TRENDS IN WEED MANAGEMENT

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As we know, Indian economy is basically an agrarian one. Agricultural production depends on various factors and any setback in these factors severely affects yield of crops. To meet the needs of growing population of India, production must be increased. There is little scope for agriculture to expand in horizontal directions, only way is to move up in vertical direction for increasing productivity

Pests are the major threat to the crop plants. Among pests, weeds have been a problem for man ever since he started domestication of crop plants.

Weeds….

  • Weeds are plants out of place
  • It causes enormous losses and suffering to human beings by way of reduction in crop yield and quality, wastage of human energy and increased expenditure to alleviate the problems caused by them.

India – Annual loss due to pests

Pests Percent loss
1995(Gautham and Mishra, 1995) 2011(Online edition of India’s National Newspaper)
Weeds 33 39
Plant diseases 26 26
Insects 20 29
Other 21 6

Need for weed control

  • Weeds compete with the crop plants for nutrients, soil moisture, solar energy, CO2
  • Yield loss will be maximum during the critical periods which can be defined as the shortest span of time in the ontogeny of crop growth during which weeding result in maximum income returns.
  • It degrades native ecosystems, foul waterways, weeds lower property values.

Critical Periods of Crop Weed Competition

Crop Critical Period Yield reduction (%)
Rice 4-6 WAT 30-40
Wheat 30-45 DAS 20-30
Maize 30-45 DAS 50-70
Green gram 15-30 DAS 25-50
Black gram 15-30 DAS 30-50
Soybean 15-30 DAS 40-60
Cotton 15-60 DAS 40-50
Sugarcane Upto 90 DAP 20-30
Potato 20-40 DAS 30-60
 
 
WAT – weeks after transplanting

DAS – Days after sowing

DAP – Days after planting

 

 

 

 

 

 

In India, till seventies, manual and mechanical removal was one of the best options available with the farmers to manage weeds in their fields and this was supplemented by cultural methods. But with the advent of labor costs herbicides started to take shape successful achievements occurred in eighties. Among these cultural method is the most indispensable and effective method. It includes tillage, crop rotation, intercropping, mulching, solarisation etc.

• With the discovery of Bordeaux mixture in 1896, scientists started looking for chemical weed control. After Second World War discovery of 2,4, D and MCPA were announced and this was revolutionized field of chemical weed control

• Now at the world level herbicides rank first when the quality of chemical used is considered.

Type Mill.lb Percentage
Herbicides 2018 39
Insecticides 955 18
Fungicides 519 10
Other 1705 33
Total 5197 100

 

 

 

 

 

 

 
 
Note:

(1) “Herbicides” include herbicides and plant growth regulators.
(2) “Other” includes nematicides, fumigants, and other miscellaneous conventional pesticides,

and other chemicals used as pesticides such as sulfur, petroleum oil, and sulfuric acid.

 

 

Relative percentage of Global Pesticide Use

Problems in Herbicide Usage

1.Shift In Weed Flora

Repeated use of same herbicides year after year in the same crop in the same area leads to shift in weed flora and appearance of resistant weeds. Eg :- Replacement of Phalaris minor in the wheat field with Lathyrus sativa, Convovulus arvensis sp., etc.

2. Herbicide residue problem

Herbicides which persist much longer than desired time several potential environmental problems. Further, they may cause injury to succeeding crop particularly in multiple cropping systems.

Persistence of herbicides in soil

Herbicide Persistence
2,4,D <7 days
Paraquat >1000days
0Glyphosate 3-130 days
Oxiflourfen 30-40 days
Pendimethalin 40 days
Simazine 28-140days
Diuron 9 days to 9 weeks
Sulfonyl urea herbicides 7-16 days

3. New problem weeds

Due to intensification and continuous use of herbicides, some new weed sp. are emerging and posing a challenge.

4. Problem of herbicide resistance by weeds Herbicide resistance is the inherited ability of weed not to be controlled by herbicide.

Eg : Phalaris minor Vs Isoproturon

New Trends in weed management

I. More effective use of herbicide

A) Development of new herbicides

New generation herbicides are more effective which are applied at lower rates and have low mammalian toxicity and reduce the risk of environmental pollution. Application rates have been reduced from 1.8 – 3 kg/ha to 20-25 g/ha.

