Importance:
Mass media enable extension workers to greatly increase their teaching efficiency. Publications, news papers articles, circular letters, radio, television, exhibits, posters etc., provide helpful repetition for those contacted personally or through groups. They also facilitate dissemination of information to a much larger & different clientele. Even though the intensity of the teaching contact, through mass media is less, the large number of people reached and the low cost per unit of coverage more than off-set the lack of intensity. The extension teaching plan which neglects the communication possibility through mass media fails to fully capitalize on what has already been invested in the more intensive contact methods.
Definitions of Communication
Leagans says, “It is a process by which two or more people exchange ideas, facts, feelings or impressions in a way that each gains a common understanding of message. In essence it is the act of getting a sender and a receiver tuned together for a particular message or series of messages.”
Communication is the imparting or interchange of thoughts, opinions or information by speech, writing or science. (American College Dictionary)
According to Rogers & Shoemaker (1970) Communication is the process by which messages are transferred from a source to receiver.
Van de Ban & Hawkins (1988) defined Communication as the process of sending & receiving messages through channels which establishes common meanings between a source and a receiver.
Communication is the process by which the message is transmitted from the source to the receiver (Rogers, 1983)
Communication is a mutual interchange of ideas by any effective means (Thayer 1968)
Communication is anything that conveys meaning that carries a message from one person to another (Brooker 1949)
Communication Methods
A method is a procedure or process for attaining an objective. The choice of channels or method of communication, also known as extension teaching method, generally depends on the number and location of the target audience, and the time available for communication.
Selection of effective combination of extension method
A combination of extension teaching methods or that of mass media and inter personal sources is perhaps the most effective way of reaching people with new ideas and persuading them to utilize these innovations.
The basic idea behind this is:
The more the senses of the learner are involved in the learning process, the greater the learning.
To ensure action on the part of learner, several exposures may be necessary in order to motivate, remind and persuade them.
It will depend on the situation as well as knowledge and experience of the extension agent.
Basically Communication Methods Are Classified As:-
1. Interpersonal Communication
2. Mass media Communication
Interpersonal Communication
It consists of a face to face exchange between two or more individuals. The message flow is from one to a few individuals. Feed back is immediate and usually plentiful, and the messages are often relatively high in socio-emotional content
Mass media Communication
It involves some type of hardware equipment that enables a source of one or a few individuals to reach a large audience. Feedback is limited and delayed and the messages are often relatively low in socio-emotional content.
[A]. Interpersonal Methods
In this method, the extension agent communicates with the people individually, maintaining separate identity of each person. This method is followed when the number of people to be contacted are few, are conveniently located close to the communicator and sufficient time is available for communication. eg. Farm & Home visit, Farmers call etc
Advantages
1. Helps in selecting demonstrators & local leaders
2. Helps in changing attitudes of people
3. Helps in teaching complex practices.
4. Facilitates transfer of technology
5. Getting feedback information
Limitations
1. This method is time consuming & relatively expensive
2. Has low coverage of audience
3. Extension agent may develop favoritism or bias towards some persons
[B]. Group Methods
It may be defined as an aggregate of small number of people in reciprocal communication and interaction around some common interest. In this method extension agent communicates with the people in groups and not as individual persons. eg. Result demonstration, Method demonstration, group meeting etc.
Objectives
1. It helps for selection of village leaders.
2. It gives the idea for the need of people.
3. Less expensive than individual method due to more coverage
4. More effective than mass method in stimulating action.
Limitations
1. Wide diversity of interest of people may create a difficulty to learning situation
2. Every body wants for their individual development.
[C].Mass Methods
In this method, extension agent communicates with vast & heterogeneous mass of peoples, without taking into consideration their individual identity. This method is followed where a large & widely dispersed audience is to be communicated within a short period of time. The size of audience may be a few hundred in mass meeting, few thousands in campaign & exhibition and millions in newspaper, radio & TV.
Advantage:
1. Suitable for creating general awareness amongst the people.
2. Helps in transferring knowledge, forming & changing opinion.
3. Large number of people can be communicated within short time
Limitations:-
1. Less intensive method.
2. Cannot be held frequently
3. Little scope for personal contact with the audience.
4. Little control over the responses of audience.
5. Difficult in getting feedback information & evaluation of results
Means of Mass Communication:
Newspaper:
Newspaper is a bunch of loose printed papers properly folded, which contains news, views, advertisements etc and is offered for sale at regular intervals, particularly daily or weekly. Newspapers are usually printed on a special type of paper known as newsprint.
Daily newspapers are resource strong and are published from national/ state capitals or big cities. Their approach is cosmopolite and the circulation may range from about a lack to several lakhs. Some of the daily newspaper are quite big and are published simultaneously from several cities.
Selection of matter for writing in news paper
Prepare a draft write up on the topic in simple language, furnishing current and important information. The lead i.e. the opening sentence or paragraph is important and should be comprehensive. Give a suitable caption
1. The draft should contain information on what, who, when, where and why.
2. Revise the draft and produce a brief, lucid, interesting and informative write up.
3. In required, enclose photographs on glossy paper with the write up.
Advantages:
1- Large circulation.
2- Cheaper than any other mass communication device.
Limitations:
1. Only literate people generally can take advantage of this medium.
2. Increase in the price of newspapers may restrict their circulation.
Television
Television is an electronic audio-visual medium which provides pictures with synchronized sound. This medium is cosmopolite in approach and can be used to create instant mass awareness. Television combines the immediacy of radio with the mobility of Cinema and can carry messages over long distance at a relatively low unit cost.
Television is multi media equipment as it can include motion picture, recording, slide, photograph, drawing, poster etc. Television can show recorded as well as live programmes.
Both recording and playback equipments are transportable, allowing flexibility of use
Purpose:
1. People learn through the eye, & will remember things better if they see them.
2. The message on the tv screen are presented in the simplest manner possible.
Advantages
1. To create a general awareness amongst the people about agricultural and rural development.
2. To provide need based programmes to the rural audience.
3. To show the rural people in general & the farmer in particular what to do,how to do,& when to do.
Limitations
1. Requires lots of planning, preparation, trained personal and availability of equipments.
2. Audience participation depends on costly receiving sets and availability of electricity.
3. TV is an electronic audio visual medium,which provides pictures with synchronize sound.
4. Seldom goes beyond creating general awareness of audience
Video Films:
Video films are really a series of still pictures on a long strip of film. Each picture is flashed momentarily on the screen and the rapid succession of still pictures-(each of which shows the subject in a slightly different position) – gives an illusion of movements. Usually 70 mm and 35 mm. films are used for commercial entertainment, 16 mm. Film for educational movies, and 8 mm. Film for domestic pictures.
