Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Growth Regulators
The growth regulator herbicides used in wheat and barley include the following herbicide families: phenoxy-acetic acids, benzoic acids, and the pyridines. Growth regulator herbicides interfere with numerous biological activities and protein synthesis of plants, causing a variety of plant growth abnormalities. These herbicides are selective against broadleaf weeds but are capable of injuring grass crops. Herbicides in this group move systemically in the plant by way of the phloem or xylem. The herbicides are quickly translocated to areas of new plant growth making them very effective against annual and perennial broadleaf weeds. Herbicide uptake is primarily through the leaves but root uptake is also possible. Injury symptoms are most obvious on newly developing leaves.

2,4-D
Application Timing (wheat and barley): Postemergence from tillering (5-leaf) to the preboot stage. Barley is considered slightly more tolerant to 2,4-D than wheat. Crop tolerance and crop injury symptoms produced are highly dependent upon crop growth stage at application. The window of safe application is slightly wider in barley than wheat (that is, damage in wheat may occur at a later seedling and earlier boot stage than with barley).
Injury Symptoms: There are two critical growth periods when injury may occur on wheat and barley with 2,4-D. These two periods are the seedling and boot stage. When 2,4-D is applied to wheat and barley before the tillering stage, rolled leaves (onion-leafing) may develop and inhibit the number of tillers produced. Additional injury symptoms that may occur at this growth stage are elongated rachis, twisted awns, and missing spikelets. Application of 2,4-D to wheat and barley after the jointing stage may result in twisted flag leaf, abnormal head emergence, and sterile spikelets. Treatments made after the soft dough stage do not result in any visual abnormalities or yield decrease. See photos 5 and 6.
Site of Action: Specific site(s) unknown, believed to have multiple sites of action.

Photo 5 Elongated rachis, twisted awns, missing spikelets, and heads not emerging from the flag leaf caused by 2,4-D applied to spring wheat before tillering.

Photo 6 Heads not emerging from the twisted flag leaf and shortened peduncles can be caused by a late application of a growth regulator herbicide.

MCPA
Application Timing (wheat and barley): Postemergence from 2-leaf to early boot stage. In comparison to 2,4-D, MCPA has greater crop safety and a wider window of application timing. Crop injury only occurs from high application rates or late applications.
Injury Symptoms: See Injury Symptoms under 2,4-D.
Site of Action: Specific site(s) unknown, believed to have multiple sites of action.

Dicamba (Banvel)
Application Timing (wheat and barley): Postemergence in spring wheat before plants exceed the 5-leaf stage and in barley before the 4-leaf stage. Plant injury from dicamba can be minimized if postemergence applications are made to wheat or barley between the 2- to 4-leaf stage. Wheat is more tolerant than barley to dicamba.
Injury Symptoms: Dicamba injury in wheat and barley is similar to that caused by phenoxy-acetic acid (2,4-D). Applications of dicamba made from the jointing to the boot stage can cause stem and leaves to layover and result in floret sterility, while late tiller stage applications can cause stunting and delay heading. See photo 7.
Site of Action: Specific site(s) unknown, believed to have multiple sites of action.

Photo 7 Sterile florets and head tips caused by a late application of dicamba to spring wheat.

Clopyralid (Stinger, Curtail)
Application Timing (spring wheat and barley): Stinger and Curtail are both postemergence herbicides labeled for wheat and barley. Stinger is applied from the 3-leaf to early boot stage. Curtail is applied from tillering to the preboot stage. Barley is more tolerant than wheat to clopyralid.
Injury Symptoms: Similar to the phenoxy-acetic acids. The premix Curtail contains 2,4-D and may produce typical phenoxy injury symptoms in small grains, therefore application and timing is more critical with Curtail than with Stinger.
Site of Action: Specific site(s) unknown, believed to have multiple sites of action.

Picloram (Tordon)
Application Timing (spring wheat and barley): Postemergence from 3- to 5-leaf stage. Picloram is labeled only at very low use rates in spring wheat and barley because of crop injury and carryover potential. Picloram is usually applied in combination with 2,4-D or MCPA.
Injury Symptoms: Similar to the phenoxy-acetic acids. See photo 8.
Site of Action: Specific site(s) unknown, believed to have multiple sites of action.

Photo 8 High rates of the pyridine herbicides can cause prostrate growth, yellowing, and stunting of spring wheat and barley.

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