Diagnosing group C herbicide damage in cereals

This category contains root pre-emergent Group C herbicides.  Post-emergent use in cereals are simazine (silver grass control), and diuron+MCPA (broadleaf weed control).

Leaf symptoms occur when the seedling emerges but roots remain normal
More damage in sandy and low organic matter soil
Leaves die back from the tip and edges.
Healthy new growth indicates plant recovery
Chemical name Example trade name Chemical name Example trade name
Triazine Triazinones
Simazine

Simazine®

Metribuzin

Sencor®

 

Geasatop®

 

Lexone®

Urea    
Diuron

Diuron®

   

These herbicides are absorbed through the roots and transported to leaves where they are activated by light. Symptoms appear as the plants emerge (for soil applied Group C herbicides) or 4 to 6 days after spray application on emerged plants.

What to look for

    Paddock

  • Yellow or dying plants with symptoms varying along or between drill rows or between soil types.

    Plant

  • Oldest leaves are affected first and most severely, followed by younger leaves until the plant recovers or dies.
  • Rapid yellowing beginning with the oldest leaf , often with inter-veinal chlorosis.
  • Leaf death beginning at the leaf tip and edges and moving down to the base.
  • Roots are not affected.

What else could it be

Condition Similarities Differences
Diagnosing Desiantha weevil in cereals Plants emerging then older leaf and plant dies Evidence of seed root and lower stem chewing and small grubs with yellow heads.
Diagnosing seedling fertiliser damage in cereals Seedling and older leaf death Group C herbicide damage differs in having shortened roots with burnt ends.
Diagnosing waterlogging in cereals Older leaf and seedling death Location specific, i.e. in low lying areas.

Where did it come from?

Wet or humid conditions
Wet or humid conditions
Low organic matter
Low organic matter
  • These herbicides are more active in moist soil. More damage is likely after heavy rainfall, particularly in shallow duplex soils that are more easily waterlogged.
  • Situations where the herbicide is close to seed. This includes shallow sowing or heavy rain washing herbicide into furrows in ridged seedbeds.
  • Topsoils with low adsorption capacity such as sands with low organic matter.

Management strategies

  • It is important to observe withholding periods for these herbicides to avoid damage to following crops.

See also

Where to go for expert help

Page last updated: Wednesday, 4 February 2015 - 2:40pm