Advantages:
1. A complete process involving motion can be shown in a short time.
2. People identify themselves with those in the films.
3. Compel attention.
4. Heighten reality.
5. Speed or slow down time.
6. Bring the distant past and the distant present into learning situation.
7. Enlarge or reduce actual size of objects.
Limitations:
1. Special equipment is necessary.
2. The equipment is costly.
3. Some sort of power is required to operate the projectors.
4. Transportation maintenance & storage of equipment & materials require special consideration & skill.
5. Suitable halls for showing motion picture are not available in many places.
6. Bring the distant past and the distant present into learning situation.
7. Enlarge or reduce actual size of objects.
Purpose:
1. To present facts in an interesting way.
2. To attract audience.
3. To arouse interest.
4. To change attitudes.
5. To bring new practices to village In a short time.
6. To reach illiterate as well as literate people.
Points to remember:
1. Be thoroughly familiar with the subject you plan to teach and how exactly the film supports the ideas you want to get across.
2. Preview the film
3. before showing the film, explain the subject, tell why it is important and stimulate viewers to look for certain thing in the film.
4. At the end of the show, have a forum
5. Follow up (capitalise the enthusiasm generated)
Mass Meeting:
Mass meeting is held to communicate interesting and useful information to a large audience at a time. The size of the audience for mass meeting may be a few hundreds, but at the time of fairs or festivals it may be few thousands
Objectives
1. To focus attention of the people on some important topic.
2. To enlist people’s participation in community work.
3. To appear personally before a large audience
Advantage
1. Reaches many who read little or none at all.
2. A means of informing non farm people (tax payers) about agricultural matters.
3. Reaches people who are unable to attend extension meetings.
4. Builds interest in other extension media.
Limitations
1. In-depth discussion of the topic not possible.
2. Can’t be held frequently.
3. Difficult to get feed back information.
Radio:
Radio is an electronic audio-medium for broadcasting programmes to the audience. This medium is cosmopolite in approach and is suitable for communication to millions of people widely dispersed and situated in remote areas. Radio is suitable for creating general awareness amongst the people, help change their attitude and reinforces learning. It reaches a large number of people at a very low cost.
Purpose:
1. To reach large numbers of people quickly & inexpensively.
2. To reach people not reach by other means.
3. To stimulate participation in extension through all other media.
4. To build enthusiasm & maintain interest.
Advantages:
1. Can reach more quickly than any other means of communication.
2. Specially suited to give emergency & timely information(e.g. weather,pest-out-break etc.)
3. Relatively cheap.
4. Reaches many who read little or none at all.
5. Reaches people who are unable to attend extension meetings.
6. A means of informing non farm people( tax payers) about agricultural matters.
7. Builds interest in other extension media.
8. Possible to do other things while listening.
Limitations:
1. Limited number of broadcasting stations.
2. Not within reach of all farmers.
3. Recommendations may not apply to individual needs.
4. No turning back if not needed.
5. Frequently loses out in competition with entertainment.
6. Difficult to check on results.
PROJECTORS:
The lantern slides is one of the most popular & versatile visual in extension education. It is a transparent picture on glass or film) which is projected by focusing light through it from electric bulb, petromax or lantern.
Reasons for the popularity of lantern slides are:
1. They can be made by the individual worker at low cost.
2. They can be made either in natural colour in black and white.
3. Both the slides and projections equipment are relatively light and can be easily transported.
4. Slide sequence can be readily changed to keep them timely and localized.
5. We can use the full set of slides or select only a few slides required for our subject.
6. Each slide can be retained for any length of time according to the teaching situation.
Limitations:
1. They do not show action.
2. They normally require live narration, unless synchronized with tape recorder.
3. They require close co-operation within a projections throughout the presentation if the speaker desires to be in front of his audience.
Wednesday, August 19, 2009
AQUATIC WEEDS AND THEIR CONTROL
INTRODUCTION
Aquatic weed are those unwanted plants which grow in water and complete at least a part of their life cycle in water. Many aquatic plants are desirable since they may play temporarily a beneficial role in reducing agricultural, domestic and industrial pollution. Letting a crop of plants grow in a lake or pond and then killing it over a period of time, and consequently releasing nutrients back into the water, may help in fish production. However, many aquatic plans are considered weeds when they deprive human beings of all facets of efficient use of water and cause harmful effects, some of which are discussed below.
Submersed, immersed, emerged and marginal weeds in and along irrigation canals, ditches, and drainage channels impede water flow, increase evaporation, cause damage to canals and structures, and clog gates, siphons, valves, sprinkler heads bridge piers, pumps, etc.
Floating and deep-rooted submersed weeds interfere with navigation. Some of the tougher and densely growing weeds, e.g. water hyacinth and alligator weed become impenetrable and prevent boats and even steamers moving through. Submersed and floating aquatic weeds in farm ponds, village tanks and water reservoirs reduce their utility for water storage and irrigation.
Aquatic weed growth also prevents or impairs the use of inland waters for fishing. The weed assimilates large quantities of nutrients from water, thus reducing their availability for desirable planktonic algae. They cause oxygen deficiency and prevent gaseous exchange with atmosphere, resulting in an adverse effect on fish production. Excessive growth of these weeds may provide excessive cover, resulting in an overpopulation of small fish and interference with fish harvesting.
Aquatic weeds provide a suitable habitat for development of mosquitoes in impounded waters, causing malaria, filariasis and encephalitis. These weeds serve as the primary vector for the disease-causing organisms.
Aquatic weeds reduce the recreational values of lakes, tanks, streams, etc. as the water is made turbid or dirty with an undesirable odour
TYPES OF AQUATIC WEED
There are two types of aquatic plants: algae and hydrophytes.
ALGAE-Algae normally inhabit the surface of fresh and saline waters exposed to sunlight. While some kinds of algae are found in solid and on terrestrial surfaces exposed to air, the majority aquatic and adapted to live in ponds, lakes, reservoirs, steams, swimming pools and oceans.
Freshwater algae are of two types: planktonic and filamentous. planktonic algae, called phytoplankton, include the truly aquatic single-celled algae and the simplest filamentous or colonial forms. A heavy growth of algae may colour the water shades of green, yellow, red and black. Hey may also form water blooms or scums. They convert solar energy into food, remove CO2 from water during photosynthesis (in day time) and produce oxygen as a by product. During the night or in cloudy water, they release co2 in the water through respiration as consume O2.
Certain planktonic algae are beneficial as they can maintain biotic balance in natural aquatic environment because of their ability to produce oxygen and maintain an aerobic condition. They are the original sources of food for most fish and aquatic animals. Although planktonic algae are beneficial, their overabundance may be undesirable for many domestic and commercial water uses.
