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Weeds are integral part of agricultural production systems. They are efficient invaders and opportunistic exploiters of disturbed soils. Interference of weeds has an adverse impact on agricultural productivity, environment and human activities. They proliferate in diverse ecosystems such as wet lands, garden lands and aquatic systems. Changing climate contributes to shift in floristic composition and weed shift has become a common occurrence, adaptive mechanisms of weeds give them competitive advantage over crop plants.

India is basically an agrarian economy where 57.8% of rural households are engaged in agriculture (Situation Assessment Survey of Agricultural Household, 2021). 48.9 per cent of the Indian work force is employed in the agricultural sector, (ILO, 2016). The estimated Indian population for the year 2050 is 1.7 billion which will raise our food demand to ∼400 million tonnes (Rao et al., 2018). In this context, Indian agricultural production system will have to take up the challenge to feed l7.5% of the global population with only 2.4% of land and 4% of the available water resources at its disposal.

Weeds are diverse plant species of the ecosystem which can survive adverse condition and reproduce quickly. The global compendium of plants has listed 28000 species as weeds. According to Holm et al (1977) about 8000 weeds species have been reported from agricultural fields in 124 countries and of these, 250 species are considered to be problematic. It is also to be noted that weeds from 12 families constitute 68% of the world’s worst weeds. Within these 12 families, weeds from poaceae, asteraceae and cyperaceae constitute 44% of the world’s worst weeds.

Weeds have specific characters which helps them to adapt to any stressful situation and successfully, compete with the crop. They have survival strategies which help them to overcome vagaries of nature like extreme cold, heat, drought, biotic stresses and soil abnormalities. Their most important characteristic is the ability to succeed in any adverse situation even in disturbed soils. The age old war between man and weeds makes it pertinent to understand the characteristics of weeds which make them better competitors in the field.

Weeds have been a problem to agriculture ever sine man started cultivation. Weed species by and large invade cropped areas and account for large economic losses to the farmer. Weeds are economically more important than insects, fungi or other pest organisms (Savary et al. 2000, 2006). Globally, weeds cause the highest potential loss to crop production (34%), with animal pests and pathogens being less important (losses of18% and 16% respectively). The loss in production due to weeds has been estimated as 5% in developed countries. It is around 10% in developing countries and 25% in least developed countries. (Oerke, 2006)

Deliberate use of natural enemies to control invasive pest in an ecosystem is known as biocontrol. To control invasive weed species, selective pathogens and weed seed predators are employed to reduce the population of the weed to non competitive numbers rather than total eradication of the weed species (Appleby, 2005) These can be insects, herbivorous fish, other animals, disease causing organisms or even competitive plants. This method is comparatively safer to the ecosystem, is cheaper and has long lasting impact. Difficulty in f inding host specific bio-agent with minimum adverse effect on other crop and the environment is a challenge.

Herbicides are chemical substances that are capable of either killing or injuring plants and so used for elimination of unwanted plants. In Latin, the term ‘herba' means plant and ‘caedere’ means to kill. There are many reasons for the increased use of chemicals for weed control. Shortage and high cost of agricultural labour prevent economic control of weeds by manual and mechanical means. Benefits offered by herbicides include efficient weed control, higher crop yield, better soil, water and energy conservation and higher income.

A large number of herbicides are currently available in the market hence selection of suitable herbicide needs judicious understanding of the product and the requirement. When large number of chemical options are available it can make the process of selection a bit confusing. So it is important to know the crops in which a herbicide can be used, the weeds it will control, the appropriate rate, adjuvants included in the formulation. The different uses of the chemical and the different situations for which it will be useful. In addition to these it is important to understand the mode of action of the herbicide and also the mechanism of action of the chemical.

Effectiveness of spray mixtures can be increased by addition of adjuvants. Weed Science Society of America (WSSA1994) have defined adjuvant as a material in a herbicide formulation or added to the spray tank to modify herbicidal activity or application characteristics. Adjuvants enhance activity of a herbicide in different ways viz., wetting, spreading, deposit building, emulsifying or deflocculating the active ingredient. In many cases cost of adjuvant will be less than that of active ingredient. In such cases it helps to reduce the cost of the final product.

