Preface Weeds pose a major threat to the productivity of agricultural and horticultural crops. Weeds are the menace in agriculture  and horticulture and cause in the lowering of yield of crops they infested. Keeping this facts in mind, Weed science is published to provide basic information of the weeds along with their control measures to Students, Teachers and persons engaged in agricultural activities. I do not pretend to claim credit for any original contribution on the subject. Rather; it should be viewed as a concise collection of various topics from different, books, bulletins, scientific paper, wave sight etc, the names of which are mentioned in the text itself or in the selected references. I am thankful to my school mates Dr. Basudeb De, Former Associate Professor in Chemistry, Sambhunath College, Lubpur, Birbhum, West Bengal for providing me books at his disposal. I am thankful to Dr. R.K. Ghosh, Professor of Agronomy, Dr. Debesh Pal, Associate Professor of Agronomy, Dr. A. Zaman, Professor of Agronomy, Dr. Kajal Sengupta, Professor of Agronomy and Dr. Koushik Brahmachari, Professor of Agronomy, Bidhan Chandra Krishi Viswa Vidyalaya, Nadia, West Bengal, Dr. A.K. Siha Roy, Professor of Agronomy, Dr. Biplab Mitra, Agronomist and Dr. Dhiman Mukherjee, Assistant Professor of Agronomy, Uttar Banga Krishi Viswavidyalaya, Coochbehar, West Bengal for their help and cooperation.

Weeds are the plants which grow where they are not wanted or simply plants growing out of place. A plant may be desirable in one situation and a weed in another situation. Those plants that interfere with human activity in crop and non-crop areas are considered weeds. Weeds compete with crops for soil nutrients, water and light; they host insects and plant pathogens harmful to crop plants, and their root exudates and/or leaf leachates may be toxic to crop plants. Weeds also hinder crop harvest and increase the costs of such operations. In addition, at harvest, weed seeds often contaminate the crop produce. Thus, the presence of weeds in crop areas reduces the efficiency of inputs such as fertilizer and irrigation water, enhances the density of other pest organisms, and finally severely reduces crop yield and quality (Labrada and Parker 1999).  Weeds are serious menace in crop production.

1. Classification of weeds based on life cycle (Ontogony) (i) Annuals: Weeds that  complete their life cycle within a year or season are called annual weeds. They are associated with annual crops and propagated mainly through seeds. Annual weeds are subdivided according to the season of prevalence as follows: (a) Kharif season  annuals or Kharif  weeds: The weeds that grow during the summer and complete their life cycle during Kharif or rainy season are called Kharif season annuals or Kharif weeds. The kharif weeds complete their life cycle during the warm wet season (i.e. from the month of June to September) and attain maturity with the onset of winter and finally dies.

Ecology is the inter-relationship between organisms and their environment. Weed ecology is study of inter-relationship between weed and their environmneent. Simply, Ecology of weeds means the inter-relationship of weeds with climate, soil, crops and other biotic factors. Terrestrial weed species persist in the soil by virtue of dormant structures, typically seeds or vegetative perennating organs such as rhizomes, tubers and taproots. In dense infestations, the banks of seeds or underground meristems from which new plants may be recruited into adult populations can be exceptionally large. Rao (1968) estimated that in Cyperus rotundus L. tuber populations of 10 million per hectare were possible, whilst Soerjani (1970) calculated that Imperata cylindrica may annually produce six tones of rhizomes per hectare. Typically seed banks of annual weeds in arable soils contain from 1000-10,000 seeds per m2whilst in grassland the upper limit to this range may extend at least to 1 million per m2 (Mortimer 1994).

Weeds have some characteristics as follows:     1.    Weeds are hardy and can grow under adverse edaphic (Soil), climatic and biotic condition.       Example: Lantana camera,Eupatorium adoratum etc.     2.    Weeds in general prolific with abundant seed production potentialities.         Example: Amaranthus viridis, Chenopodium album etc.