Excessive phytoplanktonic blooms often result in zooplanktonic (the microscopic animal forms) development that may deplete result in zooplanktonic the water of oxygen and lead to over fertilization or eutrophication and destruction of fish and other aquatic wild life. Dense growth of planktonic algae will shade bottom muds sufficiently to prevent germination of seeds and growth of many species of rooted submersed weeds, thus affecting the stability of the habitat
Generally, planktonic algae do not interfere with the use of surface waters or irrigation purposes. But some of them, the blue-green algae and green algae produce odours and scums that make unfit for swimming. Several of the blue-algae produce toxic substances that kill fish, birds and domestic animals.
Another group of algae called filametous algae (nanoplankton) consists of single celled joined end to end which may form single thread, branched filaments, nets, or erect stem like whorled branches or forked leaf like forms. They don’t have roots, stems or leaf as do higher plants. The important genera of the filamentous algae are: Chara, Nitella, Spirogyra, Hydrodictyon, Cladophora, Pithophora.
HYDROPHYTES-The hydrophytes, which represent more than 100 families, are vascular plants. They grow wholly or partially submersed in either fresh or saline water or in plaustrine areas. They are structurally different from mesophytes and xerophytes that grow in moisture-deficient situations. The protecting and conducting tissues of hydrophytes are less developed. They have extensive provision for aeration and buoyancy, particularly in the leaf mesophyll, ground tissue of the petiole and the cortex of the stem and root. Buoyancy is provided by aerenchyma or by air chambers. The air chambers may be either schiogenous or lysigenous, or both. Hydrophytes weeds can be grouped as submersed, emersed, marginal and floating weeds.
1- SUBMERSED WEEDS
Submersed weeds are mostly vascular plants the produce all or most of their vegetative growth beneath the water surface. Most submerged vascular weeds are seed plants and have true roots, stems and leaves. Abundance and density of these weeds is primarily dependent on depth and turbidity of water and physical characteristics of the bottom. A maximum depth of 3.5-4 m in clear waters is the limit for most of the submersed plants. They are capable of absorbing nutrients and herbicides through the leaves and stems as well as roots. They compete for nutrients with planktonic algae and decrease their production and a corresponding decrease in fish production.
The submersed weeds belong to the following genera: Potamogeton, Elodea, Myriophyllum, Ceratophyllum, Utricularia, Ranunculus, Heteratheral Alisma, Zannichellia, Lemna, etc.
2- EMERSED WEEDS
Emersed weeds are those plants rooted in the bottom muds with serial stems and leaves at or above the water surface. They grow in situations where the water level ranges from just below ground level to about half the maximum height of the plant. They differ in leaf shape, size and pint of attachment. Some of the weeds of this group have broad leaves, 5-50cm in diameter, and others have ling narrow leaves like grasses, less than 3-15cm or more in width; the latter are commonly called reeds. The leaves of emersed weeds do not rise and fall with water level as in the case of attached floating weeds. Some of the emersed weeds belong to the genera Nuphar, Nelumbo, Jussiaea, Myriophllum, etc.
3-MARGINAL WEEDS
Most marginal weeds are emersed weeds that can grow on saturated soil above the water surface; they grow from moist shoreline areas into water up to 60-90cm in depth. Marginal weed vary in size, shape and habitat species of this group are the most widely distributed rooted aquatic plant. Plants of this group are broad leaves herbs, shrubs, trees and some grasses. The important genera to which they belong are : Phragmitis, typha, Polygonum, Alternanthera, Populus, Tamarix, Cephalanthus, Juncus etc.
4-FLOATING WEEDS
Many water plants have leave that float on the water surface either singly or in rosettes .They have true root and leaves .Some are free floating and others rooted in bottom mud have floating leaves that rise or fall with the water level .They reproduce very rapidly under favorable conditions and are among the most troublesome of aquatic weed .Floating weed belong to the genera Eichhornia, Pistia, Salvinia , Lemna , Nymphaea and brasenia.
MAJOR AQUATIC WEEDS OF INDIA
Common Name Botanical Name
Water hyacinth Eichhornia crassipes
Cattails Typha angustata
Pond weed Potamogeton spp.
Hydrilla Hydrilla verticillata
Water lettuce Psitia stratiotes
Salvinia Salvinia molesta
Swamp morning glory Ipomea aquatic
Alligator weed Alternanthera spp.
Arrow head Sagittaria spp.
Spatter dock or Yellow lily Nuphar spp.
Pickerel weed Ponterdenia cordata
Reed weed Pharagmites communis
Swamp morning glory Ipomea aquatica
METHODS OF CONTROL OF AQUATIC WEEDS
Mechanical & Manual Methods
These methods employ physical forces to remove weeds:
1. Dredging- It is most common way of cleaning weeds in ponds and ditches.A dragline dredge may be equipped with a bucket or with a weed fork or other special tools. The bucket dreadge will also remove mud along with weeds while the weed fork will leave the mud. This method is labour expensive and slow.
2. Draining - Draining is an offseason weed management normally in drainage ditches by which weeds are cut manually or mechanically or spraying with a total weed killer or bottom ploughed to kill vegetative structures and root stock
3. Drying- Simple and inexpensive. The tops of under water weeds are exposed to sun by draining the water from ditches and ponds are allowed to dry. This method is effective in areas where the ponds are seasonal in use and remain dry during summers and rainy seasons. Drying is ineffective against emersed weeds and some of floating species.
4. Burning- It is used to control weeds in the banks above the water line. For obtaining the best results first searing the green vegetation and secondly after 10 -12 days with complete burning. In searing a hot flame is passed over the vegetation at such a rate that the plants wilt but are not charred. Mowing followed by burning the dried weeds may increase the effectiveness of mowing.
5. Chaining- This is relatively inexpensive method which is widely used. A heavy chain is attached between two tractors or teams on the opposite banks of ditch. As they move the chain drags over the weeds and breaks them off. Chaining is generally done only when the ditch is severely clogged. Chaining is primarily to ditches of uniform width and accessible from both sides with tractors. It is effective for cattails, tules, bur reed, arrow head and other emersed weeds.
The major disadvantages of chaining are its too laborious has to be repeated at regular intervals reverse chaining cannot be done.
6. Cutting- A mechanical weed cutter is used to cut the submersed weeds at 1 to 1.5 m deep in water. It consists of a sharp cutter bar operated hydraulically from a boat. The harvested weeds float to the water surface and removed manually or by sieve buckets.
Disadvantages of Mechanical Control Methods- They do not provide effective and economical weed control because of which repetitive operations are required. The weed fragment remaining proves as a source of new infestation.
HERBICIDES USED FOR CONTROLLING AQUATIC WEEDS
Asulam-Docks and bracken on banks.
