Diagnosing group B herbicide damage in cereals

Sulfonlyureas (SUs) and sulfonamides (TPS) are systemic herbicides that are used for pre- and/or post-emergent grass and/or broadleaf weed control in cereals. Imidazolinones (IMIs) are toxic to most cereals. Specific chemicals are used for non-selective weed control, weed control in legumes, or control of herbicide-resistant weeds in IMI-resistant (Clearfield®) wheat.

Pale stripes and distortion on young leaves due to pre-emergent chlorsulfuron damage in wheat (left), barley (centre) and oats (right).
Youngest leaves roll inwards giving the plant a spiky appearance
Clearfield® herbicides cause serious damage or death to susceptible crops.
Waterlogging increases herbicide damage
Sulfonylureas (SUs) chemical name Example trade name
Triasulfuron Logran®
Chlorsulfuron Glean®
Sulfosulfuron Monza®
Metsulfuron Ally®
Iodosulfuron Hussar®
Mesosulfuron Atlantis®
Imidazolinones (IMIs) chemical name Example trade name
Imazapyr Arsenal®
Imazapic Flame®
Imazamox Raptor®
Imazethapyr Spinnaker®
Imazapic + Imazapyr Intervix®
Imazapic + Imazapyr + MCPA OnDuty®
Sulfonamides (TPS) chemical name Example trade name
Flumetsulam Broadstrike®
Metosulam Eclipse®
Florasulam Paradigm®
Pyroxsulam Crusader®

What to look for

    Paddock

  • Normal crop emergence followed by paleness and stunting, particularly in wetter and alkaline areas (SUs only, the persistence of TPS products is not influenced by soil pH).

    Plant

  • Symptoms often appear 5-8 days after germination or spray application.
  • Emerging or young plants can be stunted with curled or spiky new leaves.
  • Pale to yellow red or purplish colouration of new leaves, that may show as inter-veinal chlorosis, streaks, mottles or blotches.
  • Growth of lateral roots may be reduced.
  • Imidazolamines may cause head damage.
  • Tillering may be reduced, but sulfonylurea or sulfonamide affected plants generally recover.
  • Group B herbicide damaged plants are more susceptible to root disease, nematodes and trace element deficiencies.

What else could it be

Condition Similarities Differences
Diagnosing allelopathy in cereals Weak seedlings Silly seedling plants rarely recover and the oldest leaves are first affected
Diagnosing waterlogging in cereals Weak seedlings Group B herbicide damage may be worsened by this condition but differs in that new leaves are most affected
Diagnosing salinity in cereals Weak seedlings Group B herbicide damage may be worsened by this condition but differs in that new leaves are most affected
Diagnosing deep seeding in cereals Weak seedlings Damage may be worsened by this condition but differs in that new leaves are most affected

Where does it occur?

Soil ph
Soil ph
Wet conditions
Wet conditions
  • Chemical breakdown in soil is inhibited by high pH for sulfonylureas and low pH for imidazolinones. TPS products are unaffected by pH.
  • Stressed plants are less able to metabolise herbicides and therefore more susceptible to damage. Stresses include cold, wet weather, waterlogging, deep sowing, frost, nutrient deficiency and soil compaction.

Management strategies

Time of herbicide application
Time of herbicide application
  • It is important to observe withholding periods for these herbicides to avoid damage to following crops. If damage has occurred, there is no treatment. Plants will either die or recover. As a precaution, spray when the risk of drift is low or when environmental conditions are safe.

Where to go for expert help

Topics

Page last updated: Tuesday, 11 July 2017 - 9:39am