Herbicides have received wide acceptance from farmers mainly due to ease of application and effectiveness. After application, herbicide molecules should penetrate plant tissue and get translocated to the target site. Once they reach the target protein they bind to them and disturb metabolic processes in plants by disrupting biosynthetic pathways and damaging cell structures by generating cytotoxic molecules that ultimately kill the plants. In this, the first step is penetration of herbicide into the plant system.

In a cropping systems there may be simultaneous or sequential application of herbicides, insecticides, fungicides, fertilizers, PGR during a single cropping season. The different chemicals will undergo physical and chemical changes as they interact with plant, soil and other chemical substances already applied to the plant or soil. The interaction effect of such chemical substances will be visible only at the later end of the cropping season. The responses observed in the plants can be.

Major advantage of chemical herbicides over other methods of weed control is its ease of application, which saves the cost of labour for the farmer. Effectiveness of herbicide depends on how long it remains in active form in the plant /soil. The form in which any herbicide is available in the market is known as herbicide formulation. The technical active ingredient (ai.) of the herbicide is converted to a formulations so as to improve easiness of handling, retention on leaf surface, absorption and movement in the plant. Herbicides are available in different formulations.

When a herbicide is applied to the soil, it is subjected to various interactions with soil and surrounding environment which ultimately determines its persistence. Dissipation of these compounds from soil system is by three main processes viz., physical (volatilization, leaching and runoff), chemical (photochemical decomposition, adsorption by organic and inorganic colloids and chemical decomposition) and biological (microbial decomposition and plant uptake)  1. Physical Processes a) Volatilisation

A major portion of the earth’s surface is water. Apart from the oceans, major water bodies on the earth’s surface include rivers, streams, lakes, ponds and canals. Plants adapted to these ecosystems form the aquatic species. When they pose a problem to navigation, fisheries and agriculture and also cause health hazards, they are categorised as aquatic weeds. Problems due to aquatic weeds has been reported from different parts of the world. They are common in warm waters. In Florida, it is reported that water hyacinth (Eichhornia crassipes) covers nearly 40,000 hectares of water resources.

Parasitic weeds are a major constraint in tropical agriculture. These are plants that are adapted to live on other plants sharing their nutrients and water. They constitute 1% of flowering plants, approximately 3000 - 5000 species coming under 20 families. Parasitic weeds from five families viz. Orobanchaceae, Convolvulaceae, Santalaceae, Scrophulariaceae and Loranthaceae are considered troublesome weeds as they infest economically important crops in many parts of the world and are very difficult to control by normal weed management measures.

Rice is the primary food source for half of the Asian population. Nearly 2,000 million in this region obtain 60 to 70 per cent of their calories from rice and its products. Considering the importance of the crop, United Nations General Assembly declared 2004 as the “International Year of Rice” (IYR). In India, the crop covers one-fourth of the total cropped area. About half of the Indian population use rice as their staple food. Rice based cropping systems are not only important for food security but also for maintaing an ecological balance. Low land rice fields conserve water and prevent runoff .They preserve the sustainability of the ecosystem.

Indian tea has a global reach being the second largest tea producer in the world. Indian tea is one of the finest in the world. Darjeeling tea is the first product to get a geographical indicator tag in India. 83 % of the tea produced in India is from Assam and West Bengal and 17% from the south Indian states of Kerala, Karnataka and Tamilnadu. Weeds are a major problem in tea gardens. Weed flora Drymaria cordata, Bidens pilosa,, Cynodon dactylon, Panicum repens, Celosia argentea, Ageratum conyzoides, Centella asiatica, Borreria hispida, Oxalis latifolia, Mikania micrantha, Paspalam conjugatum,

Survival strategies of some plants help them to thrive in any adverse environmental conditions. We classify them as weeds or ruderals. Such weedy species have developed unique and effective physiological traits to withstand various stresses, and unique mechanisms to regulate growth under adverse environments. Climate change is a harsh reality observed in different parts of the world which brings new challenges in crop production and weed management. Introduction of biotechnological tools in weed science and management has given new dimensions to weed research.

A  Apoplastic 113, 114, 115 Adjuvant 103, 104, 118, 158, 177, 237, 254, 257 Aquatic 5, 12, 21, 44, 49, 60, 61, 72, 76. 78, 138, 139, 140, 143, 144, 145, 146, 150, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 164, 165, 225, 226, 250, 263  Agrochemicals 117 Antagonism 118, 121 Allelopathy 34, 37, 50, 207, 209  After ripening 19 Amides 75, 78, 136 ALS inhibitors 208,