Chemical Name Trade Name a.i./ha
Bispyribac Sodium TAARAK 30.00
Bispyribac Sodium Adora 30.00
Bispyribac Sodium Nominee Gold 30.00
Carfentrazon Ethyl Affinity 20.00
Chlorimuron 10 Metsulfuron Methyl 10

Chlorimuron 10 Metsulfuranv 10

Almix

Synmuron

2 + 2

2 + 2

Ethoxysulfuran Sunrise 18.75
Fenoxaprop-P-ethyl Puma Super 56.25
Fenoxaprop-P-ethyl Ricestar 56.25
Oxidiargyl Raft 100.00
Oxidiargyl Topstar 100.00
Pyrazo Sulfuran Ethyl Sathi 30.00

Herbicide Mixtures

Combination products consisting of 2 or more herbicides have greater activity on diverse weed flora due to differential mode of action and have become popular in recent years

Eg: Londax power – Bensulfuron methyl + Pretilachlor

Almix – Chlorinmuron ethyl + metsulfuron methyl

Topshot – penoxsulam + cyhalofop-butyl 10+50 g/l

B) Herbicide safeners and adjuvants

They are compounds which protect the crop plants from possible damage by a herbicide. It will permit higher rates of herbicide application for weed control and enable farmers the use of non-selective herbicide for weed control. They act by accelerating herbicide metabolism and detoxification within crop plants. It extends the use of available herbicides on additional crops.

Eg: Dichlormid safens thiocarbamate herbicide in corn and cyclometrinil, flurazole and CGA in sorghum

Adjuvants

Adjuvants are chemicals which have no herbicidal activity as such, but when added to a herbicide enhance the activity of herbicide. It can be added to the product at the time of formulation, or by the applicator to the spray mix just prior to treatment. Adjuvants include surfactants, compatibility agents, anti-foaming agents and spray colorants (dyes) and drift control agents

Eg: addition of adjuvants like fertilizers including NH4SO4, DAP, urea etc. speeds up herbicide absorption by the plant and translocation inside the plant.

New generation adjuvants

• Velox TRS2 – adjuvants for herbicide based on natural plant extracts combining both wetting and surfactant features.

• Axial® herbicide – new formulation with built in adjuvant released by Syngenta Canada, for control of wild oats, green foxtail, barnyard grass, volunteer oats, volunteer canary seed. Active ingredient- Pinoxaden (Group 1)

C) Development of controlled release formulation

Application rates of conventional formulations of pesticides are greater than the minimum threshold concentration to counter losses from absorption, volatilization, photodecomposition, microbial, and chemical degradation and leaching. Controlled release formulation have the potential to reduce pesticide leaching and reduce the risk of ground water contamination (Gautham and Mishra, 1955). Eg: Lac coated 2, 4-D

Herbicides with long soil residual activity can pose a carryover problem when used in annual crops (Hagood et al, 1994). However, long-residual herbicides can be advantageous in perennial crops. Increasing the herbicide rate is one way to increase the length of soil residual for a given herbicide. However, this could increase the potential for crop damage, as well as concerns about off-site movement. Use of slow- release herbicides an alternative way to increase the soil residual (Verma and Smith, 1981)

Slow release tablets

These are a porous pellets containing an inert material such as plaster of paris or dicalcium phosphate, plus a pre emergence herbicide (Koncel et al, 1981). As these tablets are wetted by irrigation, small amounts of the herbicides are gradually released over an extended period of time. The herbicide release can be increased by increasing the herbicide concentration, or by changing the size or porosity of the tablet (smith and Verma, 1977). Cracks developing in a tablet could accelerate the herbicide release rate by increasing the surface area. By retaining a herbicide in the top 0.5inch (1.2 cm) of medium in a container, a longer period of weed control with less leaching should result. This reduction in leaching reduces the potential for root uptake and subsequent crop damage, along with the amount of chemical that could leach out the bottom of the container. Tablets containing metolachlor at 40 kg/ha controlled annual ryegrass better with the same rate applied in granular form. At 150 days after application, the tablet formulation reduced annual rye grass growth by 80%, while the granular formulation of metolachlor reduced shoot weight by only 10%.

Other slow release herbicide formulations

There have been other types of slow release formulations evaluated, some with implications for nursery use. Other controlled- release formulations have used starch xanthide, pine kraft lignin and various polymers to control the herbicide release rate.

eg: An ester formulation of picloram(4-amino-3,5,6-trichloro-2-pyri-dinecarboxylic acid) in a starch xanthide slow- release product had a 3-5 times slower release rate than salt xanthide formulation.

