2. 2
OUTLINE
1. INTRODUCTION
1.1. Herbicide residues
1.2.Half life of herbicide
1.3. Harmful effects of herbicides
1.4.Factors affecting herbicide carryover
1.5.Testing for herbicide residues
2. HERBICIDE RESIDUE MANAGEMENT-
2.1.Cultural and Mechanical Management Practices
2.2.Deactivation of herbicides
2.3.Reducing the availability of herbicides in soil
2.4.Removal from the site of contamination
2.5. Enhancing Herbicide Degradation
3. RESEARCH FINDINGS
4. CONCLUSION
3. “Herbicide residue” means any specified substances in
food, agricultural commodities, or animal feed resulting
from the use of a herbicide. The term includes any
derivatives of a herbicide, such as conversion products,
metabolites, reaction products and impurities considered to
be of toxicological significance.
Herbicides when applied to soil to control weeds should
not remain in soil for long period. The length of time a
herbicide remains active in soil is called “soil
persistence,” or “soil residual life”.
1.INTRODUCTION
1.1. HERBICIDE RESIDUES
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Fig. 1 Soil active herbicides
Source: Hanson (2011)
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1.2.HALFLIFEOFHERBICIDE
The gauge by which we can predict herbicide persistence is half life of
Herbicide.
Herbicide half-life is a measure of how long it takes for 50% of a chemical
to degrade.
Fig. 2 Herbicide half life
7. 1.3. HARMFUL EFFECTSOF HERBICIDES
On non-target plants
Broad spectrum herbicide affects plant biological diversity and damages
environmental balance.
It reduces plant yield and increases susceptibility to diseases.
The residues left behind can affect the growth of the next succeeding crop
grown in rotation.
On animals
Application of herbicide affects animals directly or indirectly.
A decrease in the number of rare animals and some species of endangered
birds has been seen.
On soil
Factors such as soil organisms,
soil structure, transformation, and
organization of organic substances
are negatively affected by herbicides.
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Bleeding by earthworms
8. Destroys microorganisms in the soil.
On human
Chemical herbicide poses health
dangers for everyone from field workers
to consumers.
Exposure to herbicides causes
skin irritation, while inhaling these
chemicals irritates the throat
and nasal passages.
Air, water (surface and groundwater)
and soil pollution. Rainwater can
carry these chemicals to other areas.
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Field workers spraying herbicides
Cont...
10. Fig. 3 Fate of herbicides in soil
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4. PLANT UPTAKE - Once plants absorb the herbicide, it is metabolized. This
effectively removes residues from the soil. When plant stand
densities are low, removal of herbicide residues is also low.
Cont…
11. 1.5. TESTINGFOR HERBICIDERESIDUES
There are three ways to test for herbicide residues, which include:
1. Chemical Analysis-
Many labs can test for chemical residues, but the information may be of little use to
the grower.
It is a very costly procedure.
2. Plant Bioassay-
The grower can send soil samples to the lab where a greenhouse bioassay is
conducted.
Indicator species are grown in the submitted soil and are compared to plants grown
in a herbicide-free soil sample.
3. Field Bioassay-
A field bioassay is recommended on
a number of herbicide labels.
The bioassay needs to be large
enough to accommodate differences
in landscape.
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PLANT BIOASSAY STUDIES
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2.1. Cultural and Mechanical Management Practices
2.1.1. THROUGH
INTEGRATED WEED
MANAGEMENT-
IWM involves the application of
a variety of management practices
to control weeds.
Herbicides are used only when
weed populations exceed an
economic threshold level.
Nonchemical weed control
methods are emphasized.
Fig.4 Components of IWM
14. Crop rotation:
Including crop plants that are
resistant to the particular
herbicide.
Spreads the planting and
herbicide application season,
reducing the risk of
encountering widespread
herbicide runoff during a single
runoff event.
Herbicides Re-cropping restriction
Sulfonylureas Pea, lentil
Imidazoline Soybean, sunflower
2,4-D Tomato
Fluroxypyr, Wheat, barley, oats, rye, corn, flax,
canola, mustard, lentils, peas
Clopyralid Wheat, barley, oats, rye, corn, flax,
canola, mustard, lentils, sugar beets
Metosulam Wheat, lupins
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Table 2. Herbicide and their recropping
restrictions
Source: Shobha Sondhia, ICAR-DWR,
Jabalpur(2005-2016)
15. 2.1.2 GROWING HERBICIDE TOLERANT CROPS:
Certain herbicide tolerant crops reduce herbicide residues in a
soil by absorbing and deactivating these in their tissues.
Maize and millets - triazine herbicide.
Methi, turnip, berseem and gobhi-sarson - sulfosulfuron.
2.1.3. LIGHT IRRIGATION AFTER APPLICATION:
Continuous moist soils often result in a more rapid
breakdown of herbicides
Creates favorable conditions for microbial activity.
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16. 2.1.4. PLOUGHING OR CULTIVATING THE LAND:
Tillage operations bring deep present herbicide residues to soil surface which aids
in decontamination by degradation.
The applied herbicide is mixed to a large volume of soil and gets diluted.
Minimum or no-tillage generally have higher herbicide concentration near surface
and can have carry over effect on succeeding crop.
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Ploughing the land
17. 2.2.1. APPLICATION OF FYM-
Adsorbs the herbicide molecules
thus, making them unavailable for
crops and weeds.
