Weed Management in Horticultural Crops

CHEMICAL CLASSIFICATION OF HERBICIDES

Prior to the widespread use of chemical herbicides, mechanical control and cultural controls, such as altering soil pH, salinity, or fertility levels were used to control weeds.
The first widely used herbicide was 2,4-dichlorophenoxyacetic acid, often abbreviated 2,4-D which kills many broadleaf plants while leaving grasses largely unaffected (high doses of 2,4-D at crucial growth periods can harm grass crops such as maize or cereals). The low cost of 2,4-D has led to continued usage today and it remains one of the most commonly used herbicides in the world.

In 1950s triazine family of herbicides, which includes atrazine was introduced. Atrazine does not break down readily (within a few weeks) after being applied to soils of above neutral pH. Atrazine is said to have carryover, a generally undesirable property for herbicides. Glyphosate, frequently sold under the brand name Roundup, was introduced in 1974 for non-selective weed control. It is now a major herbicide in selective weed control in growing crop plants due to the development of crop plants that are resistant to it.

Many modern chemical herbicides for agriculture are specifically formulated to decompose within a short period after application. This is desirable as it allows crops which may be affected by the herbicide to be grown on the land in future seasons. However, herbicides with low residual activity (i.e., that decompose quickly) often do not provide season-long weed control.

List of herbicides with their common name and chemical name

S.No	Common Name	Trade Name	Chemical Name
Aliphatic carboxylic
1	Acrolein	Aqualin	2-propenal
2	Allyl alcohol	Allyl alcohol	2-propen-1-ol
3	TCA	TCA
4	Dalapon	Dalapon, Dowpan
Phenoxy Carboxylic
1	2,4,D	2,4,D; Fernoxone	2,4-dichlorophenoxyacetic acid
2	2,4,5 T	Weedar; Weedone	2,4,5-trichlorophenoxy acetic acid
Phenyl acetic acid
1	Fenac	Fenac	2,3,6-trichlorophenyl acetic acid
Benzoic acid
1	Dicamba	Banvel	2-methoxy - 3,6-dichloro benzoic acid
2	2,3,6 - TBA	Trysben; Benzac	2,3,6-trichlorobenzoic acid
Phthalic acid
1	Naptalam	Alanap; Peach thin 322	N-1-naphthylphthalamic acid
2	Endothol	Aquathal; Hydrothal
Phenol
1	PCP	Premerge; DNBP	Pentachlorophenol
2	Dinoseb	Basanite	2-sec-butyl-4,6-dinitrophenol
Dinitroaniline
1	Fluchloralin	Basalin	N-propyl -N(2-chloroethyl)- 4-(trifluoromethyl) - 2,6 dinitro-aniline
2	Isopropalin	Paarlon	N,N-dipropyl- 4-isopropyl -2,6-dinitro aniline
3	Pendimethalin	Prowl; Herbadex; Stomp	N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitroaniline
4	Trifluralin	Treflan	N,N-dipropyl-4-(trifluoromethyl) - 2,6-dinitroaniline
Benzonitrile
1	Dichlobenil	Casoron	2,6, - dinitrobenzonitrile
2	Bromoxynil	Brominol; Butril	3,5-dibromo-4-hydroxybenzonitrile
Triazines
1	Atrazine	Aatres; Gesprim	2-chloro-4-ethylamino-6-isopropylamino-S -triazine
2	Simazine	Princep; gesatop; Tapazine	2-chloro-4,6-bis(ethylamino) -S -triazine
Urea
1	Diuron	Karmex	3-(3,4-dichlorophenyl)-1,1-dimethylurea
2	Monuron	Telvar	3-(4-chlorophenyl)-1,1-dimethylurea
3	Isoproturon	Tolkan; Arelon	3-(4-isopropylphenyl)-1,1-dimethylurea
Phenyl Carbamate
1	Phenmedipham	Betanol	3-(methoxycarbonyl)amino]phenyl (3-methylphenyl)carbamate
2	Propham	Chem -hoe	Isopropyl phenylcarbamate
Thiocarbamate
1	Butylate	Bolero; Saturn	S-ethyl diisobutyl thiocarbamate
2	Thiobencarb	Odram; Saturn	S-4-chlorobenzyl diethyl(thiocarbamate)
3	Metham	Vapam; VPM	Sodium methyl dithiocarbomate
Acid amide
1	Alachlor	Lasso	2-chloro-2′,6′-diethyl-N-methoxymethylacetanilide
2	Butachlor	Machete; Delchlor	N-butoxymethyl-2-chloro-2′,6′-diethylacetanilide
Organic arsenical
1	Cacodylic acid	Phyto 138; Eras	Dimethyl arsonic acid
2	MAA	Ansar	Methane arsonic acid
Uracil
1	Bromacil	Hyvar-X; Krovar -1	5-bromo-3-sec-butyl-6-methyluracil
Diphenyl ether
1	Nitrofen	TOK E 25	2,4-dichlorophenyl p-nitrophenyl ether
2	Oxyfluoren	Goal	2- choloro – 1-(3-ethoxy-4nitrophenoxy) -4-(trifluoro methyl) benzene
Bipyridilium
1	Diquat	Reglone	6,7-dihydrodipyrido[1,2-a:2′,1′- a]pyrazinediium
2	Paraquat	Gramaxone	1,1′-dimethyl-4,4′-bipyridinium
Unclassified
1	Glyphosate	Round up; Glycel	N-(phosphanomethyl) glycine
2	Picloram	Tordon	4-amino 3,5,6 –trichloro picolinic acid
Inorganic herbicides
1	Ammoniumsulphamate
2	Sodium arsenite

