Effects of Weeds in Crop Production and their Control Measures
Weeds are plants growing where they are not wanted. In general, plants are considered weeds when they interfere with the utilization of the plant and water resources or otherwise adversely intrude upon human welfare.
They range from grasses, legumes and other plants and constitute a serious constraint to food and fibre production worldwide. Estimates of crop yield losses due to weeds in developing countries are given at about 25%. Yield losses in grain crops could be much higher.
Some of the weeds commonly found across the world include goat weed (Ageratum conyzoides), Tridax (Tridax procumbens), Wire grass (Sporonbulus pyramidalis), Elephant grass (Penisetum purpureum ), Siam weed (Chromolina odoratum), Sturbbon grass (Sida acuta), Pig weed (Boerhevia diffusa), Tete (green with horns) – Amaranthus spinosus, Tete (greens with no horns)- Amaranthus viridis, Water leaf (Talinum triangular),sedge plant (Cyperus rotundantus, Emilia sognifochia, Spear grass (Imperata cylindrical) Desmodium (Desmodium spp.), Guinea grass (Panicum maximum), Carpets grass (Cynodon dactile), Northern Gambian (Andropogon gayanus),Mucuna (mucuna utiks) etc.
Effects of Weeds on Crops
1. Direct Effects of Weeds on Crops
1. Competition: Weeds compete with crops for environmental factors such as nutrient, moisture, light and space. This is because weeds like crops also require these factors for survival.
2. Chemical interactions: Some weeds produce toxic substances in the root region which can inhibit the germination of other plants. This phenomenon is called allelopathy. E.g Cyperus rotundus produces substances that can inhibit germination of seeds around it.
California thistle produces chemicals that inhibit germination of other seeds. This causes poor establishment of crop and high weed competition with the crop.
3. Parasitic weeds: Parasitic weeds depend on crop plants for their food materials. For example, dodder establishes a weak root system in the soil at first. It losses the roots when fully established on the host.
New roots, called haustoria are developed, and with which dodder taps food from the host. Dodder develops a massive network of stem around the host.
Striga is another parasitic weed with weak root system before it develops haustoria with which it establishes parasitic relationship on the host crops.
It taps food from the host plants with the haustoria hence making the plant weak and unproductive. Striga hermonthica ia also known as witch weed is parasitic on sorghum and maize in Africa, India and parts of United States.
4. Reduction in crop quality through seed contamination: The seeds of weeds contaminate crop seeds at harvest. Weeds can reduce the quality of harvested products in the following ways:
Weed seeds in harvested grains lower market value of grains.
Green plant parts in dry seeds can interfere with the wholesomeness of such seeds in storage and consequently lower their quality.
The presence of poisonous, unpalatable or even low quality plants in forages can reduce the quality of such forages with the attendant adverse effects on the animals that graze on them.
Weed control is a high cost factor in crop production. The high cost of weed control constitutes a direct loss to the farmer.
The price of seeds depends on the degree of purity, hence the more the weed seeds in crop seed lot, the lower the price of agricultural seeds. For example, in a paddy rice production, presence of Oryzabarthii reduces the quality of Oryza sativa.
The presence of striga reduces sugar cane juice in quality. Nut sedge and wild garlic in pasture field reduce the pasture quality (palatable, nutritional quality etc.).
2. Indirect Effects of Weeds on Crops
Weeds can act as alternative hosts for diseases and pests which affect crop plants e.g. acalypha ciliate acts as an alternate host to root known nematode; likewise, Sida vernocaefolia. Also, some weeds harbour polyphagous – variegated grasshopper of Zonocerus variegates.
Some weeds are poisonous and can kill livestock or produce allegic susbstance that affect both man and livestock. For example halogetonglomeratusand black night shade are poisonous to livestock.
Mucuna utilis and Fleurya aestuans produce itchy spines that affects man hence field can be abandoned in case of severe infestation by these weeds.
They can provide food, protection and habitats for several vectors of human disease e.g
Pistia stratuites and Salvilia auriculata for mosquitoes.
