| Chemical Abstract Number (CAS #) |
1918009
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| Synonyms | Dicamba |
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3,6-Dichloro-2-methoxybenzoic acid |
| Analytical Methods |
EPA Method 515.1 |
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EPA Method 515.2 |
EPA Method 555 |
EPA Method 615 |
EPA Method 8150B |
EPA Method 8151 |
| Molecular Formula | C8H6Cl2O3 |
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| Use | SELECTIVE PRE- & POSTEMERGENCE HERBICIDE; PLANT GROWTH
REGULATOR.
HERBICIDE TO CONTROL ANNUAL & PERENNIAL BROADLEAF WEEDS IN CORN,
SMALL GRAINS, PASTURES, AND NON-CROPLAND.
FOR CONTROL OF BOTH ANNUAL AND PERENNIAL BROADLEAF WEEDS IN FIELD
& SILAGE CORN, GRAIN SORGHUM, SMALL GRAINS, SUGARCANE, ASPARAGUS,
GRASS SEED CROPS, TURF PASTURE, RANGELAND & NONCROPLAND AREAS
SUCH AS FENCE ROWS, ROADWAYS & WASTELAND. ALSO REGISTERED FOR
SPOT TREATMENT OF PERENNIAL BROADLEAF WEEDS IN CROPLAND TO BE
ROTATED TO WHEAT. FOR CONTROL OF ANNUAL AND PERENNIAL BROADLEAF
WEEDS AFTER HARVEST OF ONE CROP BUT BEFORE PLANTING THE NEXT CROP
.
FOR WEED CONTROL IN CEREALS (NOT UNDERSOWN), IT IS USUALLY
FORMULATED WITH ONE OR MORE PHENOXYALKANOIC ACIDS SO THAT
DOSAGE OF DICAMBA LIES IN RANGE 80-130 G AE/HA. IT IS USED FOR
CONTROL OF DOCKS IN ESTABLISHED GRASSLAND & FOR CONTROL OF
BRACKEN.
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| Consumption Patterns | 60% AS AN HERBICIDE FOR CORN; 20% AS AN HERBICIDE FOR WHEAT; AND
20% FOR MISC APPLICATIONS (1975)
Applied to rights of way, landscape, structures and non-agricultural areas, 45.3%; Grains, 41.3%;
Forage crops vegetables and turf account for remainder applied (1984). California use, calculated
from table
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| Apparent Color | Colorless solid
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| Melting Point | 114-116 DEG C
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| Molecular Weight | 221.04
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| Density | 1.57 at 25 deg C
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| Odor Threshold Concentration | SRP: Air 250.8 PPM
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| Sensitivity Data | Irritating to skin, eyes and mucous membranes.
Severe eye injury may occur. Skin irritation may occur when handling finely ground material.
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| Environmental Impact | Dicamba is released directly to the environment by its application as a herbicide for the
control of annual broadleaf weeds. If released to soil, microbial degradation will probably be the
major removal process under most conditions. The principal soil metabolite appears to be
3,6-dichlorosalicylic acid. Dicamba is very mobile in most soils and significant leaching is possible.
Based on the results of one study, volatilization from soil surfaces may not be an important
process, although some volatilization may occur from plant surfaces. The half-life of dicamba in
soil has been observed to vary from 4 to 555 days with the typical half-life being 1 to 4 weeks.
Under conditions suitable to rapid metabolism, the half-life is less than two weeks. If released to
water, microbial degradation appears to be the important dicamba removal process; photolysis
may contribute to its removal from water. Aquatic hydrolysis, volatilization, adsorption to
sediment, and bioconcentration are not expected to be significant. If released to the atmosphere,
dicamba will probably exist in both the vapor-phase and the adsorbed to particulate phase. The
half-life for the vapor-phase reaction of dicamba with photochemically produced hydroxyl radicals
has been estimated to be 6 days. Particulate phase dicamba will be subject to wet and dry
deposition. General population exposure to dicamba may occur through oral consumption of
contaminated drinking water. Occupational exposure via inhalation and dermal routes associated
with application (spraying, loading and mixing) of dicamba as a herbicide may occur.
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| Environmental Fate | IT IS READILY LEACHED FROM SOILS ITS ABSORPTION IS
STRONGEST @ LOWER PH LEVELS (4.0-6.0) & MINIMAL AT PH LEVELS HIGHER
THAN 6.0. IT EXHIBITS INTERMEDIATE PERSISTENCE IN MANY SOIL SYSTEMS
COMPARED TO OTHER HERBICIDES. PHOTODECOMPOSITION & RUNOFF
HAVE LITTLE EFFECT ON PERSISTENCE. VERY LITTLE ((14)CARBON
CARBOXYL)DICAMBA WAS LOST OVER AN 8 WK PERIOD WHEN TREATED SOILS
WERE INCUBATED @ 35 DEG C UNDER VARIOUS RELATIVE HUMIDITIES (0-100%).
SIMILAR STUDIES WITH (14)CARBON DICAMBA ON PLANCHETS SHOWED A 50%
LOSS OF RADIOACTIVITY OVER PERIOD OF 11 WEEKS.
