eChapters

Weed Management

The productivity of crops much depends on intercultural and weeding operation. The efficient management practices ultimately increases the yield level. The productivity is much dependent on various attributes starting from the variety, climate including human resource management and other factors known well to the farmers. It is estimated that nearly 30-50 per cent of the crop is lost due to weed menace in the agricultural fields. The farmers use various methods of weed control like cultural, mechanical, chemical and manual means. Encompassing all these control measures is known as integrated weed management.

Shifts or changes in the abundance and types of weeds within agricultural systems are common. These shifts can occur for a number of reasons and may result from cultural, mechanical, or chemical weed management strategies. The right combination of weeds and relying too heavily on any one practice is often the most common cause. Some of the concepts associated with weed species shifts are discussed below.

A weed shift is the change in the composition or relative frequencies of weeds in a weed population (all individuals of a single species in a defined area) or community (all plant populations in a defined area) in response to natural or human-made environmental changes in an agricultural system. Weed shifts occur when weed management practices do not control an entire weed community or population. Some species or biotypes are killed by (or susceptible to) the weed management practice, others are not affected by the management practice (tolerant or resistant), and still others do not encounter the management practice (dormant at application).

Those species that are not controlled can grow, reproduce, and increase in the community; resulting in a weed shift. Any cultural, physiological, biological, or chemical practice that modifies the growing environment without controlling all species equally can result in a weed shift. Weed shift may be due to introduction of new species or change in relative proportion of members of the existing community. Latter is the most common.

A. Species shift: Change in density of one or more species in field

B. Biotype shift: Change in density of biotypes within a species. These shifts often found noticed since biotypes typically are similar in appearance. Herbicide resistantbiotypes are the most frequently studied.

Resistance

The intensive and continuous use of herbicides over the last five decades has resulted in the development and evolution of weeds resistant to the normally phytotoxic chemicals. Although herbicide resistance was reported as early as 1957 against 2, 4-D from Hawaii the first confirmed report of herbicide resistance was against triazine herbicide in common groundsel (Senecio vulgaris) was reported in l968 from U.S.A. Over the years, more than 150 weed species have been identified as having developed biotypes resistant to at least one or more herbicide classes. Resistance to at least 15 classes of herbicides have been noted. Moreover, the area of land infested with resistant weeds is increasing rapidly. Reports of evolution of newly resistant biotypes continue to emanate from all over the world, including developing countries, e.g., India, where extensive use of herbicides in grain crops began only in the late 1970s. These reports indicate that herbicide resistance problems are accelerating and, consequently, management of weeds is becoming increasingly more difficult and complex.

The phenomenon of herbicide resistance in plants, especially in weeds, is analogous to the evolution of changes that arise in insects and pathogens in response to the continuous use of insecticides and fungicides. Herbicide resistance is the result of selection for traits that allow weed species to survive specific herbicide usage, which would otherwise cause mortality. Herbicide resistance contributes a trait of resistance within the plant species to a herbicide and resistant weeds are those plant species that express the genetic variation required to evolve mechanisms to escape control. Like other crop pests (insects, pathogens, etc.) with reference to pesticides (insecticides, fungicides, etc.), herbicide-resistant weeds are the result of intensive selection pressure in weed populations.

Generally, resistance is defined as the ability of a plant species to withstand the phytotoxicity of a chemical. This is the trait normally found in crop plants, as against weeds, thus forming the basis for herbicide selectivity. Crop resistance to a herbicide is often different from the one that exists in the phenomenon of herbicide resistance in weeds.

Herbicide resistance in the context of weeds is defined as a characteristic of the weed species to withstand a herbicide dosage substantially higher than the wild type of the same plant species can withstand. It is the inherent or acquired ability to withstand the dosage of herbicide normally used for satisfactory weed control. Resistant weed populations are unharmed by a concentration of herbicide that completely kills all unselected populations. This herbicide concentration is the one that in field practice produces an effective kill.

Herbicide susceptibility represents the other end of the scale, encompassing natural variability in sensitivity to herbicides within the unselected populations. Tolerance, on the other hand, indicates reduced susceptibility (or conversely enhanced resistance) which may sometimes result from selection by herbicides. It is a response rather less than 100% survival accompanied by zero reduction in growth at herbicide concentrations, which normally give 100% Kill of susceptible biotypes. Tolerance may also be considered the natural or normal variability of response to-herbicides that exists within a species and can easily and quickly evolve.