Weeds can affect mechanical harvesting using combine harvester. Weesbein green can log the cutting edges of the machine thereby retarding field operations.
Weeds also clog drainage canals, causes fire in industry sites, prevent drivers from locating right ways on roads etc. in uncultivated areas, aquatic weeds block drainage ditches and irrigation canals, thereby disturbing the flow of water at the estuaries. Aquatic weeds block flow river causing flooding of banks. They can also affect recreational activities and fishing.
Weeds harbour rodents, birds etc. which feed on the crops, thus contributing to crop yield losses.
Weedy plantation of permanent crops can face severe devastation in case of fire outbreak during dry season, while the damage in clean plantation would be much less.
In addition to the above, the drudgery associated with weed control limits the land area a resource poor farmer can cultivate. This limits the farm size and consequently farm revenue.
Beneficial Effects of Weeds
Plant considered as weeds do have some positive values. Some positive effects of weeds are examined as follows:
Erosion control weeds play a vital role in soil conservation: They do not only break the force of rain drops on the soil but also stabilize the soil and reduce soil movement by rain wash. Thus, the foliage act as soil covers while the roots help to hold and stabilize the soil.
Addition of organic matter to the soil: Weeds eventually die and decay, thus adding organic matter to the soil.
Recycling of nutrients: Nutrients are transported from deep in the soil to the surface layer by fallen leaves and earthworm activities.
Food for humans: Some weeds are used as food by humans. For example, Talinumtiangulare is a weed that is used as vegetables in West African countries. Also the tubers of Cyperus tuberosusare used as food in parts of the guinea savanna zone.
Medicine: The leaves and roots of many weeds are important in the preparation of herbal medicines by traditional healers.
Host for beneficial insects: Weeds act as hosts to beneficial insects and provide nectar for bees.
Genetic materials: Some weeds are wild relative plants thus serving as a source of genetic materials for varietal development.
Management of Weeds
As discussed earlier, weeds have both harmful and beneficial effects. When and where the harmful effects of weeds are greater than the usefulness, there is a need to reduce their population and growth through management practices to such an extent that the nature and extent of damage they cause are within permissible limits.
Weed management refers to the manipulation of weeds so that they minimally interfere with the growth, development and economic yield of crops and animals. Thus, efficient weed management is a vital aspect of meaningful agricultural activities.
On the other hand, the cost of weed management must be less than that of the expected value of produce that will be reduced by the infestation of weeds. On many occasions weed growth and biomass development are more important than only the density of weeds.
The floristic composition of weeds is more important than that of the abundance of a single species in a habitat.
A specific weed management may be adopted to kill a predominant species without much or no effect on another species which may then become a predominant one (shifting flora). Thus weed management should be aimed at reducing both the population and the growth of all the species together. This is most difficult and tedious.
All the species of weeds and even individuals of the same species don not emerge, grow and reproduce, simultaneously. Thus the method employed to manage weeds must have some residual effects.
To deal with problems posed by weeds some methods of management are to be adopted. Such methods are based on some basic principles.
These principle are related to the following;
(i) Life cycle of weeds,
(ii) Characteristics of weeds,
(iii) Mode of reproduction of weeds,
(iv) Habitat, location and season,
(v) Soil and weather conditions,
(vi) Area of weeds management,
(vii) Farming and cultural practices,
(viii) Availability of resources, and
(ix) Economic of the method.
Now let’s discuss each of the above principles in details;
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1. Lifecycle of weeds and their management
The understanding of the life cycle of weeds determines the effective method and optimum time of weed management.
Annuals complete their life cycle within a season or year. If the introduction and seed production of annuals are prevented, they can be managed better than biennials and perennials.
Annuals should be killed before flowering. It is better to adopt stale seed – bed technique (allowing germinable weed seed seeds in soil to germinate by preparing a weed seed – bed and destroying them thereafter before sowing the crops), pre- sowing irrigation pre – or post – weed emergence but pre-crop emergence application of broad spectrum translocated herbicides inter-cultivation to destroy annual weeds successfully.
Annual in the early growth stages are easily and cheaply destroyed. Once the shoots are killed, the roots are unable to regenerate and rob water and nutrients, meant for crop growth and production.
