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  Pesticides - Effective Application of Herbicide  

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Herbicides are a relatively new tool in manís fight against weeds. Unlike other pesticides, herbicides demand more skill in application. Proper application is a very important aspect in herbicide use. Due to improper application, herbicide treatment may be ineffective or it can cause serious damage to crop in which it was applied. For the safe and effective use of herbicides, selection of proper equipment, techniques and methods are very important. The following points should be considered while applying herbicides.

  • Equipment
  • Spraying Pressure
  • Agitation
  • Avoidance of Drift
  • Calibration
  • Uniform Coverage
  • Cleanliness


Most of the herbicides are applied in liquid form by various types of sprayers. Under north eastern hilly region conditions, Knapsack sprayers are quite suitable. It can be carried on the back with the help of straps. It can be carried with a double action lever-operated pump mounted either inside or outside container. The operator operates the lever with his left hand and pumps the spray solution, which moves through delivery pipe, held by his right hand.

Nozzle is important art of a sprayer. The flat fan and flood jet nozzles are most suitable for herbicide application. The cone nozzles should not be used for spraying of herbicides.

Spraying Pressure

Optimum and constant pressure throughout spraying is very important for even spraying with minimum drift. For knapsack sprayer, pressure range from 0.7 kg/m to 5 kg/m may be maintained.


Agitation is important to prevent settling especially where suppressions of wettable powder (WP) are used. Due to setting there may be variation in concentration during spraying. The knapsack sprayers are equipped with either mechanical or hydraulic agitator.

Avoidance of Drift

To avoid the drift it is necessary to stop spraying when the wind is too strong (maintains 10 km/hr). For a wind velocity of 5-10 km/hr the height of nozzle may be maintained lower rather than usual. Various types of shields (hoods) are used to prevent or limit drift.


Calibration of sprayer is necessary for delivering the spray fluid at the desired rate. Too much spray is not only waste the herbicide but also has disastrous consequence on standing crop. On the other hand too little spray fluid resulting in poor weed control. This may results in wastage of herbicide, time and money.

The volume sprayed per unit surface can be calculated by the following formula:

Volume per unit surface =Discharge per unit time/Speed x Swath

The discharge depends on the number of nozzles, the size of the nozzles and the pressure. It is important to have a uniform walking speed, which can be achieved by regular practice. To get a constant swath, the nozzle should always be maintained at the same height.

Uniform Coverage

Any part of crop or the soil must receive the same dose of chemical. If the distribution is not uniform there may be risk of phyto-toxicity to the crop or ineffectiveness against the weeds. Flood jet nozzle given an even spray distribution.

To achieve the uniform coverage (i) the nozzles should be oriented so that the spray jets from each nozzle will hit each other. (ii) The height of the boom must be adjusted so that there is a 5 cm clearly in between each spray jet.


At the end of spraying the sprayer should be filled with water overnight to prevent the herbicide from drying out and forming flakes that ultimately may block the filter or nozzles. Some of the water should be sprayed out to clean the nozzles and pipe. Next day, the water should be sprayed out completely if this sprayer use for applying the same herbicide. For application of other herbicides it should be thoroughly cleaned out first by ringing with water, then washing thoroughly with a detergent and ringing again with water till no trace of detergent left. When sprayer is used for spraying soil-based herbicide, a wash with kerosene oil followed by detergent is useful.

Other important tips

  • Always use recommended dose of herbicides.

  • Always follow instructions given for the use of particular herbicides.

  • Sprayer should be calibrated properly to find out matter required to cover a hectare of land.

  • Always use clean water.

  • Always use clean sprayer for herbicide application.

  • Prepare the solution just before spraying.

  • Avoid spraying on windy weather. Be careful in applying near susceptible crops. Avoid drift hazard.

  • Be selective in selecting herbicide for multiple cropping.

  • Do not store herbicide solution for too long after mixing with water.

  • Flat fan type nozzles are well suitable for herbicides application.

  • Store herbicides separately from seeds, fertilizer, insecticides and other pesticides.

  • Generally 20-30 lbs. air pressure per square inch is good.

  • Disposal of empty container by burring down at least 18 inch deep in an isolated area away from water supply points.

  • Do not drains or flush spray equipment near source of domestic irrigation water.

  • Use gloves for applying granular herbicides.

  • Broadcast granular herbicides uniformly in 4-5 cm standing water.

  • Spray of herbicides should be done after draining the water from the field.

  • Chlorinated and phosphate type insecticides should not be applied 15 days before or after the applications of propanil herbicides.

  • Do not spray 2, 4-D herbicides before tillering stages of the drop.

  • Always see for following levels for the safety purpose.



Source: Journal of the North Eastern Council, Shillong