Herbicide chemical substance that has the ability to selectively kill herbs that grow as weeds in crop fields, gardens and lawns. In the 1950s, studies on plant growth and development led to the discovery of certain chemicals that exhibited selective toxicity on growth and development of different plants. Some common herbicides against broad-leafed weeds are 2, 4, Fosamine, Dicamba and Picloram. There are herbicides that specifically kill narrow-leafed grasses such as Dapalon. Most herbicides are sprayed as liquid but some are applied in granular form mixed with chemical fertilisers.

In Bangladesh, weeding out of unwanted herbs from crop fields has long been done by traditional methods using plough and hoe. The method is labour-intensive, but because of availability of cheap labour, weeding of many crop fields such as jute, winter crops such as tobacco, tomato, brinjal, pepper, mustard, lentils etc have presented no particular problems. Weeding of crop fields, lawns and gardens is still done manually in Bangladesh and use of herbicides is thus limited. The major application known at present is in tea gardens.

During 1990-91 Bangladesh imported about 24,000 kg of herbicides and weed killers. No herbicides are used in rice fields in Bangladesh because with little run down of water due to flat terrain, the problem of toxicity build-up in small agricultural holdings may be considerable. Control of weeds in irrigated rice fields is instead achieved by keeping the levels of water about 50% higher than what would normally be required by the plants. [Zia Uddin Ahmed]

See also weed.