The microbial population residing
in the organic matter start
decomposing the herbicide residues
at a faster rate due to high WHC of
organic matter of soils.
2.2. Deactivation of herbicides
Days after
spraying
Fluchloralin
1.0 kg/ha
without
FYM
Fluchloralin
1.0 kg/ha
with 10 t
FYM/ ha
1 0.484 0.406
7 0.352 0.231
15 0.252 0.158
30 0.104 0.053
At harvest 0.004 0.001
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Table 3. Fluchloralin residues (ppm) in
sandy loam soil under potato crop at
various intervals
Source: Patel et al., (2008)
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2.2.2.USE OF ADSORBENTS, PROTECTANTS AND ANTIDOTES:
Applied to the soil, crop seed or transplanted plant to protect the crop from
herbicide injury.
MODE OF ACTION-
Either deactivation or adsorption of the herbicide
Preventing its absorption and translocation by the crop.
Activated charcoal –
high adsorptive capacity
large surface area.
Broadcasted or applied as narrow band over the seed at the time of planting.
2.2.3. USE OF SAFENERS:
A group of structurally diverse synthetic chemicals that protect crop plants
from injury by certain herbicides.
Improves herbicide selectivity between crops and weed species.
Acts as “bioregulators”.
19. 2.3. Reducing the availabilityof herbicides in soil
2.3.1.USE OF OPTIMUM AND REDUCED DOSES OF
HERBICIDE-
More the quantity of herbicides application, more will be the residues
releasing in to the soil. Hence, more will be the persistence.
So herbicides should be applied at the least possible dose.
2.3.2.USE OF HERBICIDE IN COMBINATION AND SPLIT
DOSES-
The rate of application can be decreased by using herbicides in
combination.
Split application of herbicides will reduce the amount of herbicide
available to runoff at any one given time.
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20. Maize under sulfosulfuron 25 g/ha
Maize under sulfosulfuron 50 g/ha
Maize under sulfosulfuron 100 g/ha
Maize under control
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Source: Sondhia et al., (2005-2016)
Fig. 5 Effects of different doses of
herbicide on crop
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2.3.3. METHOD OF APPLICATION
Band application of herbicides i.e. applying herbicides in a narrow band varying
in width.
Banded application of herbicide can reduce persistence over broadcast
application.
2.3.4. USE OF ALTERNATIVES TO HERBICIDES
Plants excrete certain chemicals which inhibit the growth and germination of
other plants growing in vicinity called Allelochemicals.
Use of bioherbicides like Collego, DeVine etc.
2.3.5. SELECTION OF HERBICIDES WITH MINIMUM
CARRY-OVER POTENTIAL
Choosing a herbicide with little or no carry-over given your local soil and weather
conditions will eliminate future crop injury problems.
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2.4.RemovalfromsiteofContamination
2.4.1. PHYTOREMEDIATION:
The in situ use of vegetation in
bioremediation schemes is termed
as phytoremediation.
Emerging technology for the
cleanup of contaminated
environments such as soil, water
and sediments.
Different tolerant plants are
planted at the contaminated sites
which remove, transfer, stabilize
and destroys the main pollutant.
Fig. 6 Remediation methods adopted for
fast mitigation of sulfosulfuron in the soil,
(a) deep ploughing; (b) growing of Sesbania
in affected soil; (c and d) mixing of Sesbania
in the affected soil; (e) crop without
phytoremediation; (f) crop after
phytoremediation technique
Source: Sondhia (2014)
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2.5. ENHANCINGHERBICIDEDEGRADATION
2.5.1. BIOSTIMULATION:
The addition of electron acceptors, electron donors, or nutrients to
stimulate naturally occurring microbial populations is termed as
Biostimulation.
Introduction of adequate amounts of water, nutrients, and oxygen
into the soil to enhance the activity of indigenous microbial
degraders.
2.5.2. BIOAUGMENTATION:
The introduction of specific microorganisms (indigenous or non-
indigenous) aiming to enhance the biodegradation of target
compound can be termed as Bioaugmentation.
Microorganisms are capable of degrading the herbicide compounds
in the soil by utilizing them as a supply of nutrients and energy.
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Atrazine
(kg /ha)
Residue (ppm)
Days after herbicide application
15 30 45 90 At
harvest
1 0.294 0.118 0.291 - -
2 0.570 0.239 0.427 0.100 -
3 0.911 0.421 0.632 0.223 0.054
4 1.95 0.720 1.031 0.426 0.122
5 1.85 1.140 1.350 0.830 0.387
Table 9. Influence of optimum dose of atrazine on its
persistence in sugarcane ecosystem
Source: TNAU, QRT Report (2005)
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4. CONCLUSION
Herbicides are an indispensable part of the crop production
programme. However, in order to sustain the soil environment,
the indiscriminate use of them must be avoided. While using
herbicides, all the prevention and management aspects should
be kept in mind for bumper harvest as well as for quality food
production without deteriorating the environment. Hence,
integrating the mechanical and cultural management practices
with herbicides for managing weeds is a viable option.
Further, the combination of technologies like bioaugmentation,
biostimulation, phytoremediation along with organic matter
addition and crop rotation might be a promising technology for
minimizing the herbicide residues and persistence in soil.