Dalapon-Effective against grasses and cattails when applied on foliage. Draining of water before application is advisable. It is used in irrigation and drainage channels, lakes, ponds & ditch banks. Applied @ 15-20 Kg/ha for surface area. It is normally harmless to fish. However treated water is unsuitable for potable & irrigation purposes.
Dichlobenil-Effective on Elodea, watermilfoil, chara and potamageton species and is applied before weed starts growing. This is applied at the rate of 5 to 10 Kg/ha to the exposed bottom after draining water. This pre emergence application inhibits regeneration from roots and rhizomes. Water is let in after a month of treatment.
Diquat & Paraquat-These are generally used for control of floating weeds like water hyacinth and water lettuce with foliar application at 1 to 2 Kg/ha. There application in muddy water is ineffective. Efficiency is increased by combining with copper sulphate and triethanolamine.
Aqualin-The compound acrolein or acrylaldehyde has been named as aqualin. It is an active chemical attacks plant cells and kills them. It has lachrymatory effect on man and has to be sold by licensed operators.
Endothall-Aquatic weeds including algae without harming fish or aquatic life. It can be used for both submerged as well as emersed aquatic weeds. Granular formulation is Aquathall.
Copper sulphate-It is effective against many kinds of algae including chara and other species that causes scum. It is either applied as crystals or by placing crystals in a bag or towed behind a boat until the chemical is dissolved. This is applied at the rate of 0.5-1 ppm W. A concentration upto 2.5 ppm W is considered safe for human consumption while above 1 ppm W is considered unsafe for fish, but can be used for irrigation purpose.
Silvex-Effective for control of surface and emersed weeds like alligator weed, water lily, arrow head etc. prevalent in standing waters. Treated water is unsuitable for any purpose.
Method of application
Application is done by pumping the liquid into the water and allowing it to move as a blanket over and through the aquatic weeds. The herbicide may be introduced over a time period ranging from 45 min to 5 hrs. Temperature of water is important consideration. At 150C the dosage must be twice that at 28 0C. Hence the dosage must be adjusted with the plant population and temperature. In fast flowing streams, contact is not so thorough as in slow and hence, dosage must be increased when flow is greater than 10 cumec.
For proper control of weeds of wider water bodies power sprayers are mounted on motor boats. The spray range in this case will be 10 to 20 mts. With a discharge rate of 20 to 200 lts per min. For injection in stable water a hard pipe with several small holes is fitted. In flowing water the herbicide is injected from the shore itself. The herbicide is carried down the stream. During spraying the boats should maintain a speed of 2-5 Km/hr.
Biological control-
Name of Bioagent Weed controlled
Congo tilapia & Jawa tilapia Algae (chara & Nitella) & saw weeds (Najas)
Chinese grass carp or white amur
(Ctenopharyngodon idella) Aquatic plants
Common carp It is mud bottom feeder and it controls submersed aquatic weeds due to its uprooting plants and breaking up mats of algae in its search for food.
Marasmiellus inoderma Thread blight in water hyacinth
Flea beetles (Agasiches hydrophilla) Water hyacinth and Salvinia
Aquatic weed are those unwanted plants which grow in water and complete at least a part of their life cycle in water. Many aquatic plants are desirable since they may play temporarily a beneficial role in reducing agricultural, domestic and industrial pollution. Letting a crop of plants grow in a lake or pond and then killing it over a period of time, and consequently releasing nutrients back into the water, may help in fish production. However, many aquatic plans are considered weeds when they deprive human beings of all facets of efficient use of water and cause harmful effects, some of which are discussed below.
Submersed, immersed, emerged and marginal weeds in and along irrigation canals, ditches, and drainage channels impede water flow, increase evaporation, cause damage to canals and structures, and clog gates, siphons, valves, sprinkler heads bridge piers, pumps, etc.
Floating and deep-rooted submersed weeds interfere with navigation. Some of the tougher and densely growing weeds, e.g. water hyacinth and alligator weed become impenetrable and prevent boats and even steamers moving through. Submersed and floating aquatic weeds in farm ponds, village tanks and water reservoirs reduce their utility for water storage and irrigation.
Aquatic weed growth also prevents or impairs the use of inland waters for fishing. The weed assimilates large quantities of nutrients from water, thus reducing their availability for desirable planktonic algae. They cause oxygen deficiency and prevent gaseous exchange with atmosphere, resulting in an adverse effect on fish production. Excessive growth of these weeds may provide excessive cover, resulting in an overpopulation of small fish and interference with fish harvesting.
Aquatic weeds provide a suitable habitat for development of mosquitoes in impounded waters, causing malaria, filariasis and encephalitis. These weeds serve as the primary vector for the disease-causing organisms.
Aquatic weeds reduce the recreational values of lakes, tanks, streams, etc. as the water is made turbid or dirty with an undesirable odour
TYPES OF AQUATIC WEED
There are two types of aquatic plants: algae and hydrophytes.
ALGAE-Algae normally inhabit the surface of fresh and saline waters exposed to sunlight. While some kinds of algae are found in solid and on terrestrial surfaces exposed to air, the majority aquatic and adapted to live in ponds, lakes, reservoirs, steams, swimming pools and oceans.
Freshwater algae are of two types: planktonic and filamentous. planktonic algae, called phytoplankton, include the truly aquatic single-celled algae and the simplest filamentous or colonial forms. A heavy growth of algae may colour the water shades of green, yellow, red and black. Hey may also form water blooms or scums. They convert solar energy into food, remove CO2 from water during photosynthesis (in day time) and produce oxygen as a by product. During the night or in cloudy water, they release co2 in the water through respiration as consume O2.
Certain planktonic algae are beneficial as they can maintain biotic balance in natural aquatic environment because of their ability to produce oxygen and maintain an aerobic condition. They are the original sources of food for most fish and aquatic animals. Although planktonic algae are beneficial, their overabundance may be undesirable for many domestic and commercial water uses.
Excessive phytoplanktonic blooms often result in zooplanktonic (the microscopic animal forms) development that may deplete result in zooplanktonic the water of oxygen and lead to over fertilization or eutrophication and destruction of fish and other aquatic wild life. Dense growth of planktonic algae will shade bottom muds sufficiently to prevent germination of seeds and growth of many species of rooted submersed weeds, thus affecting the stability of the habitat
Generally, planktonic algae do not interfere with the use of surface waters or irrigation purposes. But some of them, the blue-green algae and green algae produce odours and scums that make unfit for swimming. Several of the blue-algae produce toxic substances that kill fish, birds and domestic animals.
