(A). Selective and Non-selective
Herbicides
(1).
Selective Herbicides:- A selective herbicide
is the chemical which will kill some plant species when applied to a mixed
plant population, without causing serious injury to the other species.
Selective herbicides are used in crop areas, lawns, gardens, and grasslands. 2,4-D,
Atrazine, EPTC, Trifluralin, Alachlor, Butachlor, Fluchloralin and Pendimethalin
are the examples of the selective herbicides used on crop land.
(2).
Non-Selective Herbicide:- It is the chemical
which kills plants without regard to species, for e.g, Paraquat, Diquat,
Sodium Chlorate, Weed oils and Acrolein. The non-selective
herbicides are employed for general vegetation control on industrial sites,
fallow land, and in aquatics and tennis courts.
Note:-
Certain selective herbicides when applied at high rates, can act as
non-selective plant killers, for instance, Simazine and Diuron.
(B). Contact and Translocated
Herbicides
(1).
Contact Herbicides:- The group of herbicides which
kills plants by coming in contact with the plant tissue rather than as a result
of its translocation. Thus, a contact herbicide applied to the foliage of a
plant will not kill its roots. The established weeds will regrow from their
crown buds, roots, or rhizomes, shortly after treatment with a contact
herbicide. Some common contact herbicides are Paraquat, Diquat, Propanil and
Petroleum oils.
(2).
Translocated (Systemic) Herbicides:- That group of
herbicides which moves within the plant from the point of treatment to its
other parts. It often kills the entire plant even if only a portion of the
plant was treated with the herbicide. E.g. Glyphosate.
Therefore,
the translocated type of herbicides is of particular importance in controlling
the perennial weeds. The translocated herbicides can be applied as low volume
sprays to control annual weeds because they need not wet the entire plant
foliage, where as the contact herbicides which must drench the weeds fully for bringing
about their effective kill. Some herbicides may exhibit both, contact and
translocation activities.
(C). Residual and Non-Residual Herbicides
(1).
Residual Herbicides:-
A residual herbicide maintains its phytotoxic effects in soil for considerable
time after its application. This residue period may be three to four weeks in
some herbicides, such as 2,4-D and EPTC, and much longer, up to several
months, in other herbicides.
(2).
Non-Residual Herbicides:- A non-residual
herbicide is inactivated in soil immediately, or within a few days, after it
reaches the soil, Paraquat, Diquat, Amitrole, DSMA and Weed Oils
are non-residual or very short persistence herbicides. The non-residual herbicides
are good for a quick knock down of the existing weeds.
(D). Narrow Spectrum and Broad
Spectrum Herbicides
(1).
Narrow Spectrum Herbicides:- A narrow spectrum
herbicide, upon application to a mixed population of weeds, proves active on
one, or a very limited number of species. The narrow spectrum herbicides are
very useful against specific noxious weeds. Metoxuron, Difenzoquat and Diclofop
are very successful examples of narrow spectrum herbicides. These are widely
used against specific weeds like Phalaris minor and Avena
fatua.
2.
Broad Spectrum Herbicides:- A broad-spectrum
herbicide, on the other hand, controls a wide-spectrum of weedy flora at a
time. Most of our herbicides today belong to this group of herbicides since
these find wide-spread use in agriculture.
(E). Soil Sterilants and Fumigants
A
soil sterilant prevents the growth of plants when present in the soil. The
temporary soil sterilants sterilize the soil for up to 16 weeks, whereas the
permanent soil sterilants remain active for two or more years. The temporary
soil sterilants are usually volatile compounds which leave the soil in vapour
form after a few days to a few weeks of application. During this period these
are supposed to have killed all live plant parts in the soil. Such soil
sterilants are also called Soil Fumigants. MB (Methyl Bromide),
Metham and Dazomet are good examples of soil fumigants used in
agriculture and horticulture for the treatment of seedbeds and pot soil. Permanent
soil sterilants used in causing bare ground in industrial and other non-crop
areas for one or more seasons. Sodium chlorate, Substituted Ureas,
Several Triazines, Arsenics and Borates are the major permanent soil
sterilants in use currently.
Herbicide Formulations
Herbicides
in natural state may be solid or liquid, volatile or non volatile and soluble
or insoluble. These cannot be applied in original form; these have to be made
in to suitable and safe forms for their field use. Such forms are called
herbicide formulations. The herbicide formulations are diluted by the user, in
water but sometimes in oil also, before their application in the target area.