Persistence of Herbicide in Soils

Most of the herbicides are applied to soil as pre-emergence for weed control under different situations. For effective weed control, herbicides must remain in soil in active and available form until their purpose is served. Larger persistence poses hazard to subsequent land use and is undesirable. The length of time that an herbicide remains active in the soil is called persistence or soil residual life. The quantity of herbicide that remains in the soil after its mission is accomplished is referred as residue. The term residue infers the continued presence of herbicide in the soil in very small quantities even though it may not be present in herbicidal quantities or available for plant absorption. Herbicide residues in soils are undesirable mainly because they may

Injure sensitive crops in a cropping system,

Be absorbed by succeeding crops and residues accumulated in unlawful amounts in the produce,

Result in building up of herbicide residues following repeated treatments that exceed their rate of dissipation, and

Inhibit growth of soil micro-organisms.

Knowledge of persistence and carry over residual effect of herbicides is essential to use them safely and effectively. Persistence of herbicides in soils pose hazard when

Highly persistence herbicides are used,

Crop failure necessitates replanting,

A susceptible crop follows a short term crop which received a persistence herbicide, and

Decomposition of the applied herbicide proceeds very slow.

Herbicide Mixtures

A mixture of two or more herbicides could be more effective for controlling different weed species. In horticultural and plantation crops, weed free situation is needed during the entire year over several years. In such situations, herbicides are to be rotated to take care of all the species of weeds. A mixture of herbicides, due to chemical reaction, may enhance or reduce the efûciency of one or more compounds. Hence, knowledge of herbicides interactions is essential for efûcient use of herbicides in weed control.

Mode of Action

The term mode of action refers to the sequence of events from absorption into plants to plant death. The mode of action of the herbicide influences how the herbicide is applied. For example, contact herbicides that disrupt cell membranes, such as acifluorfen (Blazer) or paraquat (Gramoxone Extra), need to be applied post emergence to leaf tissue in order to be effective. Seedling growth inhibitors, such as trifluralin (Treflan) and alachlor (Lasso), need to be applied to the soil to effectively control newly germinated seedlings.

To be effective, herbicides must 1) adequately contact plants; 2) be absorbed by plants; 3) move within the plants to the site of action, without being deactivated; and 4) reach toxic levels at the site of action. The application method used, whether pre-plant incorporated, pre-emergence, or postemergence, determines whether the herbicide will contact germinating seedlings, roots, shoots, or leaves of plants.

Mode of Action of Herbicides

Herbicides perform a vital role in the management of weeds. As the name indicates, herbicides are chemicals that kill or control vegetation. Although the ultimate effect of most herbicides is the same (usually death of weed), the way they control weeds is vastly different. Physiologists use the term mode of action to describe the way the herbicides affect weeds. It includes the entire sequence of events that occur from the time the weed absorbs the herbicide to the final plant response (usually death). The term mode of action is the broad term under which all aspects of herbicidal action including the mechanism of action is included, while the mechanism of action refers to only the biochemical and biophysical responses of plants that appeared to be associated with herbicidal action.

Thus, mode of action includes absorption, translocation to an active site, inhibition of a specific biochemical reaction, degradation or breakdown of the herbicide in the plant and soil and the effect of the herbicide on plant growth and physiology.

Although two herbicides may differ chemically, they may still possess the same mode of action example trifluralin (a dinitroaniles herbicide) and propanamide (an amide herbicide) are inhibitors of microtubule/spindle apparatus. Each herbicide family (class or group) has a primary site of action which may be different in its action from others example sulfonylureas herbicides are ALS or AHAS inhibitors while glyphosate and sulfosate are ESPS inhibitors. Some may have more than one site of action, but the mostinhibitory of these will be affected first. The other site(s) may be considered secondary. Fluometuron, a urea group herbicides act by inhibiting photosynthesis at photosystem II and carotenoid biosynthesis.

Herbicide Application

Generally, herbicides are applied in the form of solution or granules. Of the two, spraying solutions of herbicides formulated as wettable powders, soluble powders and emulsifiable and solution concentrates is more common. When herbicides are formulated as granular or encapsulated materials, they are applied by hand or with the help of a granular applicator.

