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Herbicide
has become the indispensible in weed control in crop production due to increase
of wage rates and high unavailability of agricultural laborers. Herbicides are
thought to cause many environmental and health hazards. The hazard by
herbicides could be caused by its persistence in the environment and its toxic
effect after reaching the site of action. The toxicity is indicated by the LD50 values of a herbicide and the US EPA classification of pesticides show that
most of the herbicides belong to the low to none toxic category. Although some
herbicides may cause cancer and other health complications but it must happen
due to exposure of the people to the herbicides. This problem can be minimized
to some extent by using suitable herbicides and following the appropriate
herbicide handling rules. The herbicide may deteriorate soil and water quality
if they persist in the environment. The persistence depends on the soil
factors, climatic conditions and herbicide properties. Some research reports
show that most of the herbicides used in agriculture were not present in soil,
water, plant and grain after the harvest of crops. Thus, judicious selection of
herbicide and its appropriate handling could help reduce herbicide related
environment and health hazards.
Keywords: Potential, Herbicides, Environmental
impacts, Health hazards, Toxicity, Weed control
INTRODUCTION
Herbicides are used as an alternative method of weed control as they
are less expensive, often safer, faster and sometimes more selective compared
with mechanical weeding by hand or machine. The use of herbicide has recently
been increased globally because of increased unavailability of agricultural
laborers and higher wage rates. Herbicides are chemicals designed to kill
undesired plants in a defined area. However, inappropriate use of herbicide may
cause damage to crop plants, especially if too large a dose is used or if
spraying is occurred at wrong time when the crop species is sensitive to the
herbicide. Herbicides bring change in vegetation in the sprayed site causing
changes in habitat of animals such as mammals and birds. This is especially
true for herbicide treatment in forest areas. This is an indirect effect of
herbicide use as it does not cause any direct toxic effect to most of the
animals. Broadcasting by air craft or tractor may cause spray drift beyond the
spray site or may deposit onto the intended spray area causing damage to
vegetation. People spraying the herbicides may be affected by the chemicals
during their spraying if herbicide handling safety rules are not followed. In
addition, people may be exposed to herbicides through spray drift or residues
on food, and wildlife. The activity of herbicide in the environment and its
impact on animal health depends on its persistency in the environment and its
toxicity to the animals. The present paper describes the effect of herbicides
on environment and suggests the means to alleviate herbicide related
environmental hazards.
TOXICITY
OF HERBICIDE TO ANIMALS AND HUMAN HEALTH
Weed control practices bring about a major
shift in the microbial population, rhizosphere and thereby in physico-chemical
properties of the soil. The mechanical control keeps the field bare resulting
into soil erosion caused by both wind and water. On the other hand, use of
herbicides results into evolution of various volatile compounds, fumes, gases
and other residues. The entry of the herbicides into the atmosphere by means of
application to the soil or plants, drift during spraying and vapor is the most
dangerous source of environmental pollution; however its quantity is almost
negligible. Although herbicides cause adverse effect on animal biodiversity
mostly through indirectly by destruction of non-crop species, there would be
continued reliance on herbicide until suitable alternative weed management
approach are developed. Therefore, the herbicide is an indispensible part of
modern intensively managed agriculture.
Environmental Protection Agency (EPA) classifies all pesticides into
general and restricted use categories. General use pesticides are those that
will not cause unreasonable adverse effects on the environment and are safe for
application by the general public without special training. Most of the
herbicides are relatively safe to the user, wildlife and the environment and
are classified as general use pesticides. The potential toxicity to the users
generally is expressed by the lethal dose (LD50). According to the
US -EPA most herbicides fall under low and non-toxic groups [1,2].
LD50 is a good indicator of relative toxicity and safety. The
majority of herbicides are less toxic than aspirin and DDT. For example, LD50
values of aspirin and DDT are 750 and 87 while the LD50 value of
attrazine, trifluralin, butachlor are 3080, 3700 and 2000, respectively [3].
Many herbicides may cause adverse health effect due to their properties such as
carcinogenicity, endocrine disruption, reproductive and developmental toxicity
and acute toxicity [4], but these effects are exhibited when chemicals come to
the contact to the person at a substantial amount and time. However, the toxic
effect of herbicides on human health can be avoided following herbicide safety
rules.
