Weed Management

Weed management is an important component of plant protection improving the production potential of crops. It includes management of the weeds in a way that the crop sustains its production potential without being harmed by the weeds. Weeds are the plants that grow without being harmed by the weeds. Weeds are the plants that grow without human efforts and are not wanted. They grow in the fields where they compete with crops for water soil, nutrients, light, and space, and thus reduce the crop yields. In the high-yielding cropping systems where heavy inputs are utilized for obtaining higher productivity, weed management assumes a much greater importance. Proper weed management is a pre-requisite for obtaining higher input efficiency. Weeds are also act as alternate hosts that harbor insects, pests and diseases and other micro-organisms. Some weeds release into the soil inhibitors of poisonous substances that may be harmful to the crop plants, human brings and livestock. Weeds reduce the quality of marketable agricultural produce and render harvesting difficult, leading to increased expenditure on labour, equipment and chemicals for their removal. In aquatic environment, weeds block the flow of water in canals, water-transport system and drainage system, rendering navigation difficult. The dense growth of aquatic weeds pollutes water by deoxygenating it and killing the fished. Weeds are also a nuisance and a fire hazard along railway lines, roads, right-of- ways, airports, forest and industrial sites.

Weed management is done through the mechanical, cultural and chemical means. Use of biological control methods in field crops is being considered, but still not much in use. Use of herbicides is an important method in the modern concept of much in use. Use of herbicides is an important method in the modern concept of weed-management technology. New hand-tools and implements have also been designed to assist in wed-management programme.

Every crop is exposed to severe competition from weeds. Most of these weeds are self-sown and they provide competition caused by their faster rate of growth in the initial stages of crop growth. In some crops, the yields are reduced by more than 50% due to weed infestation. The looses caused by weeds in some of the important crops are given n table 18.1


Table 18.1 Los in crop yields due to weeds
Crop Reduction in yields due to weeds (%) Crop Reduction in yield due to weeds (%)
Rice 41.6 Groundnut 33.8
Wheat 16.0 Sugarcane 34.2
Maize 39.8 Sugar beet 70.3
Millets 29.5 Carrot 47.5
Soybean 30.5 Cotton 72.5
Gram 11.6 Onion 68.0
Pea 32.9 Potato 20.1
Linseed 34.2


Characteristics of weeds: Weeds are also like other plants but have special characteristics that tend to put them in the category of unwanted plants.

  • Most of the weeds specially annuals produce enormous quantity of seeds, e.g. wild oats (Avena fatua), produces 250 seeds per plant, whereas wild amaranth (Amaranthus viridis) produces nearly 11 million seeds. It has been observed that among 61 perennial weeds, the average seed-production capacity was 26,500 per plant.
  • Weeds have the capacity to withstand adverse conditions in the field, because they can modify their seed production and growth according to the availability of moisture and temperature. They can germinate under adverse soil-moisture conditions, have short period of plant growth, generally grow faster rate and produce seed earlier than most of the crops growing in association.
  • Weed seeds remain viable for longer period without losing their viability, e.g. annual meadow grass (Poa annua) and scarlet pimpernel (Anagallis arvensis) remain viable foe about 8 years; creeping thistle (Cirsium arvense) for 20 years and field bind weed (Convolvulus arvensis) for about 50 years.
  • Weed seeds have a tremendous capacity to disperse from one place to another through wind, water and animals including man. Many of times, weed seeds mimic with the crop seeds due to their size and get transported form one place to another along with them.

Classification of weeds

Most weeds occur in the division Anthophyta (Angiosperms, which are further divided into the classes dicotyledones (dicots) and monocotyledons (monocots)).

There are approximately 250000 species of plants in the world. However, less than 250 plant species, about 10%, are troublesome enough to be called universally throughout the world.

On the basis of the habitat, weeds are divided into terrestrial and aquatic categories and on the basis of duration of life, they are divided into annual, biennial and perennial.

Annuals complete their life cycle in a year or in a single growing season. They mostly propagate through seeds. Normally, they germinate with the crop and complete their life cycle before or along with the crop. Based on the season they are encountered, weeds may be called as summer and winter annuals. Some examples are:

Summer Annuals: Echinochloa colona, E. crus-galli, Amaranthus viridis, Trainthema portulacastrum, Eclipta alba, Digitaria sanguinalis, Panicum repens.

