Weeds pose a serious problem in crop production. Because of ignorance and financial constraints, many farmers do not remove them from their fields which adversely affect their crop yield and quality of the harvest. This consequently increases produc tion cost and results in economic losses.

The weed science is the understanding of the problem and saving crops from its adverse effects. And this is only possible through development and acquisition of techniques and their practices. Weeds are companions of crop plants. Weeds and crop plants: Both extract moisture and mineral nutrient from the same soil, take 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, they compete for nutrients as a result the growth of crop is affected. Weeds are a universal problem with degrees of their infestation varying from crop to crop.

Classification: A weed may be classified as annual, biennial or perennial. An annual weed is one that completes its lifecycle in a year. It includes by far the largest number of weeds. The biennial develops roots, stems and leaves in the first year and flowers and seeds in the second year and thus completes its lifecycle in two years. The perennial nor mally continues for three or more years. Under favorable condi tions, it may live still longer. The annual and the biennial weeds multiply through their seeds, whereas the perennial multiply through seeds as well as by their vegetative parts. This group of weeds makes itself considerably difficult to eradicate.

Perennial weed 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 stolons and bulbs.

Weeds vary in their habits of growth. They may be erect, prostrate, creeping, climb ing, delicate herbs, weedy shrubs, and 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. They infest ponds, lakes, canals, rivers and seas. Aquatic weeds submerged under the surface, immersed or floating freely on the surface of water are a common sight in lakes and ponds in rural areas.

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 farms and fields of crops. Weeds choke irrigation canals, create difficulties in land preparation, and add to labour and harvesting cost of production. They harbor insects, pests, poison the livestock and are thus a great menace in modern farm management prac tices.

Weeds into crop lands: There are more than 250 weed species associated with different crops. They commonly grow in infested croplands from seeds and asexual propagules after a crop is planted. They may grow in croplands by intentional or unintentional ac tions 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 wild or domestic animals grazing in forests or by birds, winds, rains, canal irrigation and also through farm yard manure. Seeds of weeds also spread through green or dry fod der.

Losses: Weeds cause economic losses in crop production. Agricultural experts in Pakistan have experimentally as sessed the losses in yields ranging from 17 to 25, 20 to 63, 20 to 45, 13 to 41, 10 to 35 and 25 to 55 per cent in wheat, rice, maize, cotton, sugarcane and pulses, respectively. In spite of modern weed-control technology, weeds continue to cause annual losses of about 10 per cent in agricultural produc tion in the world. The annual monetary loss caused by weeds in agricultural production is estimated at more than $18.2 billion, with about $12 billion attributed to produc tion losses; $3.6 billion to chemical control and $2.6 billion to cultural, ecological and biological weed con trol methods.

Control practices: It is a common practice in many developed countries of the world to grow a single crop species on hundreds to thousands of hectares 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 adequate ly controlled by only one weed-con trol 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 or integrated weed manage ment. Weeds serve as hosts for insects and plant pathogens that subsequently move to crop plants in the area, causing adverse effects on the crop. Practices directed toward simultaneous control of more than one land of plant pest are called integrated pest-management.

Preventive weed control attempts to block the introduction, establishment or spread of specific weed species in areas not infested by these species. Preventive weed control is enforced through laws at national, state and local levels. It is practiced also by planting weed-free crop seed, by use of manure and hay free of weed propagules, by cleaning of harvesting equip ment, by screening of irrigation water, and by proper education of farmers. In essence, people are the key to preventive weed control.

Cultural weed-control prac tices include all practices common to good land, crop and water management, such as smother crops, crop rotation, row spacing, seeding rate, planting date, fer tilisation, tillage, irrigation management, weed-free crop seed, field sanitation and use of adapted crop varieties/cultivars. Any crop that is highly competitive with seeds may be used as a smother crop. Alfalfa is a superior competi tive crop. Other examples include barley, buckwheat, clovers, cow-peas, corn, millet, rye, reed canary grass, soybeans, and sunflower and forage sorghum. 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 con trolling weeds. It includes hand pulling, hoeing, machine til lage, smothering with nonliving material, burning and water management. Biological weed control is through predators and parasites as the agents of weed control. The most effective biotic agents used for weed control have been phytophagous (plant-eating) or ganisms such as insects, fish and snails. Biological weed control reduces the competitive ability of the weed and dissipates its energy reserves, while prevent ing or curtailing weed reproduc tion.

Chemical weed control in croplands is achieved by approved chemicals that effectively control weeds without significant harm to crop plants or the environment. .

Herbicides may be applied to soil before emergence of the weeds or directly to the foliage of emerged weeds. For selective weed control, herbicides may be applied to the soil before planting the crop or after planting but before emergence of the crop. A single herbicide is not the panacea to the control of all weed species in a given crop or in all croplands. Herbicide development: At present about 27 types of herbicides are being used for con trolling weeds’ growth in six major crops such as wheat, cotton, paddy, sugarcane, maize and millet. The application of chemicals to control weed infestation in cul tivated crops has been found ad vantageous and is gaining momentum.

Highly developed in the western world, herbicidal weed control technology is also becoming popular in developing countries. When fully established as an es sential part of crops management, weed control will enhance the efficiency of agriculture and free millions of people from the drudge of hand methods, enabling them to proceed with modernised way of life.