History on Classification of Herbicides

Copper sulphate (CuSO₄) was the earliest inorganic salt used as selective herbicide for controlling dicot weeds in cereal crops during 1896 in France, Germany, and USA. Sulphuric acid (H₂SO₄) was used as herbicide in cereals during 1909. In selective killing action depends on the fact that it runs off the leaves of the cereals and scorches rough surfaces of certain seeds. Sodium chlorate (NaClO₃) under the name mort-herbs is used in France for killing deep rooted perennials. Another non-selective herbicide, ammonium sulphamate (NH₄SO₃NH₂) was introduced in 1945 as ammate for the control of woody plants. Organic herbicides are more effective and selective.

Classification of Herbicides

Classification of herbicides is modified periodically as and when new herbicides are developed. Herbicides are classiûed based on their method of use (soil and foliage applied), mode of action (systemic or translocated and non-systemic or contact herbicide) and chemical key structure (aliphatic-aromatic, dinitrophenal, etc. Some herbicide molecules contain more than one classifying key structure and they are put into the group most likely to be contributing activity.

Any approved herbicide is known by three names: common name, chemical name and trade name. Common name of herbicide is a technically accepted abbreviation of its chemical name. Chemical name reads out its full molecular structure. Trade names refer to their formulations offered by the chemical companies for the consumers.

Herbicide History and Development

Common salt, ash, smelter waste, and like cheap materials have been used for centuries to control weeds on roadsides, fence rows, and pathways. But Chemical weed killing was born in 1896 when Bonnet, a French grape grower, observed that the Bordeaux mixture which he used against downy mildew of grape turned the leaves of Sinapis arvensis (a weed) black. Soon it was found that copper sulphate present in the Bordeaux mixture was responsible for its weed killing effect. Further tests in France, Germany, and U.S.A. established that copper sulphate could indeed be used for selective control of broadleaf weeds in small grains; wheat, barley, and oat. Between 1896 and 1908, several other inorganic salts, as also sulphuric acid, were found selective to small grain cereals. Of these, in Europe, a 6 to 10 per cent solution of sulphuric acid continued for considerable time as a satisfactory herbicide for broadleaf weed control in cereals, but it corroded the containers and sprayers. In the U.S.A., salts like sodium chlorate, carbon bisulphide, sodium arsenite, kainite, and calcium cyanamide were developed for non-selective eradication of perennial weeds, particularly, Convolvulus arvensis ‘field bind weed’ and Cirsium arvense ‘Canada thistle‘. In Hawaii, sodium arsenite solution was found highly effective for weed control in sugarcane plantations. Between 1930 and 1940, some boron compounds, thiocyanates, dinitro phenols (DNC and DNBP), ammonium sulphate, and certain mineral salts were developed in the west for selective and non-selective weed control. DNC and DNBP wereused extensively in Europe in small grains for broadleaf weed control, even though these chemicals dyed the hands and clothes of the users to yellow colour, almost permanently. Further milestones in weed control were introduction of organic chemical, 2-methyl-4, 6-dinitrophenoll (DMOC), in 1932 and discovery of hormone type selective weed killers. Selective residual chemicals such as substituted phenyl ureas and triazines, and non-residual chemicals diquat and paraquat are more recent discoveries.

The discovery of the herbicidal activity of 2, 4-D (2, 4-dichlorophenoxyacetic acid) first synthesized in 1941, triggered the development of modern herbicide technology. 2, 4-D proved to be an outstanding herbicide.

Integrated Method of Weed Management

Definition

1. An integrated weed management may be defined as the combination of two or more weed-control methods at low input levels to reduce weed competition in a given cropping system below the economical threshold level. It has proved to be a valuable concept in a few cases, though much is still to be done to extend it to the small farmers’ level.
2. When more than one method is employed for controlling weeds, it is termed as integrated method weed management.

FAO Definition

It is a method whereby all economically, ecologically and toxicologically justifiable methods are employed to keep the harmful organisms below the threshold level of economic damage, keeping in the foreground the conscious employment of natural limiting factors.

IWM is the rational use of direct and indirect control methods to provide cost-effective weed control. Such an approach is the most attractive alternative from agronomic, economic and ecological point of view.