LOW VOLUME HERBICIDES

Herbicides play an important role in weed control on agricultural and non agricultural surfaces. They are mainly applied by sprayers (foliar application) that consist of a herbicide tank, a pressure generator, spray nozzles, pipes and connectors. Foliar application of herbicides entails spraying the leaves of target plants during the growing season with a low concentration of herbicide in a water carrier.

Among all possible weed control methods, the use of herbicides is principally associated with risks for human health and the environment. Because of this there is much discussion about the use of herbicides. Reduction in use of herbicides will reduce associated risks. It is expected that herbicide use can be much reduced when herbicides are applied according to best possible practices i.e. application of minimal doses of herbicide adjusted to weed, weather, herbicide and sprayer conditions.

Low volume herbicide application has many benefits. Besides its cost-effectiveness for the landowner, the low volume solution is also environmentally friendly. The spray can be applied with a hand-powered, backpack sprayer or larger, motorized sprayers.

This type of application method has numerous advantages:

• With low volume foliar, only the targeted species are treated.
• Low volume foliar application is also extremely low profile. No large trucks, no noisy spray devices, only professional employees with backpacks.
• Low volume foliar also allows for a lower application cost due to the lower volume of mix being applied and is extremely effective due to the higher concentration of herbicide being applied to each individual plant.

HERBICIDE MIXTURES

It involves mixing of two or more herbicides used for effective and economical weed control.

Advantages of Mixture

• A mixture will broaden the spectrum of herbicidal action and kill a variety of weeds
• It may increase the effectiveness;
• In a mixture one herbicide may prevent rapid degradation of the other and increase its efficacy
• A mixture offers the possibility of reducing the dose of each of the herbicide necessary for weed control leading to low residue

Two types of mixtures

1. Tank mixtures made with the desired herbicides and rates before application e.g., Anilophos + 2,4-D EE – rice
2. Ready mix – formulated by the manufacturer. Ready mix available in the world market e.g., 2,4-D+Glyphosate, Paraquat+2,4,-D, Atrazine+metolachlor, paraquat+oxyfluorfen.

HERBICIDE ROTATION

The practice of following a systematic, rotational sequence of herbicide used in the same field to prevent or control formation of herbicide resistant weeds.
In a rotational programme a soil-applied or foliage applied herbicide or both are used in a sequence to take care of annual as well as perennial weeds. The choice of herbicide depends on the tolerance of crops to particular herbicides, type of weed spectrum, intensity of weed infestation, soil and climatic factors etc.,
The best rotational programme will aim at maximum cumulative cost benefit ratio and least residual problems and least build-up of tolerant weeds.

Advantages

• Helps in preventing emergence of tolerant weed species (herbicide is captured in vacuole and inactivated excluding the herbicide from site of action).
• Reduces the quantities of herbicide required for optimum weed control over the years.
• Provides most effective weed control for the duration of crop growth.
• Reduces the building up of herbicide residue problems.
• It offers high cumulative cost-benefit ratio over the years

Weed survey and mapping may be done every year and if any shift in weed flora, appropriate changes in herbicide rotation should be made.