A weed may be defined as any plant growing where it is not wanted. Weeds are the undesirable plants, which hamper the healthy growth of cultivated crops. A crop plant is any plant grown for its value to man at a given time.

Thus, both weeds and crops are plants. Both of them extract moisture and mineral nutrients from the same soil, take carbon dioxide (CO2) and light for photosynthesis from the same atmosphere and accommodate their build-up within the same space. As both live in the same biosphere, competition takes place for a particular factor when it falls short of the demand of both. As such the competition for that factor alter several other environmental factors and consequently the plant growth processes are affected.

A weed may be classified as annual, biennial and perennial. An annual weed is one that completes its life cycle in a single year. It includes by far the greatest number of weeds. Biennials produce roots, stems and leaves in the first year and flowers and seeds in the second year and thereafter. Perennial continues for three or more seasons. Under favourable conditions, it may live still longer.

The annual and the biennial weeds multiply through their seeds, whereas the perennials multiply through seeds and by their vegetative parts. This group of weeds make itself considerably difficult to eradicate. Perennial weeds species may be grouped as simple perennials, producing new plants each year from a root crown supported by a fleshy tap root and creeping perennials propagating asexually from horizontal roots, rhizomes and or stolous.

Weeds vary in their habit of growth. They may be erect prostate, creeping, climbing, delicate herbs, weedy shrubs, smooth succulent, bristly spiny, edible, inedible, toxic or medicinal. Some weeds are aggressive and thrive best in desolate places, along roadsides and in poorly cultivated lands. Infestation of weeds is not confined to land only, but weeds infest ponds, lakes, canals, rivers and seas as well.

Weeds are a great menace in orchards, vegetable gardens, lawns and in the fields of economic and industrial crops. The vegetative parts of weeds although add the much needed organic matter to the soil, check erosion in arable lands and some of them even have medicinal value, yet they have no place in the cultivated areas.

Weeds also choke the irrigation canals, create considerable difficulties in land preparation, add to labour and harvesting cost of production. Weeds harbour insects, pests and poison the livestock.

Presently, more than 250 weeds species have been found associated with different crops. Weeds commonly arise and infest croplands from seeds and sexual propagules already present in or on the soil when a crop is planted. Seeds and sexual propagules are present in weed species, are introduced into croplands by the intentional and unintentional actions of man and by natural means of plant dissemination. Dispersal of weed seeds can take place in more than one way.

It may be through the wild, domestic animals grazing in the forests, pastures lands and crop fields. Weed may spread through birds, gusty winds, rains, canal irrigation and also through under-composed farm yard manure. Seeds of weeds also spread through green or dry fodder, fruit/vegetable crates transported from one port of the country to another, through rail or road. Weed seeds may also be introduced to cropland by flood water. Wind is the principal natural means by which weeds are introduced to croplands.

Weeds are commonly introduced to irrigated cropland by seed-laden irrigation water moving in open channels. The embankments of such channels are often covered with needs and as the weeds mature their seeds drop into the flowing water and are deposited on croplands irrigated with this water.

Annual weed plants in cultivated lands grow from seeds buried in the soil, deposited there as a legacy from preceding plants. Certain perennial weeds are also perpeculated by seeds in croplands, whereas others arise freely from viable sexual reproductive parts buried in the soil. The capacity of an individual weed plant to produce seeds is prodigious. Thus, biennial wormwood can produce 1 million seeds per plant, redroot pigweed 117,000 seeds per plant, kochia 14,600 seeds per plant and foxtail barley 2400 per plant. Weed species with seeds that remain viable when buried for long periods of time become problem weeds in croplands.

Weeds cause economic losses in crop production. Agricultural experts in the country have experimentally assessed the losses in grain yields ranging from 18 to 25, 20 to 65, 20 to 45, 13 to 43, 10 to 35 and 25 to 55 per cents in wheat, rice, maize, cotton, sugarcane and pulses, respectively.

In spite of modern weedcontrol technology weeds continue to cause annual losses of about 15 per cent in the agricultural production in the world. The annual monetary losses caused by weeds in agricultural production is estimated at more than $18.2 billion with about $12 billion of this amount attributed to the production losses caused by weeds. Another $3.6 billion is spent on chemical weed control and $2.6 billion is spent on cultural, ecological and biological weed control methods. The average crop losses due to weeds in Pakistan are about 11.5 percent as compared to world losses of 9.5 percent. The estimated losses in potential crops yield due to weed alone is about Rs 300 million per annum. It is a common practice in many developed countries of the world to grow a single crop species on hundreds to thousands of hectares of land in a given crop production region. This practice subjects the crop to infestation by weed species that thrive under the cultural conditions provided for the crop.

The means by which weeds are controlled in croplands is determined largely by the growth and reproductive characteristic of the weeds and by the growth and cultural practices of the crop. Weed control practices may be grouped under five general headings: Preventive, cultural, mechanical (physical), biological and chemical. The weed populations in croplands are usually not adequately controlled by only one weed-control practice so the use of several such practices during a single crop season is common.

The use of two or more weed-control practices is referred to as integrated weed control. Weeds serve as hosts for insects and plant pathogens that subsequently move to crop plants in the area, causing adverse effects on the main crop. Practices directed towards simultaneous control of more than one kind of plant pest is called integrated pest management.

Preventive weed control is practiced also by planting weed-free crop seed, by use of manure and hay free of weed propagules, by cleaning of harvesting equipment before moving it from a woody to a weed-free area, by screening of irrigation water to remove weed propagules before the water moves into cropland. Cultural weed-control practices common to good land, crop and water management such as smoother crops, crop rotation, row spacing, seedling rate, planting date, fertilization, tillage operation, irrigation management, weed-free crop seed, field sanitation and uses of adapted crop varieties/cultivars.

Any crop that is highly competitive with seeds may be used as smoother crop. Alfalfa is a superior competitive crop. Other crops are barley, clovers, corn, cowpeas, millet, rye, canary grass, soybeans, sunflower, sorghum, etc. Crop rotation is the growing of different crops in recurring succession on the same land.

Weed control is one of the principal reasons for crop rotation. Mechanical weed control is a traditional and well established practice for controlling weeds.

Mechanical weed control includes such practices as hand pulling, hoeing, machine tillage burning and water management; biological weed control uses natural predators and parasites as the agents of weed control.

The most effective biotic agents for weed control have been the phytophageous (plant-eating) organisms such as insects, fish, and snails. Biological weed control reduces the competitive ability of the weed and dissipates its energy reserves, while preventing or curtailing weed reproduction.

Chemical weed control in croplands is achieved by approved chemicals that effectively control weeds without harm to crop plants or the environment. These phytotoxic chemicals are called herbicide. A herbicide is any chemical that kills plant or greatly inhibit growth of most green plants. Herbicides available today can control grass weeds in either grass or broad leaved crops.

Herbicides may be applied to the soil before the emergence of weeds or directly to the foliage of the emerged weeds. Properly used herbicides increase crop yields, improve quality and lower crop production costs.