User:Maurice Nsonga/sandbox

Specific outcomes
(a) Define a weed

(b) Describe the Negative/harmful effects and positive/ beneficial effects of weeds

(c) Demonstrate the method of weed control that are applicable to crop field

Weed, general term for any plant growing where it is not wanted. Ever since humans first attempted the cultivation of plants, they have had to fight the invasion by weeds into areas chosen for crops. Some unwanted plants later were found to have virtues not originally suspected and so were removed from the category of weeds and taken under cultivation. Other cultivated plants, when transplanted to new climates, escaped cultivation and became weeds or invasive species. The category of weeds thus is ever changing, and the term is a relative one.

Negative/harmful effects of weed:


 * Weeds compete with the crop plant for light, nutrients, water, space and other growth requirements.
 * Increase the cost of production by increasing the cost of labour.
 * Reduce the quality of crop products.
 * Exude inhibitors from the roots reduce the growth of the crop plants (allelopathy).
 * Serve as alternate hosts for insects and pathogens.
 * Reduce the efficiency of farm implements.
 * Harbour birds and rodents.
 * Reduce the sales value of the land.
 * Reduce fertility status of the soil.
 * Cause root and another crop damage may occur from weed control operation.
 * Cause enormous loss of water by growing in the irrigation channel.
 * Cause serious illness or death of farm animals by their poisonous constituents.
 * Interfere with navigation.
 * Injure the cattle body by their spines.
 * Increase the cost of maintenance of sports and recreation, highways, railways and other public utilities.
 * Cause annoyance to human being by plugging in cloths.
 * Aquatic weeds contaminate water bodies and on decomposition spread the odours and pollute the atmosphere.

Positive/Beneficial effects of weeds:


 * Add organic matter to the soil when incorporate into the soil.
 * Increase soil fertility when incorporated.
 * Induce soil formation by rapid weathering.
 * Improve soil structure spreading of weed roots change the soil structure and improve the physical condition of soil due to proper percolation water logging will be prevented.
 * Serve as food (Bothua, Shaknotey, Amrul etc.), feed (Durba, Mutha, Chapra etc.) and medicine (Thankuni, Durba etc).
 * Serve as raw materials for public utilities as fuel, fencing and roofing materials.
 * Help in controlling soil erosion.
 * Serve as water purifier. Eg. Water hyacinth.
 * Serve as a source of genetic materials.
 * Some weeds can be used as indicators of air pollution. Eg. Wild mustard is an extremely sensitive indicator of NH3, NO2 present in air.
 * Help in soil reclamation. Eg. Durba and Shialkata, when incorporate into the soil may reclaim alkaline soil.
 * Valued as religious and ritual purposes.
 * Valued as ornamental plant.
 * Act as a host for predatory insect.
 * Leguminous weed can be used as green manure before they set seeds.
 * Some weeds fix atmospheric nitrogen in paddy soil. Eg. BGA

Weeds interfere with a variety of human activities, and many methods have been developed to suppress or eliminate them. These methods vary with the nature of the weed itself, the means at hand for disposal, and the relation of the method to the environment. Usually for financial and ecological reasons, methods used on a golf course or a public park cannot be applied on rangeland or in the forest. Herbicide chemicals sprayed on a roadside to eliminate unsightly weeds that constitute a fire or traffic hazard are not proper for use on cropland. Mulching, which is used to suppress weeds in a home garden, is not feasible on large farms. Weed control, in any event, has become a highly specialized activity. Universities and agricultural colleges teach courses in weed control, and industry provides the necessary technology. In agriculture, weed control is essential for maintaining high levels of crop production. The many reasons for controlling weeds become more complex with the increasing development of technology. Plants become weeds as a function of time and place. Tall weeds on roadsides presumably were not problematic prior to the invention of the automobile. However, with cars and increasing numbers of drivers on roads, tall weeds became dangerous, potentially obscuring drivers’ visibility, particularly at intersections. Sharp-edged grasses are nominal nuisances in a cow pasture; when the area is converted to a golf course or a public park, they become an actual nuisance. Poison oak (Toxicodendron diversilobum) is rather a pleasant shrub on a sunny hillside in the open country; in a camp ground it is a definite health hazard. Such examples could be given ad infinitum to cover every aspect of agriculture, forestry, highway, waterway and public land management, arboretum, park and golf-course care, and home landscape maintenance. Weeds compete with crop plants for water, light, and nutrients. Weeds of rangelands and pastures may be unpalatable to animals, or even poisonous; they may cause injuries, as with lodging of foxtails (Alopecurus species) in horses’ mouths; they may lower values of animal products, as in the cases of cockleburs (Xanthium species) in wool; they may add to the burden of animal care, as when horses graze in sticky tarweeds (Madia species). Many weeds are hosts of plant disease organisms. Examples are prickly lettuce (Lactuca scariola) and sow thistle (Sonchus species) that serve as hosts for downy mildew; wild mustards (Brassica species) that host clubroot of cabbage; and saltbrush (Atriplex species) and Russian thistle, in which curly top virus overwinters, to be carried to sugar beets by leafhoppers. Many weeds are hosts of insect pests, and a number are invasive species. Modern weed control can be classified as mechanical, chemical, or biological.