Most herbicides are less hazardous to animals and humans because of the
following reasons:
1. The toxic effects
of herbicides are relatively specific to plant processes. Therefore, they tend to
be low in mammalian toxicity.
2. Most herbicides are
susceptible to breakdown by microbes, plant tissues and physical processes.
Therefore, there are few problems associated with long-term persistence.
3. Herbicides tend to
be either water soluble or are susceptible to breakdown. Therefore, there is
little or no build-up of herbicides or their residues in the fatty tissues of
animals.
4. Most herbicides are
of relatively low toxicity to fish and other aquatic organisms. Those compounds
with high toxicity tend to be soil adsorbed and seldom move into water systems.
Fish toxicity problems develop when chemicals are dumped or disposed of in
stream or applied to ditch-banks where runoff can move large quantities of
herbicide into the water.
Persistency of
herbicides
SOIL FACTORS
Soil physical, chemical and microbial factors
affect the persistency of herbicides. Physical factors include soil texture and
organic matter content. Chemical properties of soil affecting herbicide
persistence include pH, cation-exchange capacity (CEC) and nutrient status. The
microbial aspects include type and abundance of microbes present in the soil.
Soil composition affects herbicide phytotoxicity and persistence through
adsorption, leaching and volatilization. Soil high in clay, organic matter or
both increases the potential of herbicide carryover because of increase in
adsorption to soil colloids with a corresponding decrease in leaching and loss
through volatilization. Thus, soil having high clay and organic matter holds
the soil colloid and slows the plant uptake and herbicidal activity. Soil pH
affects herbicide persistence. At higher pH, herbicide adsorption in the soil
colloid is less and therefore, herbicides mainly remain in the soil solution.
Herbicides available in the soil solution are available for plant uptake.
Chemical breakdown and microbial degradation of herbicides are slow at higher
pH. Certain members of sulfonylurea group (chlorsufuron and chlorimuron)
persists for long time in higher pH soils while Low pH affects the persistence
of clomazone and the imidiazolineones (imazaquin and imazethapyr). However,
soil pH has little effect on the persistence of other herbicides.
The cation exchange capacity (CEC),
principally a function of clay type and organic-matter content, is directly
involved in herbicide absorption. There is much variation in the effects of
cations and nutrients on herbicide activity and breakdown, depending on soil
composition, nutrient type and concentration, and chemistry of the herbicide.
Soil microorganisms are partially responsible for the breakdown of many
herbicides. The type of microorganisms and their relative amount determine the
speed of decomposition.
CLIMATIC CONDITIONS
Moisture, temperature and sunlight are the
most involved climatic elements in herbicide degradation. Increased temperature
and soil moisture enhance both chemical and microbial degradation of
herbicides. Cool, dry conditions slow degradation, causing greater carryover
potential. Thus herbicide persistence is less likely in warm moist winter than
cool dry winter. Many herbicides are subjected to photo-degradation. The
dinitroanilines (trifluralin and pendimethalin) are sensitive to light
degradation. They may be lost when surface applied if they remain for an
extended time without rainfall. Thus degradation is accelerated on very sunny
days. This sensitivity to light and loss by volatility are primary reasons for
soil incorporation.
HERBICIDAL
PROPERTIES
Soil persistence of herbicide depends on its
chemical properties. Important factors include water solubility, soil
adsorption, vapor pressure and susceptibility to chemical and microbial
alteration or degradation. The water solubility of an herbicide helps to
determine its leaching potential. Leaching occurs when an herbicide is
dissolved in water and moves down through the soil profile. Herbicides that
readily leach may be carried away or carried to rooting zone of susceptible
plants. Herbicide leaching is determined not only by an herbicide’s water
solubility but also by its ability to adsorb to soil particles. Additionally,
soil texture and available soil water affect herbicide leaching. Herbicides
that are low in water solubility, are strongly adsorbed to soil colloids, and
exist in dry soils are less likely to leach and have a greater potential to
persist. The vapor pressure of an herbicide determines its volatility. Volatile
herbicides such as thiocarbamates (EPTC, butylate) must be incorporated immediately
to avoid gases losses. The herbicides with high vapor pressure are less likely
to persist than herbicides with low vapor pressure. Volatility also increases
with temperature.