Limited field tests have been shown that controlled-release herbicide-polymer formulations have potential value in aquatic weed control. Formulations of herbicides in poly-vinyl chloride and natural rubber have been used to control aquatic weeds.

Potential advantages of slow- release formulations

Overall herbicide use could be controlled

A minimum threshold concentration needed for weed control could be maintained over an extended period using slow-release technology.

  • Less labor is required because of the requirement of treatments per year.
  • For tablet formulation, no calibration is required
  • No mixing or spraying is required, thus reducing applicator exposure and chemical drift
  • Minimize pesticide residues available to the environment
  • Maintain toxic concentrations of pesticide close proximity to the target organism
  • Increase the efficacy and longevity of the pesticide by protecting it from environmental degradation
  • Decrease application costs because less frequent applications are required

Disadvantages

  • Uniform herbicide distribution from a tablet in a acontainer would be required for acceptable weed control
  • Low water soluble chemicals may not be effective in such formulations because the chemical must move laterally from the tablet to cover the medium surface.
  • The release rate must be sufficient to maintain a minimum threshold concentration needed for weed control over an extended period.
  • Marketing and regulatory hurdles also would have to be addressed in the development of slow-release herbicides.

D) Use of post emergent herbicides

Post emergence herbicides are applied to weeds after they have emerged. Since pre emergence herbicides will control emerged weeds these are important. They are generally used at lower rates than pre-emergent herbicides

New post emergent herbicides. Eg: Drive LRS, RoundUp ProMaxx,Celsius

II. Biological Control

It is the deliberate use of living organisms such as natural enemies, primarily insects or fungi to suppress growth or reduce the population of weeds sp. Foreign and native organisms that stack weeds are being evaluated for use as biological control agents.

Advantages

• High degree of specificity to the target organisms

• Absence of weed resistance development

• No effect on target organism

• Absence of residue build up on soil and environment.

• Environmental friendly approach

• Safe for applicators and consumers.

The cost of developing and conducting a biological control program varies with the target weed and the strategy selected. On an average, a biological control program will cost about $4 million. But every dollar spent in development returns at least $50 benefit.

Biological control includes both Bioherbicidal approach and classical approach.

A) Classical approach

Weeds introduced from foreign countries often require a different strategy. Insects and pathogens are collected in the area of origin and evaluated for release in North America. Insect agents often require a number of years to become fully effective. Their growth is often hindered by adverse climatic conditions. Long term monitoring is needed to determine their effectiveness. The release of biological control organisms in such manner is termed as Classical approach to biological control. It relies on the ability of the organism to multiply and spread following small scale release. The organism then remain in balance with the target weed, keeping the latter at acceptably low level. Fungi that naturally spread and infect weeds can also be used in a classical biological control strategy.

First recorded attempt of using insects to control weeds was done in 1920’s to control Lantana camera using lacebug Telenomia scrupulosa. It successfully established and controlling lantana partially.Thakur et al. (1992) recommended 3 indigenous insects as potential biocontrol agents for Lantana, viz. a flower feeder Asphondylia lantanae, flower an leaf defoliator Hypena laceratalis and a borer of ripe fruits, Homona micacerana. Mexican beetle (Zygrogramma bicolorata) caused severe impact in suppressing of Parthenium in and around Banglore (Jayanth, 1993). But later these beetle was found feeding on sunflower crop and xanthium. Jayanth et al (1997) on the basis of experiments conducted argued that Z. bicoclorata is unlikely to become a pest of sunflower.

In India, efforts are being made to control some of the aquatic weeds by use of some insects out, indigenous insects were found of little use.Water hyacinth (Eichhornia crassipes) and water – fern ( Salvinia molesta) – 2 important noxious aquatic weeds, as they are capable of colonizing large areas of water in a short time. The former can be controlled with Neochitina eichhorniae, N. bruchi and Orthogalumna tetrabrantis, and the latter with Cyrtobagus salviniae.

B) Bioherbicide approach

Some bacteria and fungi applied as biological control agents do not survive from year to year. These organisms must be applied in annual basis. This technique is called Bioherbicide strategy. The term means microbial plant pathogens which are applied as sprays that uniformly kill or suppress the growth of weeds. Biological agents are used in a manner similar to chemical herbicides.

