG

BG LG

 

BG LG

D

Pendimethalin

Stomp

 

BG LG

 

BG LG

BG LG

BG LG

BG LG

D

Chlorthal-dimethyl

Dacthal

 

BG LG

         

F

Norflurazon

Zoliar

 

BG

         

J

EPTC

Eptam

     

BG

 

BG

 

K

Metolachlor

   

BG LG

BG LG

BG LG

BG LG

BG LG

BG LG

K

Propachlor

Ramrod

   

BG LG

   

BG LG

 

C+K

Atrazine+Metolachlor

Primextra Gold

   

BG LG

   

BG LG

 

Mid-season residual herbicides

C

Fluometuron

   

BG LG

         

C

Prometryn

   

BG

         

C

Fluometuron+prometryn

Convoy

 

BG LG

         

Post-emergent selective herbicides

A

Haloxyfop

Verdict

 

BG LG

 

BG LG

BG LG

 

BG LG

A

Clethodim

Sequence

 

BG LG FTR

   

BG LG FTR

 

BG LG FTR

A

Butroxydim

Factor

 

BG LG FTR

 

BG LG FTR

BG LG FTR

 

BG LG FTR

A

Sethoxydim

Sertin

 

BG LG

 

BG LG

BG LG

   

A

Fluazifop

Fusilade

 

BG LG

 

BG LG

     

A

Propaquizafop

Correct

 

BG LG

 

BG LG

     

A

Quizalofop

Leopard

     

BG LG

BG LG

 

BG LG

B

Imazamox

Raptor

       

BG LG

 

BG LG

B

Imazethapyr

Spinnaker

       

BG

   

C

Atrazine

     

BG

   

BG

 
Table 2: Non-herbicide options for reducing the impact of summer grasses in crop and fallow

Option

Crop/s

Comments

Time of planting

Delayed planting of summer crops

Sorghum
Maize Soybeans Mungbeans

The major flushes for barnyard grass and liverseed grass occur late spring/early summer. Delayed sowing until after these flushes emerge allows other weed control options to be used (such as double knock and full disturbance planting).

Some summer crops grow slowly in cool spring conditions and therefore don´t compete well with weeds. Delayed planting until conditions are warmer will help increase the crop's competitiveness.

Early planting of winter crops

 

Winter crops that aren´t harvested until November/December may have summer grasses that have already germinated. When harvesting has been completed, these grasses may have passed their optimum time to be sprayed.

Choosing crops/varieties that can be planted earlier or are faster to mature in order to be harvested before summer grass emergences enables them to be controlled when they are small.

Crop competition

Crop and variety choice

Sorghum

Some crops and varieties are more competitive against weeds. Crops such as sunflowers have a low competitive ability and therefore are preferably not planted into fields with a heavy summer grass infestation.

An evenly established, vigorously growing crop can compete strongly with weeds.

Row spacing and plant population

Sorghum
Maize Soybeans Mungbeans

Decreasing row spacing creates less favourable conditions for competing weeds by increasing competition for moisture and light. Wider rows favour weed growth but allow inter-row cultivation.

Weed-free periods

Cotton Sorghum
Maize Soybeans Mungbeans

Keeping crops weed-free while they are getting established allows the crop to get a head start with all resources available. Once established it can then out-compete weeds by shading, and taking resources with a larger root system.

Cultivation

Full-disturbance planting

Sorghum

For high weed pressure situations at the time of planting, using equipment that creates a full disturbance to kill weeds while planting is an option that reduces the reliance on herbicides.

Strategic

 

Cultivation of major flushes either before planting, in fallow or inter-row can significantly lessen the selection pressure on potential herbicide resistant weeds.

Inter-row

Cotton Sorghum Sunflowers

Inter-row cultivation can effectively control small weeds at a reasonable cost and should be considered for control, or as a ´back up´ if chemicals are inappropriate or fail.

Salvage

 

Last resort for weeds out of control. Needs to be done before weeds have set seed or damage is already done. Inversion can be beneficial for small seeded species.

Chipping and spot spraying

 

All

Can be a very effective method to control survivors of herbicide application, particularly if they occur in low densities. Effectiveness is reliant upon good scouting to ensure escapes do not set seed.

A cultivator in a cotton crop
Inter-row cultivation in cotton

Further information

Last updated 27 August 2012