Another group of algae called filametous algae (nanoplankton) consists of single celled joined end to end which may form single thread, branched filaments, nets, or erect stem like whorled branches or forked leaf like forms. They don’t have roots, stems or leaf as do higher plants. The important genera of the filamentous algae are: Chara, Nitella, Spirogyra, Hydrodictyon, Cladophora, Pithophora.
HYDROPHYTES-The hydrophytes, which represent more than 100 families, are vascular plants. They grow wholly or partially submersed in either fresh or saline water or in plaustrine areas. They are structurally different from mesophytes and xerophytes that grow in moisture-deficient situations. The protecting and conducting tissues of hydrophytes are less developed. They have extensive provision for aeration and buoyancy, particularly in the leaf mesophyll, ground tissue of the petiole and the cortex of the stem and root. Buoyancy is provided by aerenchyma or by air chambers. The air chambers may be either schiogenous or lysigenous, or both. Hydrophytes weeds can be grouped as submersed, emersed, marginal and floating weeds.
1- SUBMERSED WEEDS
Submersed weeds are mostly vascular plants the produce all or most of their vegetative growth beneath the water surface. Most submerged vascular weeds are seed plants and have true roots, stems and leaves. Abundance and density of these weeds is primarily dependent on depth and turbidity of water and physical characteristics of the bottom. A maximum depth of 3.5-4 m in clear waters is the limit for most of the submersed plants. They are capable of absorbing nutrients and herbicides through the leaves and stems as well as roots. They compete for nutrients with planktonic algae and decrease their production and a corresponding decrease in fish production.
The submersed weeds belong to the following genera: Potamogeton, Elodea, Myriophyllum, Ceratophyllum, Utricularia, Ranunculus, Heteratheral Alisma, Zannichellia, Lemna, etc.
2- EMERSED WEEDS
Emersed weeds are those plants rooted in the bottom muds with serial stems and leaves at or above the water surface. They grow in situations where the water level ranges from just below ground level to about half the maximum height of the plant. They differ in leaf shape, size and pint of attachment. Some of the weeds of this group have broad leaves, 5-50cm in diameter, and others have ling narrow leaves like grasses, less than 3-15cm or more in width; the latter are commonly called reeds. The leaves of emersed weeds do not rise and fall with water level as in the case of attached floating weeds. Some of the emersed weeds belong to the genera Nuphar, Nelumbo, Jussiaea, Myriophllum, etc.
3-MARGINAL WEEDS
Most marginal weeds are emersed weeds that can grow on saturated soil above the water surface; they grow from moist shoreline areas into water up to 60-90cm in depth. Marginal weed vary in size, shape and habitat species of this group are the most widely distributed rooted aquatic plant. Plants of this group are broad leaves herbs, shrubs, trees and some grasses. The important genera to which they belong are : Phragmitis, typha, Polygonum, Alternanthera, Populus, Tamarix, Cephalanthus, Juncus etc.
4-FLOATING WEEDS
Many water plants have leave that float on the water surface either singly or in rosettes .They have true root and leaves .Some are free floating and others rooted in bottom mud have floating leaves that rise or fall with the water level .They reproduce very rapidly under favorable conditions and are among the most troublesome of aquatic weed .Floating weed belong to the genera Eichhornia, Pistia, Salvinia , Lemna , Nymphaea and brasenia.
MAJOR AQUATIC WEEDS OF INDIA
Common Name Botanical Name
Water hyacinth Eichhornia crassipes
Cattails Typha angustata
Pond weed Potamogeton spp.
Hydrilla Hydrilla verticillata
Water lettuce Psitia stratiotes
Salvinia Salvinia molesta
Swamp morning glory Ipomea aquatic
Alligator weed Alternanthera spp.
Arrow head Sagittaria spp.
Spatter dock or Yellow lily Nuphar spp.
Pickerel weed Ponterdenia cordata
Reed weed Pharagmites communis
Swamp morning glory Ipomea aquatica
METHODS OF CONTROL OF AQUATIC WEEDS
Mechanical & Manual Methods
These methods employ physical forces to remove weeds:
1. Dredging- It is most common way of cleaning weeds in ponds and ditches.A dragline dredge may be equipped with a bucket or with a weed fork or other special tools. The bucket dreadge will also remove mud along with weeds while the weed fork will leave the mud. This method is labour expensive and slow.
2. Draining - Draining is an offseason weed management normally in drainage ditches by which weeds are cut manually or mechanically or spraying with a total weed killer or bottom ploughed to kill vegetative structures and root stock
3. Drying- Simple and inexpensive. The tops of under water weeds are exposed to sun by draining the water from ditches and ponds are allowed to dry. This method is effective in areas where the ponds are seasonal in use and remain dry during summers and rainy seasons. Drying is ineffective against emersed weeds and some of floating species.
4. Burning- It is used to control weeds in the banks above the water line. For obtaining the best results first searing the green vegetation and secondly after 10 -12 days with complete burning. In searing a hot flame is passed over the vegetation at such a rate that the plants wilt but are not charred. Mowing followed by burning the dried weeds may increase the effectiveness of mowing.
5. Chaining- This is relatively inexpensive method which is widely used. A heavy chain is attached between two tractors or teams on the opposite banks of ditch. As they move the chain drags over the weeds and breaks them off. Chaining is generally done only when the ditch is severely clogged. Chaining is primarily to ditches of uniform width and accessible from both sides with tractors. It is effective for cattails, tules, bur reed, arrow head and other emersed weeds.
The major disadvantages of chaining are its too laborious has to be repeated at regular intervals reverse chaining cannot be done.
6. Cutting- A mechanical weed cutter is used to cut the submersed weeds at 1 to 1.5 m deep in water. It consists of a sharp cutter bar operated hydraulically from a boat. The harvested weeds float to the water surface and removed manually or by sieve buckets.
Disadvantages of Mechanical Control Methods- They do not provide effective and economical weed control because of which repetitive operations are required. The weed fragment remaining proves as a source of new infestation.
HERBICIDES USED FOR CONTROLLING AQUATIC WEEDS
Asulam-Docks and bracken on banks.
Dalapon-Effective against grasses and cattails when applied on foliage. Draining of water before application is advisable. It is used in irrigation and drainage channels, lakes, ponds & ditch banks. Applied @ 15-20 Kg/ha for surface area. It is normally harmless to fish. However treated water is unsuitable for potable & irrigation purposes.
Dichlobenil-Effective on Elodea, watermilfoil, chara and potamageton species and is applied before weed starts growing. This is applied at the rate of 5 to 10 Kg/ha to the exposed bottom after draining water. This pre emergence application inhibits regeneration from roots and rhizomes. Water is let in after a month of treatment.