Winter Annuals: Phalaris minor, Avena ludoviciana, Poa annua, Chenopodium album, Melilotus indica, Lathyrus aphaca, Anagallis arvensis.

Biennials live longer than 1 year but less than 2 years. During the first growth phase, they develop vegetatively from a seedling into a rosette. After a cold period, the growth resumes into reproductive phase. Biennials are large plants when mature, and have thick fleshy roots. Relatively few weed species are biennials, e.g. Cichorium inttybus, Raphanus raphanistrum.

Perennials live longer than 2 years and may reproduce several times before dying. They are propagated through seeds and vegetative reproductive organs like rhizomes, suckers, bulbs and tubers, e.g. Cynodon dactylon, Saccharum spontaneum, Cyperus rotundus, Sorghum halepense, Convolvulus arvensis and Cirsium arvense.

Aquatic weeds:
They have been categorized further as floating, emerged and submerged.

Floating weeds:
These rests upon the water surface. Their roots hang freely into the water or sometimes attach to the bottom of shallow ponds or streams, e.g. water hyacinth, Pistia, Salvinia, etc.

 Emergent weeds:
Typical plants of marshlands and are often found along the shorelines of ponds and canals. These plants stand erect and are always rooted into very moist soil, e.g. Typha, Scirpus, etc.

Submerged weeds:
Grow completely under water, although a few stems and leaves may exist on the water surface, e.g. Hydrilla, Chara, Potamogeton, etc.

Parasitic weeds:
These weeds parasitize on specific host plants and deprive them of water, nutrients and assimilates. They may be root parasites (Orobanche, Striga) or shoot parasites (Cuscuta). They are either partial parasites (Striga) or total parasites (Orobanche), depending on the degree of parasitism.

Crop-weed competition:
Weeds compete with crop plants for nutrients, moisture and light. The degree of competition is dependent on weed flora, time of weed emergence, relative density of crop plants and weeds, the duration of competition, etc. Competition is set in motion when supply or availability or resources are limited for the unrestricted growth of both crop and weeds. When the crop seeds are planted weeds seed also germinate along with them.


During initial stages, both crop plants and weeds grow without competing with each other. However, as the plants grow bigger and bigger, they vie with each other for resources and the one which is stronger numerically, morphologically, physiologically and biochemically will benefit more at the cost of the other. The stage at which there is maximum impact of weeds on crop growth in termed as critical period of weed competition, which usually varies between 15 to 60 days after sowing depending upon the crop, crop duration, time of weed emergence, weed flora and intensity and soil and climatic conditions. There will be le4ast or no damage done by the weeds, once the crop is established and the canopy covers the ground. Irrespective of what method of weed control is practised, it must be ensured that the weeds are kept at low level during the critical periods of weed competition. This will ensure higher efficiency of inputs used and in turn recording maximum potential crop yields.

Besides competition for resources, weeds often reduce crop growth and yield by the release of allelochemicals into the environment. The combined effect of allelochemicals (allelopathy) and competition is termed as interference.

Principles of weed Management:
In order to reduce the negative implications of weeds on crop growth and yield, weed have to be controlled efficiently. To do the job successfully, a thorough understanding of weed biology in the crop environment becomes essential. Prevention, control and eradication are the keys in weed management.

Prevention:
Prevention involves procedures that inhibit or delay weed establishment in areas that are not already inhabited by them. These practices restrict the introduction, propagation and spread of weeds on a local or a regional level. Preventive measure include cultural practices such as

  • seed cleaning,
  • use of weed-free seed,
  • manure and machinery,
  • controlling weeds on field bunds, and irrigation canals,
  • screening irrigation water,
  • restricting movement of farm animals, etc.

Prevention is highly cost effective, as establishment of any new weed is going to create problem for many year.

Eradication:
Eradication is the total elimination of a weed species from a field, area or region. It requires the complete removal of seeds and vegetative parts of a weed species in a defined area. it is usually attempted only in small area or areas with high value crops because of the difficulty and high costs associated with eradication practices.

Control:
Control practices reduce or suppress weeds but do not necessarily result in the elimination of any particular weed species. Weed control, therefore, is a matter of degree and depends upon the goals of the people involved, effectiveness of the weed control tactic used and the abundance and tenacity of the weed species present. There are 4 general methods of weed control: Physical, cultural, and biological.