Advantages

• Weed control agents utilizes naturally occurring enemies, rather than depending on man-made chemicals. It is important because agents of biological control ordinarily have many fewer, and much milder, effects on the environment than do synthetic chemicals.

• They tend not to lead to the public health problems that chemicals are associated with.

• A bio herbicide based on a fungus is called a mycoherbicide. Fungal pathogens that are virulent (capable of causing injury), host specific and genetically stable but constrained naturally by low inoculum production and poor dissemination are good candidates for development as mycoherbicides.

• In the industry, bioherbicides and other biopesticides are often referred to as “naturals”

• Utilization of this innovative technology for weed management has resulted in the commercialization of registration of 2 fungi as De Vine® (Phytophhtora palmivora) and Collego ® (Colletotrichum gloeosporioides)

Works are being conducted at Department of Plant Pathology to identify pathogens for control of water hyacinth.

• Susha (1997) identified 3 fungi suitable for our condition. They are Colletotrichum gloeosporiodes, Fusarium equiseii and Fusarium pallidoroseum.

• Saleem and Naseema (2005) evaluated the efficiency of spore suspensions and cell-free metabolites of F. pallidoroseum

• Preveena et al. (2006) analyzed bioherbicidal potential of phytotoxin produced by F. pallidoroseum

• For mass production of mycoherbicides, Praaveena and Naseema (2008) conducted a study to determine the effects of liquid substrates, which showed that wettable powder was best effective formulation with increased shelf life. The efficacy of wettable powder formulation could be enhanced by pre spraying water hyacinth with CNSL.

Drawbacks of production and use of bioherbicides include..

  • It have to be registered with the Environmental Protection Agency (E.P.A) and the registration process may be lengthy.
  • Suppression or killing of weeds by bio herbicides may be a slow process.
  • Stability under field conditions highly dependent on environmental conditions
  • Production of bio herbicides for large scale application may be an expensive process.
  • Due to high degree of specificity of these agents numerous fungi need to be discovered and developed.

Use of Aquatic mammals and rodents

Introduction of Manatee (Trichechus inunguis) and the rodent (Myocastou coypus) both known to feed on aquatic vegetation had earlier been suggested as possible bio control agents against aquatic weeds . But the slow reproductive rate of the former and the omnivorous feeding of the latter have discarded their trials.

Use of Fish

¡ The herbivorous fish Tilapia sp. is used for controlling algae- Chara sp.

¡ White amour or grass carp ,Tenopharyncodon controls aquatic weeds.

¡ A non herbivorous carp Cyprinus carpio used in the control of submerged aquatic weeds.

III. Biotechnological Applications

In broad sense, biotechnology is commonly defined as “the application of biological organisms (Micro organisms, plants, animals), systems or processes to provide desirable goods and services”.

a) Use of naturally occurring herbicides

Approach based on fermenting bacteria and fungi, testing fermentation broths for activity, and isolating active compounds from these broths. Pesticides of microbial origin (Agricultural antibiotics) are highly specific to target organisms and supposed to be inherently biodegradable because they are synthesized biologically. Of the naturally occurring compounds, BIALOPHOS is currently marketed under the trade name Herbiaceae® as commercially developed herbicide in Japan. Microbial product isolated from the fermentation broth of Streptomyces hygroscopicus and exhibits strong microbial activity against wide spectrum of grass and broad leaf weeds following the application to their foliage.

b) Synthetic derivatives of naturally occurring compounds

Many naturally occurring phytotoxins have toxicity, limited crop selectivity and instability under field conditions. These problems can be overcome by synthesis of more selective and stable analogue of these chemicals.

c) Herbicide resistance in crop plants

Ability, trait or quality of a population of plants with in a species or larger taxon, or of plant cells in culture, to withstand a particular herbicide at a dosage that is substantially greater than the wild type of that plant is able to withstand with a near normal lifecycle. Before the emergence of plant genetic engineering, Specific herbicides could be used in the crops that were naturally resistant to the herbicide. For example, monocots are naturally resistant to triazine and hence triazine could be used as selective herbicide in monocot crops to control dicot weeds. In rare cases, resistance could be induced in crop varieties through mutations (Clearfield technique)

Genetic engineering of herbicide tolerance

Isolation and introduction of a gene from another organisms, mostly bacteria, which is able to overcome the herbicide-induced metabolic blockage. For example, tolerance to the herbicide glufosinate (Basta R) is conferred by the bacterial gene bar, which metabolizes the herbicide into a non-toxic compound.

