L Nabachandra Singh & Punabati H
Among the agricultural pests causing crop loss, weeds rank first with a loss value of Rs. 2000 crores annually in India. Annual loss of nutrients due to weed amounts to a staggering 4.5 million tons. Although the extent of crop loss due to weed competition might vary from 20-50 percent depending on various conditions of rice culture, the damages caused by weeds are not so visible to create a sensation or alarm as in the case of an insect pest or disease incidence. Thus, weed control is often done as an optional practice which is generally taken up depending on the availability of labour or resources. Several valuable information regarding the control of weeds in rice has reported from many workers, but most of the workers seem to emerge on chemical weed control in the recent past. In India consumption of herbicide is around 12% of the total agrochemical consumption as against 55% in USA and 43% of the global pesticide use. In Manipur, the demand for herbicide is increasing very rapidly to the tune of 40 tons annually for rice crop alone.
The miraculous popularity of the herbicide in agriculture is due to the reason that the method is time saving, less laborious and economical. However, the efficacy of the method depends upon right choice of herbicide for the flora of weeds found in the locality/crop, right dose and time of application. Therefore, an adequate knowledge of various types of weeds found in the rice fields of Manipur is essential for effective control.
Even though chemical weed control is preferable from economical point of view, reports of environmental unsafeness are coming up due to wrong selection of the chemical, as well as of overdose application. Therefore, the user must have the following technical knowhow while using herbicides.
Choice of herbicides:
Depending upon the dominance of the weed type in a particular field, the type of the herbicides to be used will differ. Those rice fields, which are dominated by broad leaved weeds over other types of weeds (<70% population), herbicides like 2,4-D, propanil and MCPA can be used. In upland areas, narrow leaved weeds are usually found dominating other weeds. In such cases either one of butachlor, oxiflurofen, trifluralin, pretilachlor, lanilofos or thiobencarb can be used. However, when both narrow and broad-leaved weeds are grown in more or less equal proportion in the field both the herbicides have to be used. Certain weedicides like thiobencarb and bentazone are found effective over a broad range of weeds.
New generation weedicides:
Very recently so many new generation weedicides are manufactured which are effective to control a wide range of wees (both narrow and broad leaved) at very low doses and its residual effect on soil as well as on environment are also minimized. Therefore, they are becoming popular rather than the traditional weedicides as indicated above. Some of them are ethoxysulfuron 15%, oxadiargyl 80% WP, fenoxaprop-p-ethyle 9.3% w/w EC, cyhalofop-buty 10% EC, bispyribac sodium 10% SC, etc.
Dose of the herbicides:
For effective control, the user must be conscious about the dose of application. Lower dose may not be able to kill the weeds to our satisfaction. In some herbicides like 2,4-D lower dose may cause malformation of rice plant also. Usually most of the farmers use higher dose of herbicides, which are undesirable because it is unsafe to the environment as well as uneconomic.
The recommended doses for some important common herbicides of rice are given below-
Calculation of herbicide requirement:
A particular herbicide may be available in different trade names with varying concentration of active ingredient (a.i) for which, herbicides are usually recommended. The amount of commercial product (C.P) required may be calculated as follows:
Amount of C.P (kg)
Recommended dose (kg.a.i/ha x Area(ha) x100
Concentration of the product (%)
Table 2. Recommended dose and time of application of rice herbicides
*Good for direct seeded rice
Example: Calculate the amount of Delete 50% EC (trade name of Pretilachlor) required for 1 Sangam (1/4ha) area at the recommended dose of 1 kg a.i/ha.
Amount of Delete 50 EC required
1 x 0.25 x 100
= 0.5 litres
Time of application:
The effectiveness of a herbicide may be reduced drastically when they are applied at wrong time. Depending upon the mode of action, some herbicides like butachlor, pretilachlor, oxyfluorfen have to be applied before the weed seeds are germinated. That is why, they should be applied just before rice sowing or transplanting or 3-5 days after seed sowing or transplanting. They are known as pre-emergence weedicides.
While post-emergence weedicides like 2,4-D, propanil, bentazone etc. should be applied after the emergence of the weeds but before the weed plants are not too old, preferably 3 o 4 weeks after seed sowing or transplanting.
While some herbicides are effective when they are applied as pre emergence or early post emergence i.e. when the weeds have 2 to 4 leaved stages.
Mode of application:
For amorphous or granular herbicides, if the quantity is not sufficient to cover the field area uniformly, inert materials like sand, ash, saw dust, dry soil powder, paddy hull or even urea can be mixed just before application of the herbicides while liquid herbicides may be either sprayed using sprayer or broadcast after absorbing the herbicides in sand. For spraying with hand compression sprayer, about 400 liters of water may be needed for 1 ha.
Precaution to be taken up while using herbicides:
Ø Buy only the required quantity of herbicides which are not expired.
Ø Do not purchase herbicides without proper labels or torn bags or containers.
Ø Always keep herbicides away from food, feed and kitchen items and out of reach of children.
Ø Unused containers should be buried in barren field to a sufficient depth.
Ø Spray the herbicides keeping the pump nozzles as low as to reach the weeds.
Ø Avoid spraying during strong sunshine and windy days.
Ø Do not spray just before or after rains.
Ø Herbicides should preferably spray during morning and evening hours.
Ø Avoid smoking, drinking or eating while spraying.
Ø Use hand glove and mouth gag during spraying.
Ø After application of the herbicides, wash the sprayers thoroughly preferably with a detergent.
For further details contact:- Public Relation & Media Management Cell, CAU, Imphal. Email [email protected]