Physical methods:
Hand pulling or hand weeding, hoeing, tillage mowing, burning, flooding, smothering etc. are examples of physical methods of weed management, involving the use of physical energy through implements either manually or bullock drawn or power operated. Farmers mostly resort to hand weeding with the help of hand chisel (khurpi), hand hoe, spades, etc. it is one of the most commonly used methods but is back breaking, time consuming and costly. Pre-planting or post-planting tillage is practical and has been found as economical methods of weed control. The use of sol turning plough, disc and spring tooth harrows and the dug foot, blade and bukher-type cultivators are some of the implements being used to keep weed growth under control. However, adverse soil conditions such as too dry or too wet soil limit their use.

Cultural methods:
Weeds are better competitors than crop plants for light, water, nutrients and space. However, good cropping practices can change the conditions in such a way as to enable the crop plants to compete with the weeds successfully or to reduce their interference to the minimum, and thus prevent them from acting as impediments to increase in crop production. Quality seed with good germination will give the crop a vigorous and close stand, and would enable the crop plants to steal a march on the weeds. Varieties well adapted to a season will complete better with the weeds than those poorly adapted to it. The plant breeder have to evolve quick-growing and short duration varieties of crop plants with larger leaf area and good branching or tillering ability and the agronomists have to work out the proper seed rate, depth, time and method of sowing, and the use of the most appropriate method of irrigation and manuring in a given cropping system.

Some crops can compete better with weeds. For instance, the crop like sorghum, cowpea are good competitors, whereas others such as groundnut, lentil are poor competitors. Close row crops compete better with the weeds than wide row crops. Similarly, the crops and varieties having early and faster growth during the cropping season compete better than those growing slowly during the early part of their growth, e.g. pigenpea. The fodder crops in general, grow faster and thick sowing technique results in dense growth crops, thereby smothering the weeds effectively.

Crop rotations are required to minimize the dominance of a particular weed in the cropping system. Intercropping, particularly growing of a fast growing crop such as cowpea, soybean, etc. in wide spaced crops like maize, pigeon pea or sugarcane would reduce weed competition. Similarly, converting inter-row spaces with mulches (plant residues, paper, plastic, etc.) also contain weed growth.

Biological Method:
In this method, the natural enemy of a weed plant is used to control the weed. The requirements for the success in this method are:

i. The weed species must have been introduced and in the process if introduction must have been freed from its natural parasites or predators.

ii. The natural predators and parasites must be introduced to prey upon or parasitize the weeds but they in turn must have been freed from parasites in order to carry out their work for destruction unhampered.

iii. The destructive agents must be highly specialized so they these are able to thrive even under starvation condition on agricultural plants of the new habitat. Root-borers, stem-borers and internal seed-or fruit feeders are more highly specialized than the foliage feeders. 

An outstanding example of biological control of plant is that of prickly pear (Opuntia spp.) in Australia. Another noteworthy example is of destruction of Lantana with the help of Telenemia scruplosa. Attempts are under way for the biological control of nutgrass in Hawaii. Water hyacinth can also be controlled through the use of Neochetina bruchi and N. eichhorniae. Successful control of Parthenium hysterophorus has been reported with the help of Mexican beetle Zygogramma bicolorata.

Somewhat different, but an interesting example of biological control, is the destruction of aquatic vegetation in ditches by using herbivorous fish. The Hawaiian Sugar-Planters’ Association reports that Tilapia mossambica of the Sunfish family, by eating the vegetation and burrowing in the bottom of ditches to form spawning nests, destroys the roots of aquatic weeds and kills them. This method appears to have an additional merit, i.e. besides controlling submerged aquatic weeds; it can be used as human food. In Russia this method is being exploited on a large scale. Biological control through grass carp, which feeds on aquatic weeds, may be the safest, low-priced and the most effective method of dealing with water vegetation.

The use of geese to control weeds in cotton is popular in some areas in South-West America. The young geese selectively feed on Johnson grass (Sorghum halepense) without harming cotton. The eradication of prickly pear by using the cochineal insect (Dactylopius tomentosus) in Maharastra and Tamil Nadu is the best example of biological control in India. At the Indian Fisheries Laboratory, Bhopal, it was observed that grass carp (Ctenopharyngodon idella) controls effectively the submerged water vegetation comprising Najas minor, Hydrilla verticillata, Ceratophyllum demersum, Lemna minor etc. Claims are also made in India of the control of kans grass (Saccharum spontaneum) by growing basket grass, whose roots are supposed to excrete some substance or substances inhibitory to kans. 

