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Aromatic Carboxylic Acids
Benzoic Acids
Dicamba


Introduction


There are several members in this sub-family, including TBA, TIBA, chloramben and dicamba. They possess often very different herbicidal and chemical properties.
Dicamba (3,6-dichloro-2-methoxybenzoic acid) was introduced in 1965 and is used for broadleaf annual and perennial weed control primarily in maize, sorghum, cereal crops and other monocot crops, turf and pastures. It is often used to control weed species not easily controlled by 2,4-D, such as perennial broadleaf weeds species. It is also used to control triazine, and other types of, resistant weeds. It is applied preemergence and postemergence.


Physiology & Metabolism of Dicamba

Mode of Action & Lethality

The mode of action of dicamba is probably similar to that of the phenoxy carboxylic acids. Rapid, abnormal, cell growth then leads to the disruption of the phloem system and normal auxin balance in the plant. As with the phenoxy carboxylic acids, these herbicides act by stimulating abnormal cell growth in meristematic cells. This can result in the blockage of phloem vascular tissue. Extensive destruction of cambial, phloem cells near meristems occurs within days of treatment. The plant is killed by starvation resulting form an inability to translocate needed energy in the phloem.


Uptake and Movement of Dicamba in Plants


Dicamba is rapidly absorbed by plant root and shoot tissue. Dicamba translocates readily in the xylem and phloem. Dicamba taken up by plant roots is translocated mostly in the xylem initially, but over a longer time it moves to areas of high metabolic activity.


Basis of Selective Toxic Action between Susceptible & Resistant Species


Selective dicamba toxicity amongst plant species appears to be a function of uptake, distribution in the plant, and metabolism. In some tolerant species, translocation is limited to the xylem. Tolerant species metabolically degrade dicamba rapidly. The primary degradation pathway is by metabolism of the herbicide (ring hydroxylation followed by very rapid glucoside conjugation).
been found to occur.


Fate of the Dicamba in the Environment


Soil.
Dicamba will persist in the soil for up to 3 months, or longer, in the soil. Breakdown rates in the soil are primarily due to volitalization losses from the soil, and microbial degradation in warm, moist soils. As such it is an effective soil-applied herbicide for weed control after the spring is over. Dicamba is highly mobile in soils and can leach readily in the soil, especially in sandy soils. It can injure plant roots in these lighter soils. Little soil adsorption of dicamba to the soil colloidal fraction occurs. This adsorption is a function of soil pH: adsorption increases with a decrease in soil pH, decreased adsorption with calcareous soils.

Air.
One of the problems encountered with the use of dicamba is volatility leading to drift of the herbicide in the air. It is more volatile than 2,4-D and can cause injury in more instances than that herbicide on adjacent susceptible plants. Often dicamba drift can travel farther than 2,4-D also.

Toxicology.
Dicamba is excreted by animals with relatively rapid uptake, with little metabolism occuring to the parent molecule.


Dicamba Injury Symptomology


In general, dicamba injury is identitical to that caused by 2,4-D. There are some ways to differentiate between injury caused by these two herbicides, but no symptom is conclusive evidence of one or the other.


Epinasty.

The most typical injury symptom of dicamba is epinasty, or curved and twisted stems and leaves. This symptom is caused by differences in growth on different sides of an organ.

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Meristem Inhibition.

When leaf edge meristems are inhibited by dicamba they often force the leaf form a cup-shape. This cupping is often associated with a darker green color and a bunched, or puckered, appearance. Injured leaves appear cupped (upward) with the upper leaf surface the outside of the cup usually. 2,4-D can also cause leaf cupping, but usually the cupping is with the upper leaf surface forming the inside of the cup. The physiological basis of this difference is unknown, and may not be a consistent diagnostic difference. Monocot plant leaves can form tightly bunched shapes, "onion-leafing" or "buggy-whipping".

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Abnormal Plant Part Development.

Due to its auxin-like activity, dicamba can cause growth abnormalities similar to those caused by 2,4-D. A variety of morphological malformations can occur including leaf malformations and increased branching. Corn can form fasciated, or fused, abnormal brace roots. Other parts of the root system can be abnormal, with grotesque roots growth near the soil surface. Stems can become brittle and break. They can also become weakened and form a curved, or "goose-neck", shape. Often, dicamba can cause monocot, normally tolerant, species to lay flat for a time just after treatment. Often, leaves can be curled or twisted in addition to the procumbent habit of the leaves. This "relaxed" leaf position assumed by grasses treated with dicamba often disappears hours or days after treatment.

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Reproductive Interference.

Dicamba can interfer with reproductive activities of many species is a similar manner as does 2,4-D. Corn pollenation and seed set can be affected by late treatments of dicamba.


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Differences in Injury due to 2,4-D and Dicamba.


It is hard, if not impossible, to definitely differentiate between injury caused by these two similar herbicides. There are several keys that often can be helpful though.

----Dicamba translocates more completely throughout a plant and better control of woody and brushy species can help: these species will be injured more by dicamba than 2,4-D.

----1360T.JPG (12767 bytes) Another difference is the tendency of dicamba to flatten and twist monocot leaves just after treatment, a symptom not encountered as much with 2,4-D.

----Dicamba can drift in the air to greater distances than does 2,4-D.

----Dicamba injury often will develop over a longer period than 2,4-D: dicamba is slower acting.

----2,4-D often will cause slightly more corn stalk injury under the same conditions than that caused by dicamba.

----Dicamba has a longer residual effect in the soil than that from 2,4-D.

----Dicamba costs more than 2,4-D, look for thrifty farmers using 2,4-D.

----Dicamba may also act so quickly that it doesn't form the "strap-shaped" leaves, narrow and puckered, that 2,4-D does. Dicamba injured leaves instead are shorter than those with 2,4-D.


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