An
herbicide formulation is prepared by the manufacturer by blending the toxicant
(active ingredient) with substances like Solvents, Invert Carriers,
Surfactants, Antifoaming Agents, Stickers, Stabilizers etc.
A
herbicide formulation may be in one of the following forms:-
1.
Emulsifiable Concentrate (EC)
2.
Water Soluble Concentrate (SC)
3.
Wettable Powders (WP)
4.
Dry Flowables (DF)
5.
Flowable Liquid (FL)
6.
Granules (G)
7.
Others – Capsules, Wax Bars, Soluble Mulches, Foam Pieces, Aerosols etc.
(A).
Sprayable Concentrates:- A sprayable
concentrate may be in the form of (i) emulsifiable concentrate, (ii) wettable
powder or (iii) dry flowable. Water as carrier these herbicide concentrates
form solutions, emulsioon, or suspensions. They are collectively designated as
sprayable concentrates.
(a).
Emulsifiable concentrate (EC):- An herbicide emulsion
is a heterogeneous system. The active ingredient or herbicide concentrate is
dissolved in solvent, where each component maintains its original identity as
minute globules. An emulsifying agent must be added to it for uniform
distribution of chemical in water. Eg: 2,4-D Ester, Alachlor, Diallate etc.
(b)
Gels (GL):- Gels are relatively new products that
are thickened emulsifiable concentrate packed in water soluble bags. Gel can be
formulated so they resist leaking from pinhole size tears in the bags. The bags
are pre-measure so that user knows exactly how much herbicide is being added to
the spray tank.
(B)
Water-Soluble Formulations:-
(a).
Soluble Liquid (SL):- Formulations are in the form of soluble
liquids. It is a physically homogenous mixture of herbicide concentrate and the
carrier (usually water). Amine Salts of 2,4-D, 2,4 5- T, Diquat, Paraqut
and Isopropyl Amine Salt of Glyphosate and Imazethapyr.
(b).
Soluble Powder (SP):- Soluble powder
formulations are similar to solutions (S) in that, when mixed with water, these
dry formulations dissolve readily and form a true solution. The formulation is
dry and consists of the active ingredient and additives.
(c).
Soluble granules (SG):- Soluble granules are
dry and larger particle size than soluble powder. Considerable stirring or
agitation may be needed to dissolve these herbicides, but once in solution they
remain in that state indefinitely. They form clear solutions in the sprayer
tank and require a surfactant for maximum foliar activity. Typical formulation
contain 40 to 95 % active ingredient.
(C).
Dry Solid To Be Suspended In Water
(a).
Wettable Powders (WP):- In an herbicide
suspension the fine particles of the wet table powder are dispersed in a
suitable carrier. To prepare a suspension, first the wet table powder is turned
into slurry with limited amount of the carrier and then it is extended to
required volume by adding remainder of the carrier to it. Herbicides sold as
wettable powders are Atrazine 80%WP, Diuron 80%WP and Isoproturon 70%
WP and Almix20% WP.
(b).
Water Dispersible Granules (WDG, WG, DG) Or Dry Flowables (DF):-
Dry flowable and water-dispersible granule formulations are much like wettable
powders except that the active ingredient is formulated on a large particle
(granule) instead of onto a ground powder. Lexus (50DF) and Carfentrazone
Ethyl (Affinity 40DF).
(c).
Flowable Liquids:- Herbicides like Acrolein and
Aromatic Solvents are applied in water bodies as flowable liquids, direct
from the container under pressure, without any dilution. These are known as
flowable liquid formulations.
(D).
Microencapsulated Formulations (ME) Or Capsule Suspension (CS):-
Microencapsulated formulations are small particles consisting of a herbicide
core surrounded by a barrier layer, usually made up of a polymer shell. They
also are refereed to as capsule suspensions because the capsules are suspended
in a liquid medium. Microencapsulation greatly reduces the amount of solvent
needed.
Dry
Applications
(a).
Granular Herbicides:- These are made by
loading the toxicant on some dry, inert material, usually attaclay. The
herbicide granules vary in size from 0.04 mm to 1.0 mm in diameter. Herbicide
granules smaller than 0.04 mm are not used because they drift easily with wind.
E.g. Butachlor, 2,4- DEE.
(b)
Pellets (P) or Tablets (TB) Pellets:- These are dry
formulation of herbicide and other components in discrete particles usually
larger than 100 mm3 , tablets are in the form of small flat pellets.
Pellets and tablets frequently are used for spot applications. Herbicide
concentrations typically are 5 to 20 %.
Thank You
Vikas Kashyap💜:)
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