No plant in the earth is completely worthless. Weeds have also some beneficial effects although they do much harm in different ways. The beneficial effects of weeds are as follows:     1.    Most of weeds absorbs nutrient from the deeper layer of soil. When such weeds are incorporated in the soil, they add organic matter content of the soils. As a result, the fertility of plough layer from where plants absorb most of their nutrients is enriched.     2.    Weeds help in controlling erosion by the soil binding effects of their roots and by forming a living mulch, which resist beating action of rain drops and sweeping action of wind.

Dispersal means the spreading of weeds from one place to another place. Dispersable of weeds generally takes place through seeds and vegetative parts. Dispersal of mature seeds and live vegetative parts of weeds is nature’s way of providing non-competitive sites to new individuals.     (a)    Dispersal of weeds through seeds: Seeds are protector and propagator of their kind. Weeds seeds remain viable for a long time and their dispersable mainly takes place through natural and human agencies.

There are some weeds which prefer to in crop field and some of them have a distinct association with some crops.

Weeds interfere with man’s utilization of land and water in various ways. The extent of losses depends on the intensity, infestation, time of occurance and type of weeds. 1. Competition between weeds and crops for growth factors Weeds compete with crop plants for  other factors in the micro-enviroment. Weeds are capable of absorbing more amount of nutrient from soil than the crop plants. In dry lands, weeds exhaust the soil moisture and put the crop under water stress earlier and thus reduce the yield. Weed can compete with productive crop or pasture land or convert the productive land into unusable scrub. Weeds are also often poisonous, produce burrs, thorn or other damaging body parts or otherwise interfere with the use and managemet of desirable plants by contaminating  harvest or excluding livestock.

Plant competition is a powerful force for the elimination of some weaker plants. Plants require nutrients, water, light, Oxygen and Carbondioxide for their growth and development. Weeds compete with the crop for growth factors and the competition is maximum at the early growth stages of the crop. But the competiting ability of the weeds becomes practically nil as soon as the crops attain maturity. Weeds need more nutrients and water than the crop plants. The weeds in general grow faster and shade the crop plant if they are not controlled at proper time and afterwards interfere in the growth of crop plant. Weeds have allelopathic effect on crop growth. Some weeds excrete certain chemicals into the soil which inhibit the germination and growth of other plants in their vicinity and the phenomenon is called allelopathy. Allelopathy is defined as the direct or indirect harmful effects of one plant on another plant through the production of chemical compounds that escape in the environment.

Weed control methods may be classified into two groups as follows:     1.    Preventive method     2.    Curative or remedial measures. Curative measures are sub-divided into eradication and control. (a) Preventive Methods Prevention is the measures taken to prevent the introduction or spread  of the weeds to new areas and to check the infestation prior to the germination of weed seeds. To achieve this, a wise step to be followed the principle- “Prevention is better than cure”.

A single active ingredient often is sold in several different kinds of formulations as follows: (a) Dry Formulations 1. Granule (G) Granule is, a ready-to-use, dry mixture of a small amount of active ingredient with inert carriers. Most are made by applying a liquid formulation of the active ingredient to coarse particles (granules) of some porous material such as clay, sand or plant material. Granule particles are much larger than dust particles; will pass through a 4-mesh sieve but not through an 80-mesh sieve (the number of wires per inch). The herbicide is absorbed into the granule, or coats the outside of it, or both. Inert ingredients may be added to make the formulation handle well. The amount of active ingredient usually ranges from 1 to 15 percent.

Aquatic weeds are those unwanted plants which grow in water and complete at least a part of their life cycle in water. Aquatic weed problem is a global one but more pronounced in tropical and subtropical zones, where warm weather supports profuse growth of aquatic plants. In India, large irrigation projects have been reported to be adversely affected by profusely growing aquatic weeds, that block canals reducing water flow by as much as four-fifths. (Dr. K.T. Chandy,  2010). 1. Harmful Effects of Aquatic Weeds The important harmful effects of aquatic weeds are as follows       (i)    The aquatic weeds mainly Submerged, immersed, and marginal weeds in and along irrigation canals, ditches, and drainage channels impede water flow, increase evaporation, cause damage to canals and structures and clogs gates, siphons, valves, sprinkler heads, bridge piers, pumps, etc.

