Weed management

Avoiding and managing herbicide resistance

Key points on resistance

  1. Weed numbers - the higher the numbers the greater the risk of resistant weeds being present.
  2. Each herbicide application will increase the proportion of resistant individuals in the population.
  3. Resistance is generally not reversible (particularly for Group A and B herbicides).
  4. The proportion of resistance in subsequent populations is not increased after the use of a herbicide if seed set is prevented.
  5. Resistance occurs quickest where there is repeated use of only a limited number of weed control methods.

Management tactics

Tactic

Management

Practical issues

Deplete weed seed bank in the soil

Burn residues

Increase erosion risk on some soils

Invert seed bank by ploughing

Only practical in deep soils

Autumn tickle

Useful to increase weed germination

Delay sowing

Only practical in years with an early break

No till seeding

Keeps seed on surface from germinating

Control seedling weeds in the target area

Fallow and cultivate pre-sowing

Can increase germination of dormant weed seeds

Herbicides

Knockdown herbicides

Glyphosate or paraquat plus spike for difficult to control weeds

Double knockdown

Glyphosate followed by paraquat three to four days later

Pre-emergent herbicides

Trifluralin (Group D) + Dual® Gold (Group K) or Avadex® (Group E) mix

Selective post-emergent herbicides

Group A herbicides applied when weeds small and actively growing

Clearfield™ system

Atrazine and simazine application to triazine tolerant varieties

Non-selective post-emergent herbicides

Glyphosate application to Roundup Ready® varieties Glufosinate application to InVigour® varieties

Crop competition

Sow early using narrow row spacing

Ensure correct sowing depth and good seed soil contact for rapid and even emergence

Stop weed seed set

In crop weed management

Cut for hay

This may be the best longer term option where there are high numbers of resistant weeds

Windrowing

Will reduce seed set

Spraying with non-selective herbicides

Apply glyphosate during the windrowing operation to kill surviving plants/stop seed set

Prevent viable weed seeds being added to seed bank

Weed seed collection at harvest

May be impractical; slowing harvest, demand on header power, disposal of weed seeds

Weed seed destruction at harvest

Not yet available; greater power requirement; degree of effectiveness

Narrow header rows

Enables header rows to be burnt

Prevent introduction of viable weed seed

Sow weed-free seed

 

 

 

Clean farm machinery and vehicles

Prevent introduction in hay and grain