Herbicides vary in their susceptibility to
microbial decomposition. For example, microbial degradation of 2,4-D occurs
very quickly in the soil, whereas microbial degradation of atrazine is slow.
Chemical decomposition of an herbicide not only depends on the chemistry of the
herbicide but also on soil and climatic factors. Chemical breakdown of an
herbicide involves reactions such as hydrolysis, oxidation and reduction. The
speed of these reactions depends on soil type and climatic conditions. These
chemical reactions along with microbial degradations are important processes in
the decomposition of herbicides.
Means of avoiding herbicide carryover/persistence
Herbicide carryover problems can be overcome
by the following ways.
1. Dose,
method and soil type influence herbicide persistence. Thus use of correct rate
and method for any herbicide for specific soil and weed problem would reduce
persistence.
2. The
amount of tillage affects herbicide persistence. Tillage encourages herbicide
decomposition indirectly through increased microbial and chemical breakdown.
3. Herbicide
combination may reduce the risk of carryover problem. Lower dose of each
herbicide may broaden the weed control spectrum and potentially reduce
carryover.
4. Herbicide
application to the crop determines the toxicity to the succeeding crop. For
example, soybean may tolerate a certain level of atrazine residue but
application of metribuzon to soybean could increase injury when soybean is
grown after atrazine treated corn.
5. Plants
absorb herbicides from soil. The removal of plants after harvesting from the
field may reduce the risk of herbicide residue carryover.
6. Use
of tolerant crops in the rotation may reduce the risk of carryover problem.
Generally smaller seeded crops are more likely to herbicide sensitivity than
larger seeded crops.
CONCLUSION
1. Radosevich S, Holt J, Ghersa C
(1997) Weed ecology: Implications for management. 2nd Edn. John
Wiley & Sons, Inc.: New York, pp: 387-411.
2. Lingenfelter D (2016) Safe
herbicide use. Penn State College of Agricultural Sciences. Available at: http://agsci.psu.edu
3. Zimdahl RL (1993) Fundamentals of
weed science. Academic Press Inc.: New York, pp: 329-360.
4. Damalas CA, Eleftherohorinos IG
(2011) Pesticide exposure, safety issues and risk assessment indicators. Int J
Environ Res Public Health 8: 1402-1419.
5. Crosby DG (1983) The fate of
herbicides in California rice culture. In: Miyamoto J, Kearney PC, ed. Herbicide
chemistry, human welfare and the environment. Oxford (UK): Pergamon Press, pp:
339-346.
6. Sondhia S (2014) Herbicides
residues in soil, plants and non-target organisms and human health
implications: An Indian perspective. Indian J Weed Sci 46: 66-85.
7. Hager AG, Nordby D (2007)
Herbicide persistence and how to test for residue in soils. In. Illinois
Agricultural Pest Management Handbook, pp: 343-350.
2. Lingenfelter D (2016) Safe
herbicide use. Penn State College of Agricultural Sciences. Available at: http://agsci.psu.edu
3. Zimdahl RL (1993) Fundamentals of
weed science. Academic Press Inc.: New York, pp: 329-360.
4. Damalas CA, Eleftherohorinos IG
(2011) Pesticide exposure, safety issues and risk assessment indicators. Int J
Environ Res Public Health 8: 1402-1419.
5. Crosby DG (1983) The fate of
herbicides in California rice culture. In: Miyamoto J, Kearney PC, ed. Herbicide
chemistry, human welfare and the environment. Oxford (UK): Pergamon Press, pp:
339-346.
6. Sondhia S (2014) Herbicides
residues in soil, plants and non-target organisms and human health
implications: An Indian perspective. Indian J Weed Sci 46: 66-85.
7. Hager AG, Nordby D (2007)
Herbicide persistence and how to test for residue in soils. In. Illinois
Agricultural Pest Management Handbook, pp: 343-350.
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