Integrated weed management: In nature a balance has been struck among all the components, both living (biotic) and non-living (abiotic) of an environment. This ecosystem concept has much relevance in that all the components co-exist in perfect unison, each component satisfying its own niche. The introduction of a foreign element, e.g. a herbicide, may create some serious upheavals in the ecosystem. Further, the continuous use of a herbicide may eliminate all the susceptible weed species and its place may be taken over by some resistant ones, or the existing ones may develop resistance. All these eventualities have to be borne in mind when recourse is taken to chemical weed control. One of the ways to avert such a situation consists of using low doses of herbicides, in rotating herbicides just like a crop rotation and alternating intelligently chemical and non-chemical methods in weed management. An integrated weed management may be defined as the combination of two or more weed-control methods at low input levels to reduce weed competition in a given cropping system below the economical threshold level. It has proved to be a valuable concept in a few cases, though much is still to be done to extend it to the small farmers’ level. 

Integrated Weed Management (IWM) approach aims at minimizing the residue problem in plant, soil, air and water. An IWM involves the utilization of a combination of mechanical, chemical and cultural practices of weed management in a planned sequence, so designed as not to affect the ecosystem. The nature and intensity of the species to be controlled, the sequence of crops that are raised in the rotation, the standard of crop husbandry, and the ready and timely availability of any method and the economics of different weed-management techniques are some of the potent considerations that determine the success for the exploitation of the IWM approach.

Aquatic Weeds and their Management:
Almost all the water bodies have plants growing in them. Presence of plants in water bodies is essential for the conversion of solar energy into chemical energy for the development of aquatic fauna like fish, prawns etc. and for the continuous addition of oxygen to water during photosynthesis. If the water plants due to overgrowth make such water bodies unfit and take the shape of noxious aquatic vegetation, these may be referred as aquatic weeds Aquatic weeds are the greatest problem in fishing, irrigation and efficient water supply. Because of scarce water supply and high population it has almost become imperative in every country to save water from the ravages of aquatic weeds. Since the beginning of this century, greater efforts are being made using the variety of implements, chemicals and bio-agents. Of the 800,000 ha of fresh water available in India for pisciculture, about 40% rendered unsuitable for fish production because of invasion by aquatic weeds. 

An integrated and to the extent possible an environment-friendly approach is to be employed for management of aquatic weeds. Aquatic weed-control measures can broadly be grouped into the following categories.

Preventive: The success of preventive weed-management programmes varies with the weed species, its means of dissemination and the amount of efforts applied. Preventive weed-management programmes usually require community action through the enactment and enforcement of appropriate laws and regulations.

Mechanical:
This has both, advantages and limitations. Advantages include utilization of available man-power; is environment friendly, yields immediate results, is non-selective with fewer chances of permitting ecological shifts in aquatic flora; lessens mass nutrient load of eutrophic water bodies, helping indirectly in diminishing the future weed populations; reduces dependence on import of herbicides; harvested weeds may have various utilities as feed, manure, energy source etc; and most importantly can be exercised in any localized areas of water bodies. The limitations include limited effectiveness as in some cases the weeds re-grow from their rootstocks, rhizomes and the like spreading weeds new areas; labour-intensive and expensive and sometime removal of weeds may deplete water bodies of their nutrients limiting growth of planktons.

The methods include, netting, erecting barriers, chaining, dredging, draining, use of water-weed cutters, submergence, shading, cleaning of irrigation waters etc.

Biological:
Biological methods of management, require the use of organisms that have been used for biological control, are diverse and include various types of animals and plants like insects, fishes, pathogens, nematodes etc. Biological management is more complex than chemical weed control because it requires (a) long-term planning, (b) multiple tactics, and (c) manipulation of cropping system and direct interaction with the environment.

Use of several species of herbivorous fishes which feed on submerged aquatic weeds include Tilapa sp., Ctenopharyngodon idella, and other species. Observations are also available for rodents, snails etc. The use of insects like Neochetina bruchi and N. eichhorniae for control of water-hyacinth, and Cyrtobagous salviniae for control of Salvinia molesta has been found effective in India.