Methods of Weed Control

Prevention, eradication and control are the fundamental principles of weed control.First basic step is to prevent the infestation of a weed(s) in a locality in order to avoid their further spread. If one fails in preventing spread of weeds, eradication step must be adopted. Eradication is limited on a very small scale and adopted only when suitable methods of complete weed removal are available as eradication is cent per cent control of weeds alongwith its root system and other propagative parts. Further, if one fails in prevention and eradication techniques then different weed control methods must be strictly followed in which the objective is to bring down the weed population (with any method) to a level that economical crop yields are possible. The aim of each weed control method is to achieve reasonable good control of weeds. Following methods for controlling weeds are very common:

Preventive Methods

Try to prevent spread of weed seeds/vegetative parts with the adoption of following preventive techniques.

Weed monitoring is conducted to detect significant weed problems, rank them for relative severity and sort them by management type annuals, biennials, perennials, grasses and broadleaves. Weed monitoring provides information to tailor weed management efforts that reduce the impact of the weeds. The primary objective of weed survey and mapping is to accurately identify and delineate land with populations of unwanted plants. These surveys are conducted so that scientists and managers can predict those areas potentially subject to weed invasion; understand the biology of the invasion process and determine means by which weeds spread; develop, implement and evaluate weed management plans; assess the economic impact of weed invasion and increase public awareness, education and weed management efforts. Weed survey information is collected and compiled into maps showing the distribution and severity of infestation. Weed monitoring involves repetitive surveys to track weed populations over time. A standardized system of weed surveying and mapping is necessary to provide consistently reliable information that can be compared from year to year. Further, a standardized system allows weed survey data to be incorporated into a state wide weed survey database for the production of state wide noxious weed maps.

Weed Ecology

Ecology is the study of reciprocal relationship between organisms and their environments. The ecology of weed is thus concerned with growth characteristics and adaptations that enable weeds to exploit the ecological niches left open in those environments altered by man for his use and the survival mechanisms where by weeds persist in such environments. In simple words weed ecology is reciprocal relationship of weeds with their environments. Weeds grow voluntarily in all crops raised by human beings since ancient ecology of weeds is divided into two categories (types).

1. Weed Auto Ecology

It is study of only one population i.e. how an individual species in a population interacts with its environment at all stages of its life cycle. So, in case of auto ecology competition within same species (intraspecific) or with crop plants (inter specific competition) is studied.

2. Weed Synecology

The study of relationships between communities of different weed species with their environment is called weed synecology. Different weed species grow in certain combinations and in specific patterns in the ûeld. The population of different weed species interact with the external environmental conditions and among themselves. Weed communities differ from field to field and crop to crop in the same locality. The community character is dynamic and changes in form and composition of weed flora takes place in course of time due to some external and internal factors prevailing in the community. This process is called ecological succession. With the passage of time there will be series of changes in the composition of weed community which ultimately results in stable community. This is termed as climax community.

Weed Biology

Weed biology is the study of the establishment, growth, reproduction, and life cycles of weed species and weed societies/vegetation. Weed biology is an integrated science with the aim of minimizing the negative effects, as well as using and developing the positive effects, of weeds.

Life Cycle - Based on life cycle weeds are classified as annuals, biennials or perennials.

Annuals

Annuals complete their life cycle from seed in less than one year. There are two types: summer and winter annuals. Summer annuals germinate in the spring, mature, produce flowers and seeds and die before fall. Winter annuals germinate in the fall, overwinter in a seedling or rosette stage, mature, produce flowers and seeds, and die in the spring or early summer. Because of the seedling stage, annual weeds are generally easy to control. There are usually few annual weeds present in low bush blueberry fields.

Biennials

Biennials generally complete their life cycle over two years. The first year the seeds germinate and form a basal cluster of leaves and a tap root. The plant overwinters in this stage. During the second year the weed produces a flower stalk, sets seed and dies. Examples of biennial weed are evening primrose and wild carrot.

The association of weeds with field crops was started when ancient man started cultivating plants as crops commercially. However, type of weed flora went on shifting with the passage of time, due to changing climatic conditions, cropping pattern, agronomic practices, crop cultivars etc. When crops are cultivated invariably a variety of volunteer undesirable vegetation come up simultaneously which competes with the crop for all growth factors viz., nutrients, soil moisture light, space etc.

Definition of Weed

Weed is a plant that originates under a natural environment and in response to imposed and natural environments, evolved and continues to do so as an interfering associate with our desired plant and activities.

