Weed control is as old a practice as agriculture itself. It is the method of limiting weed infestations so that crops can be grown profitably. Therefore, the degree of weed control depends on its' costs/benefits and the resources available. In recent years, different chemicals have been increasingly used as an efficient, economical and time saving method of controlling weeds. These weed killing chemicals are called weedicides. People are resorting to chemical methods of weed control, because they experience a number of limitations in their traditional practices of weed control.
Before introducing weedicides for the control of weeds, the tradional methods, especially, mechanical methods have to be evaluated.
1. Poor efficiency of mechanical control
The implements used for mechanical weed control shear and tear the surface of the soil resulting in the uprooting of plants. But many of the weeds have deep and extensive roots that cannot be uprooted and majority of them can regenerate. The most common examples are grasses like kans grass, thatch grass and quack grass. To kill such weeds, chemical weedicides are necessary.
The implements used in mechanical weed control also scrap the soil and destroy it in the long run. In shallow rooted crops, especially, cereal crops, sometimes they injure roots of crop plants. Under such situations, chemical weed ,control comes in handy.
2. Adverse weather
The sowing time of some crops coincides with or just precedes periods of heavy rainfall which soaks the soil. Soaked soils don't permit efficient mechanical weed control when the crop requires it. Under such a situation, there is no other option except the application of herbicides if one wants to save one's crop from weeds.
3. Morphological similarity
Certain weeds have morphological characters quite similar to crop plants, such as, colour, shape and size of leaves and even the whole plant. For example, weeds like kanki, (Phalaris minor) and jungli jai (Avena fatua) and similar to weat. Sawan grass (thinochloa spp), wild rice or karga (Oryza sativa) and fatua resemble rice plant, and gokhru (Xanthium strumarium) resembles cotton plant. More than half of such weed population is commonly left out during traditional weed control practices as the weeds look similar to crops and cannot be distinguished.
4. Parasitic weeds
Parasitic weeds are plants that live on or in crop plants and extract nutrients from them. Striga in jowar and bajra, Orobanche in tobacco and brinjal and Loranthus and dodder in fruit trees are the most common and widely distributed parasitic weeds causing considerable amount of economic loss. Their roots penetrate into different parts of crop plants like root, stem and branches and suck the nutrients, seriously affecting the growth of crop plants. When there is a large scale infestation of such parasites, mechanical methods are not effective.
5. Perennial weeds
The weeds that remain alive for more than two seasons are called perennial weeds. They are more persistent and difficult to control than annuals due to their underground rhizomes, stolons and tubers. In their underground parts, these weeds store food material and can regenerate several new plants. Some of the most harmful perennial weeds are motha (Cyperus rotundus) and doob-grass (Cynodon dactylon), kans grass (Saccharum spontaneum), sawan grass (EChinochloa spp.) and hirankhuri (Convolvulus arvensis). They regenerate easily at soil depths beyond the reach of conventional hand tools and other mechanical implements.
6. Labour scarcity
Generally, weed flourishes during a particular period in every season. So, every farmer has to practice weed control measures during that time. Mechanical methods, especially hand weeding, are labour intensive though they are the most common methods. But scarcity of labour due to various reasons limits the mechanical weed control forcing the farmer to use weedicides