Genetically engineered microorganisms may be useful for solving a number of other difficulties involved in modern weed management and herbicide technology. These areas include

1.Microbial degradation of soil applied herbicides.

2.Development and use of microbial herbicide safeners.

3.Use of micro organisms as biocatalysts in the production of synthetic herbicides.

Roundup Brand Herbicides

Roundup brand herbicides are non-selective herbicide mixtures of glyphosate, water and a patented surfactant system. Glyphosate resistant crops : soybean and canola in 1996, cotton in1997 and maize in 1999. The use of Roundup® brand agricultural herbicides on Roundup Ready® crops has allowed farmers to conserve fuel and decrease the overall amount of agricultural herbicides used. Roundup agricultural herbicides can be sprayed in crop from emergence to flowering for unsurpassed weed control, proven crop safety and maximum yield potential

IV. Allelopathy and associated effects

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil surface. Allelochemicals are present in all types of plants and tissues and are released in to the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization and root exudation. Under the appropriate environmental conditions, these phytotoxins may be released to the environment in sufficient quantities to affect the growth of neighboring plants. Allelochemicals structures and modes of action are diverse, and may offer potential for development of future herbicides.

Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are more highly weed suppressive in managed settings.

Allelopathic potential for cover crops

Legume cover crops have the ability to fix atmospheric nitrogen that potentially provides a nitrogen source to the subsequent crop without need for additional fertilizer application. In addition, they provides a weed control potential. Due to the rapid degradation of legume residue on the soil surface in comparison to cereal grain residue, weed control through a physical barrier may not last as long into the season as other cover crops. Determining allelopathic effects of legume crops extracts concluded that legume covers did inhibit and cotton radicle elongation; however, cotton root exhibited less inhibition than that of radish for all included crops. While comparing the hairy vetch and winter pea, latter had the least effect on germinating seedlings. Different varieties of cover crops are available for use in agricultural systems and the varieties of one species may differ in level of Allelopathy. Although under field conditions, allelopathic performance of these species may fluctuate, it is apparent that these cover crops can provide additional weed control measures over systems that do not include a cover crop. Cereal grain crops such as black oat (Avena strigosa Schreb), rye, triticale and wheat are utilized frequently in conservation systems as cover crops with effective ground cover and weed suppression. Rye is a commonly used cereal cover crop due to its ability to be sown later in the season while maintaining successful growth and its biomass production capability, which increases allelopathic effect also. Cereal crop decay more slowly providing some ground cover and allelochemicals release, further into the growing season.

Allelopathic potentiality of crop plants

Different crops such as beet (Beta vulgaris L.), lupin (Lupinus lutens L.), maize (Zea mays L.), wheat (Triticum aestivum L.), oats (Avena sativa L.) and barley (Hordeum vulgare L.) are known to have allelopathic effect on other crops (Rice, 1984b).

Table

Crops Scientific Name Allelochemicals
Rice Oryza sativa L. Phenolic acids
Wheat Triticum aestivum L. Hydroxamic acids
Cucumber Cucumis sativus L. Benzoic and Cinnamic acids
Black mustard Brassica nigra L. Allyl isothiocyanate
Buck wheat Fagopyrium esculentum L. Fatty acids
Clovers and Sweet clover Trifolium spp.

Melilotus spp.

Isoflavonoids and

Phenolics

Oat Avena sativa L Phenolic acids

& Scopoletin

Sorghum Sorghum bicolor L. Sorgoleone
Cereals   Hydroxamic acids

RICE (Oryza sativa L.)

  • Chung et al. (2003) described the effect of allelopathic potential of rice (Oryza sativa L.) residues against Echinochloa crusgalli P. Beauv. var. oryzi-cola Ohwi (barnyardgrass), an associated weed of paddy.
  • Early and late maturing varieties showed less inhibitory effect of 50.2% and 56.1% respectively and intermediate rice varieties with 59.3% inhibition
  • Both laboratory screening and field experiments reveal that rice allelopathy is active against both monocot and dicot weeds (Olofsdotter and Navarez, 1996).
  • A rice cultivar (Taichung Native 1) has also shown activity against most of the weeds including barnyardgrass, desert horsepurslane (Trianthema portulacastrum L.), ducksalad, and toothcup (Ammannia coccinea Rottb.) (Dilday et al., 1998; Olofsdotter and Navarez, 1996), and is therefore considered to be a suitable choice for both identifying allelochemicals and studying allelopathy genetics (Olofsdotter, 2001).