Mechanical control

Mechanical weed control began when humans first pulled weeds from their cereal crops and attempted to grow single plant species, free from all plant competition. That was the start of monoculture, a method that since has come to dominate agriculture. Contrary to the principles of ecology, farmers throughout the world grow the major food, fibre, and forage crops in a monoculture because experience has shown that the highly improved modern crop species give their highest yield under this system. From hand pulling, humans devised simple tools such as the spud, the knife, and the hoe to eliminate weeds. For thousands of years, from the Egyptian culture to the Renaissance, those simple methods were used. The first efforts to turn away from simple hand methods and mechanize the arduous task of weed control began in 17th-century England. Since then there has been continuous improvement of agricultural tools used to destroy weeds and of cultural methods employed to minimize weed growth. The principal virtue of cultivation of row crops is the control of weeds. Any method of weed control that minimizes tillage tends to conserve soil structure and maintain fertility. In addition to tillage, other mechanical methods of weed control involve burning, grazing, and the use of ducks or geese in certain crops (in cotton and mint especially). All of those methods have drawbacks: there is the arduous, painful nature of hand weeding; the repetitious and often harmful nature of clean tillage with machinery; the slow, fuel-consuming nature of burning; and the costly requirement of livestock or fowl for the biological grazing methods. Tillage, still the most widely used method of row-crop weed control, has been greatly improved by development of precision seeding and close preadjustment of tiller tools, allowing the passage of weed knives within an inch or less of the young crop plants. Despite these improvements it is known that weed knives injure crop roots, especially late in the tillage season. Additionally, tillage tools can spread perennial weeds rapidly, bringing about rapid infestation of whole fields. Such methods as crop rotation, use of smother crops, use of weed free seed, mulching and covering, and cleaning of machinery to prevent spread of weed seeds are also classified as mechanical.

Chemical control

Chemical weed control (see herbicide) has been used for a very long time: sea salt, industrial by-products, and oils were first employed. Selective control of broad-leaved weeds in fields of cereal crops was discovered in France in the late 1800s, and this practice soon spread throughout Europe. Sulfates and nitrates of copper and iron were used; sulfuric acid proved even more effective. Application was by spraying. Soon sodium arsenite became popular both as a spray and as a soil sterilant. On thousands of miles of railroad right-of-way, and in sugarcane and rubber plantations in the tropics, the hazardous material was used in tremendous quantities, often resulting in the poisoning of animals and occasionally humans. Diesel oil, as a general herbicide, and sodium dinitrocresylate (Sinox), as a selective plant killer, were introduced during the first three decades of the 20th century.

Biological control

Efforts to eradicate weeds through biological control are a fairly recent development. An early report from 1902 described the importation of insects from Mexico to Hawaii in an effort to control Lantana, an imported shrubby climbing weed that had spread over thousands of acres of pastureland, rendering them useless for grazing. Prickly pear cacti have been very effectively controlled in Australia; some 24 million hectares (60 million acres) have been converted from cactus thicket to plowland and pasture by the cactus moth (Cactoblastis cactorum), which was introduced from Argentina in 1925. By 1933 the major cactus areas were under control. Another successful use of biological weed control was in the western United States, where Saint-John’s-wort, or Klamath weed (Hypericum perforatum), was subjected to depredation by three insect species, beginning in California in 1945. The release of two insects of the genus Chrysolina and one of the genus Agrilus continued for a number of years, and the effort was carried to Oregon, Washington, and Idaho by 1950. The insects spread rapidly after introduction and became established, ultimately reclaiming some 1 million hectares (2.5 million acres) of rangeland from the invasive plant. A number of vertebrate animals have been used to control certain specific weeds. Sheep and goats have been employed to control brushy plants on rangelands in many countries. Their effectiveness is evident in parts of the Middle East and Africa where dry range and desert lands have been almost completely denuded by grazing goats. In these cases, however, the destructiveness of the goats far outweighs their usefulness in plant pest control, which indicates the need for rational management in all efforts at weed control. Additionally, biological weed control tends to be only periodically effective. Weed populations may be reduced initially to very low levels by the control insects; however, they recover when the insects die off for lack of food. The insect predators may then flourish again in reciprocal cycles, but complete control may never be achieved. Finally, the introduction of alien organisms as biological control is hazardous in that these same organisms may become pests in the new habitat. Kikuyu grass, which was introduced into California to prevent soil erosion on hillsides and roadways, soon spread into orchards, turf, and crop areas, where it became a serious weed.

CONCLUSION

Mechanical methods cause soil disturbance and possible erosion while chemical herbicides lead to pollution of the environment and the aftermath

Some weed species have developed resistance to some chemical herbicides and biological control readily comes as a viable alternative

Classical method of biological weed control has been the most popular and widely adopted and practiced; it involves the introduction and release of agents in form of exotic insects, mites or pathogens to give permanent control

Inundative method of biological weed control involves the releases of predators, use of bioherbicides and other integrated pest management which usually are not as widely used as the classical method.

Biological weed control is presently widely adopted in the USA, Canada, Australia, South Africa and New Zealand.

The biological approach to weed control holds great prospects for sustainable, environmentally friendly and economically viable control of exotic weeds and should be explored further through research, development and legislation.