Diquat & Paraquat-These are generally used for control of floating weeds like water hyacinth and water lettuce with foliar application at 1 to 2 Kg/ha. There application in muddy water is ineffective. Efficiency is increased by combining with copper sulphate and triethanolamine.
Aqualin-The compound acrolein or acrylaldehyde has been named as aqualin. It is an active chemical attacks plant cells and kills them. It has lachrymatory effect on man and has to be sold by licensed operators.
Endothall-Aquatic weeds including algae without harming fish or aquatic life. It can be used for both submerged as well as emersed aquatic weeds. Granular formulation is Aquathall.
Copper sulphate-It is effective against many kinds of algae including chara and other species that causes scum. It is either applied as crystals or by placing crystals in a bag or towed behind a boat until the chemical is dissolved. This is applied at the rate of 0.5-1 ppm W. A concentration upto 2.5 ppm W is considered safe for human consumption while above 1 ppm W is considered unsafe for fish, but can be used for irrigation purpose.
Silvex-Effective for control of surface and emersed weeds like alligator weed, water lily, arrow head etc. prevalent in standing waters. Treated water is unsuitable for any purpose.
Method of application
Application is done by pumping the liquid into the water and allowing it to move as a blanket over and through the aquatic weeds. The herbicide may be introduced over a time period ranging from 45 min to 5 hrs. Temperature of water is important consideration. At 150C the dosage must be twice that at 28 0C. Hence the dosage must be adjusted with the plant population and temperature. In fast flowing streams, contact is not so thorough as in slow and hence, dosage must be increased when flow is greater than 10 cumec.
For proper control of weeds of wider water bodies power sprayers are mounted on motor boats. The spray range in this case will be 10 to 20 mts. With a discharge rate of 20 to 200 lts per min. For injection in stable water a hard pipe with several small holes is fitted. In flowing water the herbicide is injected from the shore itself. The herbicide is carried down the stream. During spraying the boats should maintain a speed of 2-5 Km/hr.
Biological control-
Name of Bioagent Weed controlled
Congo tilapia & Jawa tilapia Algae (chara & Nitella) & saw weeds (Najas)
Chinese grass carp or white amur
(Ctenopharyngodon idella) Aquatic plants
Common carp It is mud bottom feeder and it controls submersed aquatic weeds due to its uprooting plants and breaking up mats of algae in its search for food.
Marasmiellus inoderma Thread blight in water hyacinth
Flea beetles (Agasiches hydrophilla) Water hyacinth and Salvinia
Cultivation or Package of practise for Fodder Sorghum (Sorghum bicolor (L)) Moench
Botany-
Annual, erect, single stalked or tillering; leaves alternate, ligules short membranous, inflorescence a panicle, contracted or loose, spikelets in clusters of 2 or 3 one sessile and hermaphrodite and the other/(s) pedicelled staminate or sterile; stamens three; stigmas two; styles terminal; fruit is a caryopsis.
Growth Stages-
Sorghum has five distinct growth stages:
1. Seedling Stage- Germination takes place in 4-5DAS. Emergence of coleoptiles indicates seedling stage.
2. Flag Leaf Stage-3-4 leaves with fully expended leaf area. Stage reaches in 40-55 DAS.
3. Boot Stage-Ear head covered within sheath i.e. under flag leaf stage reaches in 45-60 DAS.
4. Soft Dough-Endosperm filled with watery fluid. It’s called milky stage. Stage reaches in 70-85DAS.
5. Physiological Maturity- Grain with maximum accumulation of dry matter. Stage reaches in 85-95DAS.
Climate:-
It is a plant of warm climate. It remains dormant under unfavorable conditions, leaves posses waxy coating and presence of motor cells in leaves rolls the leaves under moisture deficit conditions. It is also posses more no of secondary roots/ unit of primary roots. Thus crop can withstand drought better than maize. Crop is grown in areas receiving 800-1000mm annual rainfall isohyets. High rainfall is undesirable though crop can withstand water logging. It is a Short day plant and a 10 hr photoperiod enhances early flowering. Increase in day length delay flowering and benefit dry matter production. During drought the cyanide content remains high.
Minimum temperature required for germination 7-10 degree C
Optimum temperature for growth 25-30 degree C
Soil Requirement-
The crop grows on every type of soil except very sandy soil. It is predominantly grown in vertisols. As a rainfed crop it is better suited to heavier soils retentive of moisture. It grows best in deep alluvial soils with irrigation. Black cotton soils of central India are considered as best soils for its cultivation. It can withstand moderate salinity.
Field Preparation-
Sorghum is a shallow rooted crop and does not require deep cultivation. 2 to 4 operations by desi plough or blade harrow, alternatively 2 to 4 times harrowing to pulverize the soil to a fine tith OR deep summer ploughing with mould board plough and two ploughings by country plough with the onset of monsoon are enough to prepare the land for sowing. Crop is grown in pH range of 7 to 8.5.
Manures & Fertilizers
The application of FYM @ 10 t/ha for drylands and 25 t/ha in irrigated areas plus 50 Kg Ammonium Sulphate at the time of planting and remaining 50 Kg at knee height stage (30-35 DAS) proves beneficial (Top dressing only when moisture is adequate). Nitrogenous fertilizers increased forage yields significantly. Forage yield is good when N coupled with P and K is supplied. Hence fertilization with balanced N, P & K is very important.
In irrigated are 100:80:50 Kg NPK/ha is applied. Source of fertilizer’s are urea, SSP & MOP. For multicut varieties 75-100 Kg urea along with 20 Kg P2O5 and 20 Kg MOP is applied after every cut.
Under dry farming conditions 50:40-80:40 Kg NPK/ha is applied. Instead of fertilizer dressing, 15 to 20 kg urea dissolved in 750 to 1000 lts of water may be sprayed every fortnight after establishment of crop for quick utilization of fertilizers and better succulence of crop. Care is taken not to apply excess of N fertilizers for free accumulation of nitrates.
In case of Zinc deficiency apply 10-15 kg of ZnSO4 or 0.2 % ZnSO4 with half quantity of lime to prevent burning of leaves. Iron chlorosis is a common problem is sorghum as it is very sensitive to iron deficiency. Spraying ferrous sulphate with lime is efficient to overcome deficiency.
Bio Fertilizers- Seed inoculation with Azospirillum brasilense.
Sowing Time-
Sorghum can be grown in Kharif (June-September), Maghi (late Kharif) September - December), Rabi (October-January) and in summer (January - May). Summer crops can be multicut in nature. Sowing of seed crop is so adjusted that it flowers at a time when there is no rains.