The best time to destroy biennials is during the seedling stage of the year when they behave like annuals. Once they form underground storage organs they escape a number of management practices, and they may even multiply due to the shearing and tearing action of tillage implements.
The application of translocated herbicides at the beginning of the aerial growth in second year or digging out of storage organs help destroy them.
They can also be killed by creating extreme adverse conditions, such as impounding water for a fortnight at their resumption of growth in the second year, particularly for biennials of aerobic. Heating soil by burning trashes (rabbing) after exposing the storage organs also kills the weeds successfully.
Perennials may be grouped according to forms of vegetative reproduction: simple, bulbous or rhizomatous and creeping perennials.
In combating perennial weeds two problems are to be encountered;
(a) To check the spread or reinfestation by seeds or vegetative propagules, and
(b) To destroy the established plants.
They should be prevented from seeding and forming vegetative propagules by the repeated destruction of aerial shoots and underground roots by cutting, ploughing, digging, drying, and flooding during their regeneration period when food reserve tend to be exhausted.
Some rhizomatous weeds (such as imperata cylindrical and sorghum halepense) hide their storage organs well below the plough layer. Such organs and to be dug out and destroyed or translocated herbicides should be applied during their regeneration period.
The burning of grown up bushes destroys the shoot and then digging or the application of translocated herbicides after the beginning of regeneration makes it easy to kill them.
If required, perennial weeds may be controlled by leaving the land uncropped for the season when several; measures, such as repeated ploughing, pudding, flooding, green manuring, digging and uprooting of storage organs, stocks and stumps, burning, grazing, and shifting of soils may be adopted.
The cultivation of cleaning crops, fodder crops for repeated cutting at shorter intervals and rational cropping reduces the infestation of perennial weeds. A combination of several methods accelerates the destruction of such weeds and thus gets rid of the perennial problems posed by them.
2. Characteristics of weeds
Predominance of a particular group of weeds (such as broad – leaved/grasses/sedges) determines the method of management. Individual species of the group may also be important in a particular habitat because of their noxious, problematic, poisonous or predominant nature.
Such individual weed species may have special characteristics by which they severely react to the environmental conditions.
For example, cyperus, rotundus and Ahhagi camelorum cannot tolerate waterlogged conditions. They may be killed by flooding or growing wet land rice. Some weeds (such as E. colonum, E. indica) can re- establish even if a single root is left undisturbed.
Complete uprooting and feeding to cattle or composting and the application of herbicides are the possible measures to destroy them.
Some weeds particularly during their grown up stages (such as Trianthema portulacastrum) take a few days to desiccate even under hot dry conditions. There should be a considered gap between mechanical uprooting and irrigation to them up completely.
3. Mode of reproduction of Weeds
While selecting a suitable method of management detailed information regarding the production of a prevalent weed species is essential.
Information pertaining to the propagation by seeds or vegetative means by both, number and time of production of propagules, the method for their dispersal, their dormancy, viability, time of germination or sprouting, association with crop plants, mode of perennation, stabilization and adaptation under adverse environmental conditions dictate the methods and the scheduling of them from their control.
4. Habitat, Location and Season
In crop field habitat, targeted plants (weeds) are to be killed or suppressed without affecting crop plants, beneficial organisms and their activities.
Such selective treatments are essential in crop fields considering the standing crop and crops in sequence and also the surrounding environment.
The relative age and tolerance of crop plants and weed are to be considered. Such considerations may not be required for non–cropped areas where other factors, such as soil erosion and degradation may be considered.
The crops and weeds of uplands are quite different from those of low lands. Cultural practices and the workability of such land also differ, thus weed management practices vary.
The occurrences, composition and abundance of weeds vary from season to season and therefore suitable with management practices also differ. In the Kharif season a large number of terrestrial weeds can be suppressed by continuous submergence in water of a dept of 5cm to 10cm.
5. Soil and Weather Conditions
Soil texture and moisture are the important factor in selecting a suitable method of weed management. Light soils have a poor water holding capacity and therefore flooding cannot be adopted.