Weed is a plant growing out of place. They are unwanted not useful, persistent and prolific, effectively competing with the beneficial and desirable crop plants for space, nutrients, sunlight, and water, interfere with agricultural operations and thereby reducing the yield and quality of produce.

Weeds are unwanted and undeserved plants that interfere with the utilization of the land and water resources and thus adversely affect crop production and human welfare. Thus a plant out of its place or a plant growing where it is not desired is a weed. This definition was given by Buchholtz in 1967. E.g. Bajra is a weed in pulse, pulse is a weed in bajra, tomato is a weed in brinjal field. Weed is defined as the unwanted, undesirable plant growing out of their proper place which interfere with the utilization of natural resources, prolific, persistent, competitive, harmful and even poisonous in nature and can grow in adverse climatic conditions (Jethro Tull: Father of Weed Science).

Weediness: Weediness is defined as the state or condition of a field, flower beds, lawns and so forth in which there is an abundance of weeds.

Weeds are plants that are unwanted in a given situation and may be harmful, dangerous or economically detrimental. Weeds are a serious threat to primary production and biodiversity. They reduce farm and forest productivity, displace native species and contribute significantly to land and water degradation. The costs of weeds to the natural environment are also high, with weed invasion being ranked second only to habitat loss in causing biodiversity decline.

Growth: Growth involve cell division subsequently cell elongation. Increase in volume that is associated with fruit growth largely result of cell division or cell elongation or both. In some fruit such as apple, extension of intercellular space may also contribute the growth. Generally growth by cell division in early stage whereas, cell elongation during later stage.

Two distinct type of growth curve are observed when increase in such variable such as fresh weight, volume and dry weight of fruit.

1. Single sigmoid curve
2. Double sigmoid curve

In case of single sigmoid growth curve firstly there is rapid increase in size then after decline so there is only one rapid growth is observed e.g. Apple, pear, tomato, cucumber and strawberry.

In case of double sigmoid two rapid growth period are separated by an intermediate period that is less growth or no increasing growth curve. Double sigmoid growth curve can be used as two successive sigmoid curve thus there are three clearly define stage of growth.

Jam: Jam is a product made by boiling fruit pulp with sufficient sugar to a reasonably thick consistency. Jam is mainly prepared by apple, pear, sapota, apricot, peach, papaya, karonda etc. It can be prepared from one kind of fruits or from two or more kind of fruits. In general jam contains 0.5-0.6% acid and invert sugar should not be more than 40%.

Drying- Removal of water from product by influence of non conventional energy source like sun and wind.

Dehydration- Process of removal of moisture by the application of artificial heat under controlled condition of temperature, humidity and airflow etc.

Advantage of dehydration over sun drying

The process of dehydration is much more rapid than sun drying.

Dehydration requires less floor area.

Dehydration is done under very hygienic condition.

Sun drying is not possible in cloudy weather or during rain while dehydration or mechanical drying is not dependent on weather.

The colour of dehydrated or mechanically dried fruit and vegetable remain uniform due to uniform drying temperature.

Canning: Canning is the process by which sealing of the food product and sterilizing them by heat for long storage is known as canning.

History: The term canning was first invented by N. Appert (1804) in France. In honour of the inventor canning is also known as appertization (Appart is known as the father of canning) and Saddington (1807) in England first described the method of canning. Peter Durand (1810) got first british patent on canning of food in tin container and William under wood introduced canning of fruit on a commercial scale in U.S.A.

Principle of canning: The main principle of canning is destruction of spoilage organism by means of heat.

Proper handling, packaging, transportation and storage reduce the post harvest losses of fruits and vegetables. For every one percent reduction in loss will save 5 million tons of fruit and vegetable per year. Processing and preservation technology helps to save excess fruit and vegetable during the glut season (off season). The technology has become a necessity to improve the food safety and strengthen nation’s food security. The technology helps to boost export of agricultural commodities in the form of preserved and value added products e.g. mango, pineapple, citrus, grapes, tomatoes, peas, potato and cucumber being processed on a large scale.

Principles and Methods of Preservation of Fruits

Preservation: Preservation means just protect the foods against the spoilage, but scientifically it may be defined as a science which deals with the process for prevention of decay or spoilage of the food is called preservation. In other words, just controlling the physical, chemical or microbial changes in the foods is called preservation.