Seed Rate-
Since close spacing is necessary seed rate is usually higher 20-25 kg/ha for bold seeded varieties & 10-15 kg/ha small seeded varieties. Sometimes more than 100 kg/ha seed rate is used for producing thin stalked stems for hay making or to feed it without chaffing. In mixtures 25 kg sorghum and 20 Kg of cowpea or guar seeds/ha may be sown either in crosswise lines or 2 lines of sorghum alternated with 2 lines of legumes.
Spacing- For fodder crop spacing is kept Row to row: 25 to 30cm &Plant to plant: 10 to 15 cm.
Plant population-3.33 lakh plants /ha for fodders are considered optimum while for grain sorghum a plant population of 1.5-2.0 lakh plants /ha is considered optimum.
Method of Sowing-
Generally seeds are sown by broadcasting and covered with desi plough or harrow to achieve quick sowing in accordance with monsoon. The seeds must not be sown at 2-5cm depth. If germination is gappy seedlings from thick patches can be uprooted and transplanted in the evening.
Irrigation-
Sorghum is drought tolerant. An irrigated fodder crop of sorghum needs about 50 to 70cm of water in 5 to 7 irrigations. In hot and dry summers the crop requires irrigation every fortnight, but during rains excess water should be drained out of the field. During the post monsoon season irrigation water may be required every 3 weeks. Early seedling stage and flower primordial stages are considered most critical for moisture stress. Depth of irrigation is generally 6-8cm and water requirement of crop is around 80-480mm. Crop should be irrigated after each cut for better regeneration.
Cutting Management-
At the flowering stage the herbage contains low to moderate amounts of HCN. In young herbage, the HCN content may be as high as 750 ppm or more which is dangerous to animals. The plants at heading stage can be safely grazed or fed. So harvesting is to be done after 50 % flowering. After this stage crude protein content, digestibility, Ca & P content decreases sharply however crude fibre content increases. The stems become hard and woody with maturity. In multicut varieties, the first cut is taken after 2 months and subsequent cuts after 35 to 40 days. A long duration variety cut 2 to 3 times at 15 cm, better yields of quality forage then cut once at flowering. Crop should be cut 5-8 cm above the ground level for fast sprouting.
Sorghum Poisoning-
Young sorghum plants (30-40 days old) contain poisonous chemical cynogenic glucoside ‘Dhurrin’. ‘Dhurrin’ in the stomach of animals is converted to hydrocyanic acid. Thus when cattle, sheep and goats graze such green and growing sorghum crop (about 5 Kg) may result in hydrocyanic acid (HCN) or Prussic acid poisoning leading to carcinogenic death of animal. HCN content is more in leaves. Concentration of HCN is more in morning and in summers. Danger of HCN poisoning is greatest at immature stages of sorghum growth and decreases with maturity. The toxic limit of HCN is 200ppm, concentration decreases after 50 days. Hay and silage are generally free from HCN.HCN levels are most potent in:
young plants under 6 weeks old;
young regrowth;
plants stressed due to drought;
high soil nitrogen level;
Imbalance between soil nitrogen and phosphorus.
Generally, cutting the plants and chopping, and sun-curing can reduce HCN levels, because the HCN can evaporate with the loss in forage moisture.
Management to reduce HCN poisoning:
Do not turn hungry livestock out on lush regrowth of sorghum. Feed some dry fodder first, then turn out in the afternoon;
Do not graze until the regrowth is 18 to 24 inches tall;
Drying or ensiling or allowing the forage to mature will reduce its HCN content.
Sorghum or Yellow Effect or Soil Sickness-
The nutrient status of soil is exhausted by growing sorghum. The crop residue of sorghum having wide C: N ratio takes a long time to decompose. During this process of decomposition, part of soil nitrogen is temporarily immobilized affecting the succeeding crop. Such effect is more pronounced in low fertile soil causing to temporary deficiency of nitrogen in the soil for succeeding crop. To reduce sorghum effect, 25% more nitrogen is applied at the time of first fertilizer dose of the succeeding crop. It hastens the process of decomposition and overcomes the immobilized nitrogen. Ragi is the exception because its residue decomposes rapidly resulting in mineralization of soil nitrogen.
Weeds & Weed Control-
Sorghum is badly infested with grassy and broad leaf weeds because of congenial weather. 15-45 DAS of crop growth is critical period of crop weed competition. Sorghum halepense barru mimics with sorghum particularly before appearance of inflorescence.Inter row weeds can be controlled mechanically by running blade harrow, but intra row weeds remain.Striga (Striga lutea) causes 15-100 % loss depending on severity of infestation.
Constraints of Production-
Lack of demand for human consumption, non availability of improved seeds, cultivation of crop on poor soils and low acerage under HYV. Poor emphasis on plant protection and moisture stress are other factors leading to poor yields.
Annual, erect, single stalked or tillering; leaves alternate, ligules short membranous, inflorescence a panicle, contracted or loose, spikelets in clusters of 2 or 3 one sessile and hermaphrodite and the other/(s) pedicelled staminate or sterile; stamens three; stigmas two; styles terminal; fruit is a caryopsis.
Growth Stages-
Sorghum has five distinct growth stages:
1. Seedling Stage- Germination takes place in 4-5DAS. Emergence of coleoptiles indicates seedling stage.
2. Flag Leaf Stage-3-4 leaves with fully expended leaf area. Stage reaches in 40-55 DAS.
3. Boot Stage-Ear head covered within sheath i.e. under flag leaf stage reaches in 45-60 DAS.
4. Soft Dough-Endosperm filled with watery fluid. It’s called milky stage. Stage reaches in 70-85DAS.
5. Physiological Maturity- Grain with maximum accumulation of dry matter. Stage reaches in 85-95DAS.
Climate:-
It is a plant of warm climate. It remains dormant under unfavorable conditions, leaves posses waxy coating and presence of motor cells in leaves rolls the leaves under moisture deficit conditions. It is also posses more no of secondary roots/ unit of primary roots. Thus crop can withstand drought better than maize. Crop is grown in areas receiving 800-1000mm annual rainfall isohyets. High rainfall is undesirable though crop can withstand water logging. It is a Short day plant and a 10 hr photoperiod enhances early flowering. Increase in day length delay flowering and benefit dry matter production. During drought the cyanide content remains high.
Minimum temperature required for germination 7-10 degree C
Optimum temperature for growth 25-30 degree C
Soil Requirement-
The crop grows on every type of soil except very sandy soil. It is predominantly grown in vertisols. As a rainfed crop it is better suited to heavier soils retentive of moisture. It grows best in deep alluvial soils with irrigation. Black cotton soils of central India are considered as best soils for its cultivation. It can withstand moderate salinity.
