Drift
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external residue which subsequently volatized. The carbamate carbofuran
did not leave a residue in grain at harvest, so the rice could be safely consumed. Tilapia fsh reared in the same paddy did not show ill effects from
carbamates and were free of carbamate residue at harvest. Thus, consumers
benefted from increased yields of rice along with a protein (fsh) source for
their diets from the same paddy.
10.4 Dicamba Drift, an Ongoing Debate
A current as yet unresolved issue involves alleged vapor drift of dicamba
herbicide from soybeans to nontarget crops located some distance from the
initial soybean-treated area.
Dicamba (3,6-dichloro-2-methoxy benzoic acid) is a post-emergent, growth
regulator herbicide used in agriculture for control of broad leaf weeds. The
herbicide is volatile (4.5 mPa, 25°C). So, it must be applied as a salt, which, by
defnition, is normally nonvolatile. Drift to nontarget felds during application can be minimized by using ground rigs with height-adjusted (~24 in.)
spray nozzles that produce aerosols of suffcient size that quickly deposit
onto the target plants under optimum environmental conditions of temperature (<30°C) and wind speed (<10 mph). However, in practice, regardless
of the dicamba formulation, damage to nearby, nontarget felds continues,
implying that the herbicide volatilizes post application from treated felds,
sometimes called “vapor drift.” This was observed early on for the dimethylamine salt and sodium salt formulations and continues to be observed for
the newer diglycolamine formulations. A number of feld studies have identifed factors that contribute to post-application volatilization: (1) temperature (Behrens and Lueschen, 1979; Mueller and Steckel, 2019), (2) pH of the
tank mix (Mueller and Steckel, 2019), (3) pH of the soil surface (Oseland et al.,
2020), and (4) leaf surface in the area treated (Behrens and Lueschen, 1979).
Only the factors that had a direct bearing on the stability of the dicamba salt
are included here. Other factors that have been associated with volatilization
and nontarget damage include the following (Behrens and Lueschen, 1979;
Bish et al., 2019): (1) application rate; (2) application time of day; (3) effect of
dicamba formulation; (4) rainfall and relative humidity; (5) time after application; and (6) wind speed and atmospheric stability.
Obviously, it’s not the salt that is volatilizing. A strong contender is the
dicamba acid. Support for this comes from studies that have measured
dicamba in air (ng and ng/m 3 ) post application (Bish et al., 2019; Mueller and
Steckel, 2019). Acid pH will cause a dicamba salt to dissociate. For example,
mixing Roundup PowerMax® with Xtendimax® VaporGrip® Technology
(diglycolamine salt) will lower the pH of the tank mix, leading to greater
dicamba volatilization; also, an acid pH soil surface will lead to greater
185
external residue which subsequently volatized. The carbamate carbofuran
did not leave a residue in grain at harvest, so the rice could be safely consumed. Tilapia fsh reared in the same paddy did not show ill effects from
carbamates and were free of carbamate residue at harvest. Thus, consumers
benefted from increased yields of rice along with a protein (fsh) source for
their diets from the same paddy.
10.4 Dicamba Drift, an Ongoing Debate
A current as yet unresolved issue involves alleged vapor drift of dicamba
herbicide from soybeans to nontarget crops located some distance from the
initial soybean-treated area.
Dicamba (3,6-dichloro-2-methoxy benzoic acid) is a post-emergent, growth
regulator herbicide used in agriculture for control of broad leaf weeds. The
herbicide is volatile (4.5 mPa, 25°C). So, it must be applied as a salt, which, by
defnition, is normally nonvolatile. Drift to nontarget felds during application can be minimized by using ground rigs with height-adjusted (~24 in.)
spray nozzles that produce aerosols of suffcient size that quickly deposit
onto the target plants under optimum environmental conditions of temperature (<30°C) and wind speed (<10 mph). However, in practice, regardless
of the dicamba formulation, damage to nearby, nontarget felds continues,
implying that the herbicide volatilizes post application from treated felds,
sometimes called “vapor drift.” This was observed early on for the dimethylamine salt and sodium salt formulations and continues to be observed for
the newer diglycolamine formulations. A number of feld studies have identifed factors that contribute to post-application volatilization: (1) temperature (Behrens and Lueschen, 1979; Mueller and Steckel, 2019), (2) pH of the
tank mix (Mueller and Steckel, 2019), (3) pH of the soil surface (Oseland et al.,
2020), and (4) leaf surface in the area treated (Behrens and Lueschen, 1979).
Only the factors that had a direct bearing on the stability of the dicamba salt
are included here. Other factors that have been associated with volatilization
and nontarget damage include the following (Behrens and Lueschen, 1979;
Bish et al., 2019): (1) application rate; (2) application time of day; (3) effect of
dicamba formulation; (4) rainfall and relative humidity; (5) time after application; and (6) wind speed and atmospheric stability.
Obviously, it’s not the salt that is volatilizing. A strong contender is the
dicamba acid. Support for this comes from studies that have measured
dicamba in air (ng and ng/m 3 ) post application (Bish et al., 2019; Mueller and
Steckel, 2019). Acid pH will cause a dicamba salt to dissociate. For example,
mixing Roundup PowerMax® with Xtendimax® VaporGrip® Technology
(diglycolamine salt) will lower the pH of the tank mix, leading to greater
dicamba volatilization; also, an acid pH soil surface will lead to greater