In heavy soil with poor drainage, mechanical weed management is impeded. Also, dry soil resists mechanical weed management. Types of weed flora differ with soil reaction and therefore weed management practices vary.
The prevalent weather condition determines the workability of the soil, the affectivity of operation and the efficiency of treatment. Continuous rain or drought, and the high velocity of wind affect filed operation.
Under such conditions the management operation are either pre– or postponed or an effective method is sought for combating the infestation of weeds.
6. Area of weed management
In localized spots or small area (such as nursery) labour intensive but most effective methods, such as hand pulling may be adopted. In greater area (say up to two to five ha), mechanical methods and in large areas, chemical methods of weeds management may be adopted.
In many cases an integrated approach may be more effective and remunerative than that of individual methods.
7. Farming and cultural practices
Methods of weeds management in farming with seasonal field crop differ from farming with fodder or seed production and multiplication. Similarly, weed management practices in rain fed or dry land farming differ from that of irrigated farming.
Methods of weed management in mixed or intercropping differ from that of pure cropping. Weed management methods in direct seeded upland crops differ from transplanted low land crop.
Management practices such as good land preparation, sowing at the optimum time and by the best method, intercultural operation, such as earthing up, insect – pest and disease management at the appropriate time and by the correct method, favour crop growth and development and at the same time suppress the population and growth of weeds.
Weed management in rations and repeatedly cut crops, differ from that in plantation crops. The best time for weed management in rations crops is the pre – regenerative stage of the crops.
In green manure crop, weed population and weed management is an unnecessary expenditure and rather a loss.
In potato, groundnuts and sugar beats, earthing up and the preparation of irrigation and drainage channels are the essential intercultural operations when weed management becomes simultaneous, while in cotton and jute, inter – culture is directed mainly for weed management. Therefore, the method and time of weed management differ in these types of crops.
The method and time of weed management vary with different methods of harvesting crops. Potato, sweat potato and groundnut are harvested by digging when all the weeds are uprooted and destroyed. This is not possible with reaping, picking or uprooting methods of harvesting.
In mechanized farming, weed infestation increases as they are not identified separately by machines. Weeds that come up in the crop rows grow and produce seeds which are harvested with crop plants and remain as an admixture with crop seeds for the next sowing.
Under certain conditions mechanical weed management becomes impossible. In such cases, chemical weed management becomes indispensable, effective and cheap.
The same crop species may be grown for different purposes, such as maize, oat and cowpea for grains, fodder or seeds. Some species of weeds may be allowed to grow for fodder, still less for grain and no weed in seed crops. Therefore, weed management practices differ with the purpose of the crop grown.
8. Availability of resources
Farm labour, implement, power and herbicides are the major inputs in weed management. The availability and cheapness of these inputs determine the method of weed management.
Although other factors such as the availability of time, the effectiveness and work ability are also contributing factors in selecting the suitable method.
Agriculture is very dependent on the whims of nature. Therefore, the demand for labour reaches its peak during sowing, intercultural and harvesting as every farmer wants to complete such operation in a short span of time. During such period skilled labourers are in short supply; thus wage rate also high.
Apart from some hand tools, there are a few improved speedy and effective implement for weed management in the country. This also have limitation of use under the various conditions of soil and crop.
The availability of appropriate herbicides with the desirable formulation is rare in some countries. Sometimes they are costly. The knowledge and the experience of the farmer are also lacking regarding herbicides use. The availability of resources therefore determines the method of weed management.
9. Economics of the methods
Most of the farmers are poor and live at substance level. Even though some methods are found to be profitable, they do not care for them. The level of production is in general, low and therefore the use of costly inputs for weed management becomes risky.
Except in areas with commercial crops, farmers prefer multi – purpose traditional operations such as hoeing which improves the physical conditions of the soil, earthing up and the incorporation of top dressed manures and fertilizers, and at the same time weed management which on apportioning the cost is found to be the least expensive and most effective, though time consuming and tedious.
In the recent past, the introduction of integrated weed management involving non – monetary, less monetary and monetary inputs becomes popular, effective and less risky.
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