• In Philippines, 111 rice cultivars have been evaluated for weed suppression capability against barnyardgrass under field conditions over three seasons claimed that allelopathy can give 34% of the reduction in total weed dry weight after 8 wks of seeding.

• There appears to be a higher frequency of allelopathic varieties among tropical Japonicas within Oryza sativa and among O. glaberrima accessions than in other varietal groups

• Microscope studies revealed that allelopathic rice cultivars seem to inhibit secondary growth in barnyard grass roots besides reducing root elongation

• Numerous phytotoxins such as cytokinins, diterpenoids, fatty acids, flavones, glucopyranosides, indoles, momilactones (A and B), oryzalexins, phenols, phenolic acids, resorcinols and stigmastanols have been identified as growth inhibitors in rice.

WHEAT

• Oueslati (2003) examined the allelopathic effect of diluted extracts of roots, leaves and stems of two durum wheat varieties viz., Karim and Om rabi on barley (variety Manel) and bread wheat (variety Ariana). Guenzi and McCalla (1966) found phytotoxicity of phenolic acids, particularly p-coumaric acid, from residues of wheat and other cereals.

• Allelopathic effect of wheat straw to corn (Zea mays L.) and cotton (Gossypium hirsutum L.) (Hicks et al., 1989) was also reported by Opoku et al. (1997).

SORGHUM (Sorghum bicolor L.)

Netzley and Butler (1986) isolated sorgoleone {2- hydroxy- 5- methoxy- 3- [(8’Z, ll’Z)- 8′ ,1l’ ,14′ – pentadecatriene]- p- benzoquinon} from hydrophobic root exudates of sorghum. Sorgoleone, the major pbenzoquinone, and three other structurally related minor p-benzoquinones together constitute 90% or more of the root exudates (Netzly et al., 1988). According to Cheema (1988) nine water soluble allelochemicals of sorghum (Sorghum bicolor L.) are phytotoxic to the growth of certain weeds like Phalaris minor Retz., Chenopodium album L., Rumex dentatus L. and Convolvulus arvensis L. He also found that incorporation of sorghum roots into soil suppressed the weed biomass by 25–50% and increased wheat yields by 7–8%. A single spray of 5% sorgaab (water extract of mature stalk of Sorghum bicolor L. Moench plants obtained after soaking in water for 24 h and sprayed as a natural herbicide) solution applied 30 days after sowing increased wheat yields by 14% and suppressed weed biomass by 20–40% (Cheema et al., 1997).

BLACK MUSTARD (Brassica nigra L.)

Brassica spp. contains high amounts of glucosinolates (Fenwick et al., 1983). Allelopathic effect of extracts of different plant parts like leaf, stem, flower and root of black mustard was experimented. These authors found that germination and radicle length were affected by extract solutions and the inhibitory effect on germination increased with increasing concentration of extract solution of the fresh plant parts .They also observed that the protease enzyme activity was suppressed causing reduced water uptake, which led to poor seed germination of wild oat. They found that residue incorporation affected the germination, plant height and dry matter accumulation per plant and the effect was greater for both root and shoot incorporation than only root incorporation.

LEGUMES

• Allelopathic effect of aqueous extracts of perennial legume Pueraria thunbergiana leaves on the germination and growth of lettuce was reported by Fujii (1994).

• Caamal-Maldonado et al. (2001) examined the toxic effect of four legumes velvetbean (Mucuna deeringiana (Bort) Merr.), jackbean (Canavalia ensiformis (L.) DC.), jumbiebean (Leucaena leucocephala (Lam.) de Wit), and wild tamarind (Lysiloma latisiliquum (L.) Benth.) on growth of three weeds viz., barnyardgrass (Echinochloa crusgalli L. P. Beauv.), alegría and amaranth (Amaranthus hypochondriacus L.). The aqueous leachates (1%) of all four legumes exhibited strong phytotoxic effect on the radicle growth of the weeds.

CUCUMBER (Cucumis sativus L.)

