1) The use of herbicides as pre-plant and pre-emergence treatment can control weeds, before their emergence from the soil so that crop can germinate and grow in weed free environment or with minimum competition during their tender and seedling stage. This is not possible with other methods of weed control.
2) In broadcast sown and narrow spaced crops hebicides prove very effective in reaching every weed. Mechanical methods are not so effective in such crops.
3) In wide spaced crops mechanical methods are effective for controlling weeds in rows but it is leaves the intra-row weeds. Herbicides reach to all places and control the weeds i.e inter row and intra-row weeds.
4) Weeds with similar morphological characters like crop are escaped from mechanical method. But now herbicides are available which can kill such weeds without damaging the crop.
5) Herbicides withhold the weeds for considerable period after their application. In mechanical methods weeds tend to grow back soon.
6) Deep rooted, vegetatively propagated weeds can be controlled by using translocated herbicides. The mechanical methods like weeding or hoeing are not so effective for their control. Sometimes the suitable combination of mechanical methods ( deep ploughing or digging) and chemical methods is more effective for controlling such weeds.
Limitation of Herbicides (Chemical Method) :
1) The use of herbicides requires technical know how regarding choice of particular herbicide, time of application safe dose method of application etc in the particular crop.
2) Over and under-dose of herbicides can make a market difference between the success or failure of weed control.
3) Certain herbicides because of their long residual effect limit the choice of next crop in the crop rotation.
4) Herbicides drifits may harm the neighbouring crops. E.g Ester form of 2,4,-D may harm the neighbouring crop of cotton, soybean, okra, etc.
5) Herbicides use may cause environment pollution.
(Source – http://www.agriinfo.in/default.aspx?page=topic&superid=1&topicid=2198)