Depending on the target site, the herbicides are classified into soil applied herbicides and foliage applied or foliar herbicides.
(a) Surface application – Soil active herbicides are applied uniformly on the surface of the soil either by spraying or by broadcasting. The applied herbicides are either left undisturbed or incorporated into the soil. Incorporation is done to prevent the volatilization and photo-decomposition of the herbicides. e.g., Fluchoralin–Left undisturbed under irrigated condition and incorporated under rainfed condition.
(b) Subsurface application – It is the application of herbicides in a concentrated band, about 7–10 cm below the soil surface for controlling perennial weeds. For this special type of nozzle is introduced below the soil under the cover of a sweep hood. e.g., Carbamate herbicides to control Cyperus rotundus and Nitralin herbicides to control Convolvulus arvensis.
(c) Band application- Application to a restricted band along the crop rows leaving an untreated band in the inter-rows. Later inter-rows are cultivated to remove the weeds. Saving in cost is possible here. For example when a 30 cm wide band of a herbicide applied over a crop rows that were spaced 90 cm apart, then two-third cost is saved.
(d) Fumigation – Application of volatile chemicals into confined spaces or into the soil to produce gas that will destroy weed seeds is called fumigation. Herbicides used for fumigation are called as fumigants. These are good for killing perennial weeds and as well for eliminating weed seeds. E.g., Methyl bromide, Metham, etc.
(e) Herbigation- Application of herbicides with irrigation water both by surface and sprinkler systems. In India farmers apply fluchloralin for chillies and tomato, while in western countries application of EPTC with sprinkler irrigation water is very common in Lucerne.
(i) Blanket spray – Uniform application of herbicides to standing crops without considering the location of the crop. Only highly selective herbicides are used here. e.g., Spraying 2,4-Ethyl Ester to rice three weeks after transplanting
(ii) Directed spray – Application of herbicides on weeds in between rows of crops by directing the spray only on weeds avoiding the crop. This could be possible by use of protective shield or hood. For example, spraying glyphosate in between rows of tapioca using hood to control Cyperus rotundus.
(iii) Protected spray – Applying non-selective herbicides on weeds by covering the crops, which are wide spaced, with polyethylene covers etc. This is expensive and laborious. However, farmers are using this technique for spraying glyphosate to control weeds in jasmine, cassava, banana etc.
(iv) Spot treatment – It is usually done on small areas having serious weed infestation to kill it and to prevent its spread. Rope wick applicator and Herbicide glove are useful here.
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