Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Lipid Synthesis Inhibitors
Lipid synthesis inhibitors used in small grains consist of one herbicide family, the aryloxyphenoxypropionates. Herbicides in this family inhibit the production of fatty acids within plants. Fatty acid synthesis in plants is an essential process in the development of plant lipids that are required for cell membrane integrity and normal plant growth. The lipid synthesis inhibitor herbicides prevent the activity of acetyl-CoA carboxylase enzyme, which is involved in fatty acid biosynthesis (Figure 1). Broadleaf plants are tolerant to these herbicides; however, almost all perennial and annual grasses are susceptible. Herbicides in this family are taken up by foliage and are translocated via the phloem to areas of new growth. Injury symptoms are slow to occur (7 to 14 days) and first appear on new leaves emerging from the whorl of the grass plant.

Figure 1. Enzymes function as steps in biological processes. Enzymes are also extremely specialized in their function. As a result, different enzymes are involved with the many different biological processes that occur within a plant. Some herbicides can stop specific enzymes from functioning, resulting in a disruption of specific plant processes; this often leads to the death of the plant. This herbicide-enzyme relationship is very specific and any chemical modification of the herbicide or enzyme can eliminate herbicidal activity. Reprinted with permission from Herbicide Mode of Action, J. Gunsolus and W. Curran, NCR Extension Publication 377, University of Minnesota Extension Service, St. Paul, 1996. PLANTS ARE HEALTHY PLANTS ARE KILLED

Diclofop (Hoelon)
Application Timing (spring wheat and barley): Hoelon can be applied to wheat postemergence before the first node (jointing) moves above ground and while wild oats are in the 1- to 3-leaf stage. It also can be applied postemergence prior to tillering in barley.
Injury Symptoms: Hoelon applied to spring wheat and barley can cause yellowing of leaf tips and blades soon after application. Wheat is more tolerant to Hoelon than barley. Suppression of shoot and root growth may also occur. Applications made after the jointing stage may result in stem pinching approximately one inch above the soil line. This can weaken the stem and make it susceptible to breakage or lodging. Environmental conditions, such as wet soils or cold temperatures below 35°F that precede or follow application, can also result in crop injury. See photos 9 and 10.
Site of Action: Acetyl-CoA carboxylase enzyme.

Photo 9	Photo 10
Diclofop can cause barley injury when applied at high rates and during cool temperatures. Note leaf yellowing, browning, and stunting.

Fenoxaprop
(Cheyenne) = fenoxaprop + MCPA ester + thifensulfuron + tribenuron
(Dakota) = fenoxaprop + MCPA ester
(Tiller) = fenoxaprop + MCPA ester + 2,4-D amine
Application Timing (spring wheat, not labeled for barley): Cheyenne, postemergence from the 3-leaf stage to the end of tillering (6-leaf stage) and before jointing; Dakota, postemergence from the 3-leaf stage to the end of tillering and before jointing; Tiller, postemergence after crop begins to tiller (3- to 4-leaf stage) until the 6-leaf stage and before jointing.
Injury Symptoms: Any of the three fenoxaprop mixtures applied to spring wheat can cause yellowing of leaf tips and blades soon after application. Suppression of shoot and root growth could occur and last through the season. Applications made after the jointing stage may result in stem pinching approximately one inch above the soil line. This can weaken the stem and make it susceptible to breakage or lodging. Temporary leaf yellowing or plant stunting also may occur if cool, wet conditions exist at the time of application. With Cheyenne, these types of plant injury may also result under prolonged cool weather (daily temperatures <50°F) or widely fluctuating temperatures just before or after treatment. See photos 11 and 12. Site of Action: Acetyl-CoA carboxylase enzyme.

Photo 11	Photo 12
Fenoxaprop can cause injury to barley. Note leaf yellowing and stunting.

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