One of the first studies on varietal differences in allelopathic strength was made in cucumber, Cucumis sativus. A screening of 526 cucumber accessions, originating from 41 countries, revealed several accessions showing strong growth inhibition of Panicum miliaceum and Brassica hirta. In the experiment, 26 accessions caused 50-87% growth inhibition of the species tested (Putnam and Duke, 1974).

ALLELOPATHIC POTENTIALITY OF WEEDS

Many weeds are now achieving importance as an agent of weed control for having special types of allelochemicals. These allelochemicals are capable of suppressing germination and growth of several other weeds, some of which are herbicide resistant.

CONGRESS GRASS (Parthenium hysterophorus L.)

The allelopathic effect of this weed is mainly due to the presence of parthenin, a sesquiterpene lactone of pseudoguanolide nature in various parts of the plant (Kanchan and Jayachandra, 1980b; Kohli et al., 1993; de la Fuente et al., 2000), having greatest concentration in the leaves followed by inflorescence, fruits, roots and stems (Kanchan, 1975). Parthenin is known to have specific inhibitory effects on root and shoot growth of Crotalaria mucronata L., Cassia tora L., Oscimum basilicum L., Oscimum americanum L. and barley (Hordeum vulgare L.) (Khosla and Sobti. 1979, 1981).

CHENOPODIACEA SPECIES
They found that germination of lettuce seed was inhibited at concentrations ranging from 3.12 and 6.26 g l-1.

CANARY GRASS (Phalaris minor Retz.)

Allelopathic potentiality is in the following order: Chenopodium album L.< Medicago denticulate L.< Melilotus indica L.< Convolvulus arvensis L. (inhibiting 100% germination over control) < Vicia hirsute L. (inhibited 86.33% germination) < Cirsium arvense L. (47.85% inhibition) < Lathyrus aphaca L. (37.98%) < Rumex acetosella L. (9.36%).

Allelopathy In India

• Kathiresan (2005) reported that dried powder of the leaves of Coleus amboinicus at 40g/l as a water suspension killed water hyacinth within 25 hours reducing the fresh weight by 80.72% and the dry weight by 75.63% within one week.

• Kathiresan and Dhavabharathi (2008) screened residues of 60 rice cultivars for their allelopathic inhibition on water hyacinth in lab bioassays as well as in micro pond tests. The cultivar BBT proved highly allelopathic and caused reduction of 45.67% in lab assays in micro ponds. Cultivar ADT-36 was moderately allelopathic and reduced weed biomass by 33.4 and 32% in lab bioassay and micro pond, respectively.

Constraints in the utilization of allelochemicals

  • Not stable, highly photo decomposing when applied to field situations.
  • Allelochemicals are not as phytotoxic as commercial herbicide and they are inactivated quickly in soil.
  • When applied to the crop environment, crop plants are also bound to absorb the applied chemicals and store in root, leaf, economic parts and may prove carcinogenic in the long run.
  • Allelochemicals of higher plants are not selective and cause auto toxicity to donor plants.
  • Identification and extraction of allelochemicals is difficult
  • Many allelochemicals are very much expensive to synthesize inspite of having excellent herbicidal properties as for example, tentoxin
  • Some allelochemicals are toxic to human beings and are carcinogenic, e. g. AAL-toxin and fumonisin are toxic to mammalian cells. Sorgoleone, for example, is reported to cause dermatitis

V. Integrated Weed Management System

What is IWM?

Integrated Weed Management (IWM) is the combination of multiple management tools to reduce a weed population to an acceptable level while preserving the quality of existing habitat, water, and other natural resources.IWM is a science based decision making process, which coordinated the use of environmental information, weed biology and ecology,and all available technologies to control weeds by the most economical means (Sanyal, 2008).

Advantages

  • By using several techniques to control weeds we reduce the chance that weed species will adapt to the control techniques, which is likely if only one technique is used.
  • The long term approach to integrated weed management should reduce the extent of weeds and reduce the weed seed stock in the soil. The development of both species shift and herbicide resistance can be effectively managed by the practice of Integrated Weed Management (IWM).
  • IWM provides a framework for effective and sustainable weed management that ultimately prevents weeds from propagating (setting seed or vegetatively reproducing).
  • This will lead to a reduction in the weed population over time, resulting in reduced crop competition and an improvement in crop productivity.

Different components in IWM

a) Weed prevention

Weed prevention comprises all measures which deny the entry and establishment of weed is an area.