(1) Striga   Striga, commonly known as witchweed or witches weed, is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. Witchweeds are characterized by bright-green stems and leaves and small, brightly colored and attractive flowers, Striga has been given the common name of “witchweed” because of attaching to the roots and robbing the host of water and nutrients. The genus Striga (Family: Scrophulariaceae) includes a large number of tropical parasitic or hemi-parasitic plants as follows

The mode of action is the overall manner in which a herbicide affects a plant at the tissue or cellular level. The mode of action is the way in which the herbicide controls susceptible plants. It usually describes the biological process or enzyme in the plant that the herbicide interrupts, affecting normal plant growth and development. Herbicide may act on the life process of plant and dislodge the normal functioning of plants resulting in the death of plants. Herbicides  do damage of plants by inhibiting the biological process of plant as follows:

Herbicides degrade over time. The persists of herbicides is dependent on several factors including light, temperature, and substrate moisture. Several processes are responsible for herbicide degradation and include the following:     1.    Photodegradation: The decomposition of herbicides caused by duration or intensity of sunlight is called as Photodegradation. Photodegradation occurs when ultraviolet (UV) light breaks chemical bonds of the herbicide active ingredient. Secondary molecules resulting from the cleavage of the parent molecule are generally less effective in providing weed control. Herbicides such as Monuron, Trifluralin, PCP, Chloramben etc are prone to photodegradation. So these herbicides should be applied in evening or incorporated in to the soil just after application in order of avoiding photodegradation.

Herbicides are applied to the soil in hopes of obtaining season-long weed control. It is desirable for the chemicals to control weeds during the season of application, but they should not remain long enough to affect subsequent crop growth.     1.    Selection of herbicides: The type of herbicide and the method of application can influence the extent of the residual carryover. Herbicides differ in their physical and chemical properties such as water solubility, volatility and susceptibility to breakdown by sunlight, and microbes. When applied at recommended rates, most herbicides breakdown within a few days or weeks after application and impose no restrictions on cropping options in the next year. Some herbicides however do not degrade quickly and can persist in the soil for weeks, months or years following application. The use of residual herbicides can be beneficial as the residues prevent growth of sensitive weed species throughout the season. These residues however can restrict the crops that can be grown in rotation. Some herbicides can remain active in the soil for weeks, months or years. This can be an advantage as it ensures good long term weed control. However, if the herbicide stays in the soil longer than intended, it may damage sensitive crop or pasture species sown in subsequent years. For example, chlorsulfuron is used in wheat (Triticum sp) and barley (Hordeum vulgore), but can remain active in the soil for several years and damage legumes and oilseeds.

1. Herbicide –herbicide interaction Herbicides cause some effects as follow: (i) Synergistic: Synergism is defined as the cooperative action of different chemicals in a mixture such that the total effect is greater than the sum of the independent effects (Green et al., 1997). In relation to herbicides, the total  induced by an herbicide mixture is greater than the sum of the responses by each herbicide alone (Anderson, 1996). Synergistic  is often used to describe the effect of drugs working together — where one drug increases the other’s effectiveness. Synergy is the interaction of multiple elements in a system to produce an effect different from or greater than the sum of their individual effects.

Glossary Acidophiles or Acidophilic weeds: The weeds that grow acidic soil having a pH between 4.5-6.5 are known as Acidophiles. Example: Borreria hispida, Pteridium sp. Rumex acetosella, Digitaria sanguinalis, etc. Acid equivalent: The active ingredient or actual chemical in a commercial material which is responsible for required chemical reaction.          Acid Equivalent     =     Molecular weight of the acid-1      x 100                                 Molecular weight of the salt or ester Active ingredient: An active ingredient (AI) is the substance in a herbicide that is biologically active.  Aquatic weeds: Unwanted plants that grow in water and complete atleast a part of their life cycle in water are called as aquatic weeds.