TERRESTRIAL FATE: The persistence of bromoxynil, (14)C-dicamba, and propanil at 1 kg/ha
were studied under lab conditions in a clay loam, a heavy clay and a sandy loam at 85% of field
capacity and at 20 degrees, both singly and in the presence of herbicides normally applied with
these chemicals as tank-mixture or split-mixture components. The breakdown of (14)C-dicamba
in a particular soil was unaffected by being applied alone or in the presence of diclofop-methyl,
flamprop-ethyl, MCPA, metribuzin, propanil or 2,4-D. The times for 50% of the applied dicamba
to be degraded were approximately 16 days in both the clay loam and sandy loam, and about 50
days in the heavy clay.
TERRESTRIAL FATE:The degradation of (14)C-dicamba was studied under field conditions at
rates of 1 kg/ha in small sandy loam plots. Duplicate plots were samples to a depth of 10 cm after
45 and 95 weeks and extracted with aqueous acetonitrile to determine amounts of extractable
radioactivity. After 45 weeks, soluble radioactivity recovered from the dicamba treated plots was
less than 1% of that applied, whereas the nonextractable activity accounted for 2% of that
applied. After 95 weeks, less than 1% of the applied radioactivity was, on the average, extractable
from the dicamba, whereas 3% remained in a solvent nonextractable form.
DICAMBA MAY PERSIST SIGNIFICANTLY LONGER UNDER CONDITIONS OF LOW
SOIL MOISTURE & RAINFALL. UNDER CONDITIONS AMENABLE TO RAPID
METABOLISM, DICAMBA HAS A HALF LIFE OF LESS THAN 14 DAYS.
TERRESTRIAL FATE: Microbial degradation is probably the major fate process for the removal
of dicamba from soil under most conditions . The principal soil metabolite of dicamba appears
to be 3,6-dichlorosalicylic acid(1-2). Dicamba is very mobile in soil and significant leaching is
possible. Based on the results of one study, volatilization from soil surfaces may not be an
important process, although some volatilization may occur from plant surfaces. In one review of
literature of dicamba persistence in soil, the percent lost/day was found to range from 0.09 to
12.50% (which corresponds to half-lives of 4 to 555 days assuming first-order kinetics), with
most of the data indicating a half-life of two to four weeks . In another review of persistence
literature, the mean half-life under laboratory conditions was 14 days while the mean half-life
under field conditions was 8 days . It has been reported that under conditions suitable to rapid
metabolism, dicamba has a half-life of less than two weeks(1,SRC).
AQUATIC FATE: Based on the results of various studies, microbial degradation appears to be
the important dicamba removal process in natural water . Photolysis may contribute to dicamba
removal from water . Aquatic hydrolysis, volatilization, adsorption to sediment, and
bioconcentration are not expected to be significant.
ATMOSPHERIC FATE: With a vapor pressure of 3.4X10-5 mm Hg at 25 deg C , dicamba
released to the atmosphere will probably exist in both the vapor-phase and the adsorbed to
particulate phase(2,SRC). The half-life for the vapor-phase reaction of dicamba with
photochemically produced hydroxyl radicals has been estimated to be 2.42 days . Particulate
phase dicamba will be subject to wet and dry deposition.
The herbicide dicamba has a half life of 31 days with a first-order rate constant of 0.0224/day in a
typical midwestern agricultural soil under aerobic conditions. The half life of dicamba in the same
soil after the soil was made anaerobic at 30 days is 58 days with a first-order rate constant of
0.012/day. Dicamba is completely mineralized to CO2 under aerobic conditions with
3,6-dichlorosalicylic acid as the only major metabolite. Low levels of
2,3-dihydroxy-3,6-dichlorosalicylic acid were detected. Metabolism under anaerobic conditions is
similar to that which occurred in aerobic soil except the rate of dicamba metabolism is reduced
under anaerobic conditions.
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| Drinking Water Impact | DRINKING WATER: Dicamba has been detected in drinking water concentrates taken
from Cincinnati OH (Oct 1978; Jan 1980), New Orleans LA (Jan 1976), Philadelphia PA (Feb
1976), and Seattle WA (Nov 1976) .
SURFACE WATER: Dicamba has been qualitatively detected in tributaries feeding Lake Erie,
Lake Huron, and Lake St. Clair . Dicamba was detected in one of 949 stream waters analyzed
from 11 agricultural watersheds in Ontario Canada, at a concn of 0.7 ppb, during 1975-76
monitoring; 1976-77 monitoring detected no dicamba(2-3). During a 1981-1985 survey of
Ontario rivers, dicamba was detected in 3.9% samples from Grand River at a maximum
concentration of 0.4 ug/L, in 2.1% samples from Saugeen River at a maximum concn of 13 ug/L
and 18.1% samples from Thames River at maximum concn of 1.1 ug/L .
GROUNDWATER: Based on its sorption properties in soils, it was estimated that a mean
concentration of 1X10-5 ug/L of dicamba will leach into groundwater resulting from a 1.0 kg/ha
application of the herbicide under conventional soil-till conditions .
EFFL: Dicamba was detected in effluents from a chemical plant and a sewage treatment plant
discharging into the Chattanooga Creek in Feb 1973 . It has also been identified in effluent
concentrates taken from advanced waste treatment facilities in Lake Tahoe CA .
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