Field Preparation-
Sorghum is a shallow rooted crop and does not require deep cultivation. 2 to 4 operations by desi plough or blade harrow, alternatively 2 to 4 times harrowing to pulverize the soil to a fine tith OR deep summer ploughing with mould board plough and two ploughings by country plough with the onset of monsoon are enough to prepare the land for sowing. Crop is grown in pH range of 7 to 8.5.
Manures & Fertilizers
The application of FYM @ 10 t/ha for drylands and 25 t/ha in irrigated areas plus 50 Kg Ammonium Sulphate at the time of planting and remaining 50 Kg at knee height stage (30-35 DAS) proves beneficial (Top dressing only when moisture is adequate). Nitrogenous fertilizers increased forage yields significantly. Forage yield is good when N coupled with P and K is supplied. Hence fertilization with balanced N, P & K is very important.
In irrigated are 100:80:50 Kg NPK/ha is applied. Source of fertilizer’s are urea, SSP & MOP. For multicut varieties 75-100 Kg urea along with 20 Kg P2O5 and 20 Kg MOP is applied after every cut.
Under dry farming conditions 50:40-80:40 Kg NPK/ha is applied. Instead of fertilizer dressing, 15 to 20 kg urea dissolved in 750 to 1000 lts of water may be sprayed every fortnight after establishment of crop for quick utilization of fertilizers and better succulence of crop. Care is taken not to apply excess of N fertilizers for free accumulation of nitrates.
In case of Zinc deficiency apply 10-15 kg of ZnSO4 or 0.2 % ZnSO4 with half quantity of lime to prevent burning of leaves. Iron chlorosis is a common problem is sorghum as it is very sensitive to iron deficiency. Spraying ferrous sulphate with lime is efficient to overcome deficiency.
Bio Fertilizers- Seed inoculation with Azospirillum brasilense.
Sowing Time-
Sorghum can be grown in Kharif (June-September), Maghi (late Kharif) September - December), Rabi (October-January) and in summer (January - May). Summer crops can be multicut in nature. Sowing of seed crop is so adjusted that it flowers at a time when there is no rains.
Seed Rate-
Since close spacing is necessary seed rate is usually higher 20-25 kg/ha for bold seeded varieties & 10-15 kg/ha small seeded varieties. Sometimes more than 100 kg/ha seed rate is used for producing thin stalked stems for hay making or to feed it without chaffing. In mixtures 25 kg sorghum and 20 Kg of cowpea or guar seeds/ha may be sown either in crosswise lines or 2 lines of sorghum alternated with 2 lines of legumes.
Spacing- For fodder crop spacing is kept Row to row: 25 to 30cm &Plant to plant: 10 to 15 cm.
Plant population-3.33 lakh plants /ha for fodders are considered optimum while for grain sorghum a plant population of 1.5-2.0 lakh plants /ha is considered optimum.
Method of Sowing-
Generally seeds are sown by broadcasting and covered with desi plough or harrow to achieve quick sowing in accordance with monsoon. The seeds must not be sown at 2-5cm depth. If germination is gappy seedlings from thick patches can be uprooted and transplanted in the evening.
Irrigation-
Sorghum is drought tolerant. An irrigated fodder crop of sorghum needs about 50 to 70cm of water in 5 to 7 irrigations. In hot and dry summers the crop requires irrigation every fortnight, but during rains excess water should be drained out of the field. During the post monsoon season irrigation water may be required every 3 weeks. Early seedling stage and flower primordial stages are considered most critical for moisture stress. Depth of irrigation is generally 6-8cm and water requirement of crop is around 80-480mm. Crop should be irrigated after each cut for better regeneration.
Cutting Management-
At the flowering stage the herbage contains low to moderate amounts of HCN. In young herbage, the HCN content may be as high as 750 ppm or more which is dangerous to animals. The plants at heading stage can be safely grazed or fed. So harvesting is to be done after 50 % flowering. After this stage crude protein content, digestibility, Ca & P content decreases sharply however crude fibre content increases. The stems become hard and woody with maturity. In multicut varieties, the first cut is taken after 2 months and subsequent cuts after 35 to 40 days. A long duration variety cut 2 to 3 times at 15 cm, better yields of quality forage then cut once at flowering. Crop should be cut 5-8 cm above the ground level for fast sprouting.
Sorghum Poisoning-
Young sorghum plants (30-40 days old) contain poisonous chemical cynogenic glucoside ‘Dhurrin’. ‘Dhurrin’ in the stomach of animals is converted to hydrocyanic acid. Thus when cattle, sheep and goats graze such green and growing sorghum crop (about 5 Kg) may result in hydrocyanic acid (HCN) or Prussic acid poisoning leading to carcinogenic death of animal. HCN content is more in leaves. Concentration of HCN is more in morning and in summers. Danger of HCN poisoning is greatest at immature stages of sorghum growth and decreases with maturity. The toxic limit of HCN is 200ppm, concentration decreases after 50 days. Hay and silage are generally free from HCN.HCN levels are most potent in:
young plants under 6 weeks old;
young regrowth;
plants stressed due to drought;
high soil nitrogen level;
Imbalance between soil nitrogen and phosphorus.
Generally, cutting the plants and chopping, and sun-curing can reduce HCN levels, because the HCN can evaporate with the loss in forage moisture.
Management to reduce HCN poisoning:
Do not turn hungry livestock out on lush regrowth of sorghum. Feed some dry fodder first, then turn out in the afternoon;
Do not graze until the regrowth is 18 to 24 inches tall;
Drying or ensiling or allowing the forage to mature will reduce its HCN content.
Sorghum or Yellow Effect or Soil Sickness-
The nutrient status of soil is exhausted by growing sorghum. The crop residue of sorghum having wide C: N ratio takes a long time to decompose. During this process of decomposition, part of soil nitrogen is temporarily immobilized affecting the succeeding crop. Such effect is more pronounced in low fertile soil causing to temporary deficiency of nitrogen in the soil for succeeding crop. To reduce sorghum effect, 25% more nitrogen is applied at the time of first fertilizer dose of the succeeding crop. It hastens the process of decomposition and overcomes the immobilized nitrogen. Ragi is the exception because its residue decomposes rapidly resulting in mineralization of soil nitrogen.
Weeds & Weed Control-
Sorghum is badly infested with grassy and broad leaf weeds because of congenial weather. 15-45 DAS of crop growth is critical period of crop weed competition. Sorghum halepense barru mimics with sorghum particularly before appearance of inflorescence.Inter row weeds can be controlled mechanically by running blade harrow, but intra row weeds remain.Striga (Striga lutea) causes 15-100 % loss depending on severity of infestation.
Constraints of Production-
Lack of demand for human consumption, non availability of improved seeds, cultivation of crop on poor soils and low acerage under HYV. Poor emphasis on plant protection and moisture stress are other factors leading to poor yields.
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