Persistence and Biodegradation of Herbicides
127
factors to obtain maximum efficacy and minimal persistence beyond the required
period of weed control. Burnside (1974) has prepared an excellent review on the
prevention and detoxification of pesticide residues in soils. He suggests that the
following steps can minimize pesticide residues in soils: (1) use an integrated
pest control system, (2) use alternative pest control methods, (3) improve efficacy, (4) select pesticides with short residual properties, and (5) rotate pesticides
and crops.
Most introductory textbooks of weed science stress several methods of weed
control, i.e., mechanical, crop competition, crop rotation, biological, fire, and
chemical control (Klingman and Ashton, 1975). These are the approaches to
integrated pest management (IPM) and alternate pest control methods of weed
science. Often the best and most economical way to control weeds is a combination of two or more of these methods. Since most herbicides are used to
reduce the weed competition to the desired plant and excessive rates often injure
the desired plant, minimal rates are mandatory. In row crops like orchards and
vineyards, the amount of herbicide required can be further reduced by using a
band or strip treatment rather than treatment of the entire area. The weeds in
the areas between the bands can be controlled by cultivation. Rotation of crops
has several advantages in weed control including different cultural practices,
crop competition aspects, crop-weed associations, and rotation of herbicides
that can prevent an increase in the population of a tolerant weed and accumulation
of a single herbicide and/or its degradation products. Rotation of herbicides
should also be used in monoculture for this same reason.
In general, long-term herbicidal residues have not been a problem in environmental pollution. However, even short-term residues can be a problem with
crop failure, errors in application, or unpredicted changes in land use when it
is desirable to plant a susceptible species. Under these conditions, man can
occasionally utilize certain methods to effectively reduce herbicide residues.
Although each situation requires specific methods, the following approaches may
be considered. Physical or chemical methods should be more rapid than biological
methods. In general, chemical methods have not been effective under field
conditions (Burnside, 1974). Physical methods such as adsorption by activated
charcoal, dilution by tillage, and leaching with water have been effective with
certain herbicides. Total removal of the contaminated soil has been useful for
small areas when the herbicide is limited to the surface few centimeters of soil.
However, under many conditions biological methods are the most practical. The
primary biological method is the use of a tolerant plant species to absorb and
degrade the herbicide, i.e., maize for simazine or atrazine. The promotion of
the growth of soil microorganisms can be helpful. The inoculation of the site
with microorganisms which have been shown to degrade the herbicide is a
technique that appears promising but has had little utilization. However, the
degradation of DDT by this means has been studied with some success (Kearney,
127
factors to obtain maximum efficacy and minimal persistence beyond the required
period of weed control. Burnside (1974) has prepared an excellent review on the
prevention and detoxification of pesticide residues in soils. He suggests that the
following steps can minimize pesticide residues in soils: (1) use an integrated
pest control system, (2) use alternative pest control methods, (3) improve efficacy, (4) select pesticides with short residual properties, and (5) rotate pesticides
and crops.
Most introductory textbooks of weed science stress several methods of weed
control, i.e., mechanical, crop competition, crop rotation, biological, fire, and
chemical control (Klingman and Ashton, 1975). These are the approaches to
integrated pest management (IPM) and alternate pest control methods of weed
science. Often the best and most economical way to control weeds is a combination of two or more of these methods. Since most herbicides are used to
reduce the weed competition to the desired plant and excessive rates often injure
the desired plant, minimal rates are mandatory. In row crops like orchards and
vineyards, the amount of herbicide required can be further reduced by using a
band or strip treatment rather than treatment of the entire area. The weeds in
the areas between the bands can be controlled by cultivation. Rotation of crops
has several advantages in weed control including different cultural practices,
crop competition aspects, crop-weed associations, and rotation of herbicides
that can prevent an increase in the population of a tolerant weed and accumulation
of a single herbicide and/or its degradation products. Rotation of herbicides
should also be used in monoculture for this same reason.
In general, long-term herbicidal residues have not been a problem in environmental pollution. However, even short-term residues can be a problem with
crop failure, errors in application, or unpredicted changes in land use when it
is desirable to plant a susceptible species. Under these conditions, man can
occasionally utilize certain methods to effectively reduce herbicide residues.
Although each situation requires specific methods, the following approaches may
be considered. Physical or chemical methods should be more rapid than biological
methods. In general, chemical methods have not been effective under field
conditions (Burnside, 1974). Physical methods such as adsorption by activated
charcoal, dilution by tillage, and leaching with water have been effective with
certain herbicides. Total removal of the contaminated soil has been useful for
small areas when the herbicide is limited to the surface few centimeters of soil.
However, under many conditions biological methods are the most practical. The
primary biological method is the use of a tolerant plant species to absorb and
degrade the herbicide, i.e., maize for simazine or atrazine. The promotion of
the growth of soil microorganisms can be helpful. The inoculation of the site
with microorganisms which have been shown to degrade the herbicide is a
technique that appears promising but has had little utilization. However, the
degradation of DDT by this means has been studied with some success (Kearney,