• prevention of weeds that disperse with crop seeds can be achieved in 2 ways.

• By production of weed free crop seeds at govt farms or at farmers’ field itself

• By cleaning the crop seeds of weeds before storage as well as at the time of sowing.

b) Weed control

Weed control is the process of limiting weed infestation so that crops could be grown profitably and other activities of man conducted efficiently.

Crop husbandry control of weeds (Ecological control of weed)/ cultural method

1. Proper crop stand and early seedling vigour

2. Selective crop stimulation

3. Proper planting method

4. Proper planting time.

5. Crop rotation

6. Stale seed bed

7. Smother cropping

8. Summer fallowing

9. Minimum tillage

10. Lowering area under bunds

11. Flooding and drainage

12. Ecofallow

c) Physical control

Physical control is the removal of weeds by physical or mechanical means, such as mowing, grazing, mulching, tilling, burning or by hand. The method used often depends on the area of weeds to be managed, what the land is used for, physical characteristics and the value of the land.

Methods :

Mowing, cutting, chaining and dredging, Soil solarisation, Burning & flaming, Hand weeding, Tillage systems, Mulching.

d) Biological Control

e) Chemical control (Herbicidal control)

Emerging Technologies In Weed Management

Laser Treatment for weed control

A laser beam directed towards weeds can be an efficient weed control method as an alternative to herbicides. Lasers may deliver high-density energy to selected plant material, raising the temperature of the water in the plant cells and thereby stop or delay the growth. A commercial use of lasers for weed control, however, require a systematic investigation of the relationship between energy density and the biological effect on different weed species, growth stages, etc.was obtained using the 5W, 532nm laser and 18mm spot diameter. Laser exposure of the apical meristem of weed species can be used as a method of physical weed control. The efficiency of the laser weed control is related to wavelength, exposure time, spot size and laser power. The efficiency also varies between weed species. It is possible to improve the laser application method and to obtain a better performance by increasing the laser power and exposure time. The experiment also indicated that the efficacy can be improved by a proper selection of wavelength and spot diameter. In order to improve the performance and to document the efficacy on a broader spectrum of weed species and growth stages, further research and development are needed.

Nanoherbicides

Herbicides are encapsulated in nano particles. This can serve as magic bullets which can target particular plant parts to release their contents because they are very tiny (10,000 mm) and are able to blend with the soil prevent weed seed germination while destroying weed seeds.

“Nano-size Manganese Carbonate hollow cores are made and coated with water-soluble and bio-degradable polymers. Then a hole is made in the core into which the herbicide is filled. These cores are applied to the soil after sowing. They remain in the soil for a long time and the core cracks open to release the herbicide when there is moisture. The shell is of 40 to 80 nano metre size and the herbicide is of 16.9 nano metre size. It is being tested in laboratory conditions for resistance to light, temperature and microbes. Once the laboratory testing is complete, it will be experimented on the field to check for release pattern for at least two years

Advantages

• The nano-herbicide is expected to increase the yield of these crops considerably by destroying weeds effectively.

• It is expected to save many man hours involved in manual weeding.

• Since farmers under rain-fed areas are found to be economically week and unable to invest in weed management, chemical weeding using nano-herbicides is seen as an economically viable alternative.

Conclusion

No single weed control technique is perfect because weed population constantly adapts to its physical environment. There is no need to reduce dependability on herbicides. Weed control strategies must balance the use of herbicides with environmental protection and the production of food safe for human consumption. So, new weed management strategies must be found which are cost effective, environmentally friendly to sustain food production.

Reference

Bhadoria, P. B. S. 2010. Allelopathy: A natural way towards weed management. American J. Experimental Agrl.1 (1) :7-20.

Derr, J. F. 1994. Innovative herbicide application methods and their potential for use in the nursery and landscape industries. HortTechnology.4 (4) :345-350.

Harris, F.W. Controlled release herbicide-polymer formulations for aquatic weed control.

Mathiassen, S. K., Bak, T., Chritensen, S. and Kudsk, P. 2006. The effect of laser treatment as a weed control method. Biosystems Engineering. 95 (4) : 497-505.

Sushilkumar. 2011. Aquatic weed problems and management in India. Indian J. Weed Sci. 43 (3&4): 118-138.

Weston, L. A. 2005. History and current trends in the use of Allelopathy for weed management. HortTechnology. 15 (3) :529-534.

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