128
Floyd M. Ashton
et al .• 1969). The addition of large amounts of plant residues and/or animal
manure has also been useful; this probably not only increases microbial activity
but may also adsorb some of the herbicide. Combinations of these treatments
should be considered.
5.5. CONCLUSION
The persistence and degradation of herbicides in the environment have been
reviewed and key references cited. In general, we know the approximate period
of persistence of most herbicides in the soil based on their phytotoxicity to higher
plants. This information has been primarily developed by empirical testing and
field observations. Future research should stress model systems under controlled
environmental conditions and the development of mathematical equations or
computer models. The use of physical models should also allow us to study
techniques that have the potential of acclerating herbicide degradation more
efficiently than field investigations. Of course, any model system must ultimately
be field tested to prove its worth in the real world. More attention should be
given to the persistence of herbicide degradation products in the environment.
Although investigations appear to be limited, the author is unaware of any
evidence that suggests that any herbicide or its degradation products undergo
bioaccumulation. However, the potential for bioaccumulation exists and should
be subjected to further study.
There are some situations where we wish to increase the persistence of a
herbicide. For example, some herbicides are degraded too rapidly to give seasonlong weed control. The use of controlled-release formulations has been investigated to increase the period of effectiveness of such herbicides (Cardarelli,
1976); however, acceptance of this technique has not been widely adapted. A
nonphytotoxic chemical, p-chlorophenyl-N-methylcarbamate, has been shown
to approximately double the period of weed control with chlorpropham (E. K.
Plant, personal communication, 1979). It appears to block the effect of the
degradative enzyme(s) of several microorganisms but does not alter their growth.
It delays the initiation of degradation but does not alter the rate once degradation
has started. EPA approval for this use in the United States is expected soon.
Future research on the biodegradation of herbicides should also include (1)
identification of higher plants and microorganisms that rapidly degrade specific
herbicides and develop techniques for their use in the field, (2) search for suitable
nonphytotoxic substrates that will promote the growth and activity of these
degradative microorganisms, (3) study ofthe mechanisms of degradative enzyme
induction in higher plants and microorganisms, (4) investigation of the environmental impact and mammalian toxicity of the metabolites of herbicides, and (5)
Floyd M. Ashton
et al .• 1969). The addition of large amounts of plant residues and/or animal
manure has also been useful; this probably not only increases microbial activity
but may also adsorb some of the herbicide. Combinations of these treatments
should be considered.
5.5. CONCLUSION
The persistence and degradation of herbicides in the environment have been
reviewed and key references cited. In general, we know the approximate period
of persistence of most herbicides in the soil based on their phytotoxicity to higher
plants. This information has been primarily developed by empirical testing and
field observations. Future research should stress model systems under controlled
environmental conditions and the development of mathematical equations or
computer models. The use of physical models should also allow us to study
techniques that have the potential of acclerating herbicide degradation more
efficiently than field investigations. Of course, any model system must ultimately
be field tested to prove its worth in the real world. More attention should be
given to the persistence of herbicide degradation products in the environment.
Although investigations appear to be limited, the author is unaware of any
evidence that suggests that any herbicide or its degradation products undergo
bioaccumulation. However, the potential for bioaccumulation exists and should
be subjected to further study.
There are some situations where we wish to increase the persistence of a
herbicide. For example, some herbicides are degraded too rapidly to give seasonlong weed control. The use of controlled-release formulations has been investigated to increase the period of effectiveness of such herbicides (Cardarelli,
1976); however, acceptance of this technique has not been widely adapted. A
nonphytotoxic chemical, p-chlorophenyl-N-methylcarbamate, has been shown
to approximately double the period of weed control with chlorpropham (E. K.
Plant, personal communication, 1979). It appears to block the effect of the
degradative enzyme(s) of several microorganisms but does not alter their growth.
It delays the initiation of degradation but does not alter the rate once degradation
has started. EPA approval for this use in the United States is expected soon.
Future research on the biodegradation of herbicides should also include (1)
identification of higher plants and microorganisms that rapidly degrade specific
herbicides and develop techniques for their use in the field, (2) search for suitable
nonphytotoxic substrates that will promote the growth and activity of these
degradative microorganisms, (3) study ofthe mechanisms of degradative enzyme
induction in higher plants and microorganisms, (4) investigation of the environmental impact and mammalian toxicity of the metabolites of herbicides, and (5)