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Weed management

Weed management:

Weeds cause more yield losses in rice field than any other pests. Transplanted crop is affected less by weeds than direct sowing method. Weed management is very important component in rice production. Weeds grow vigorously and compete for resources. The crop weed competition varies with the type of rice culture, variety, cultural practices, like plant density, fertilizer, application, land preparation, time and method of planting, water management etc. Weeds can reduce rice yield varying from 20 to 80 per cent if not controlled even at early stage. However, crop damage is depending upon the intensity of weeds and cultural practices adopted by the farmers. Weeds of the rice fields are grouped into three categories such as grassy, sedge and broad leaved weeds .Among different weed species, grassy weeds like Echinochloa, Digiteria, Paspalum, Ichochaemum rugosum, Dactyloctenium are predominant in rice field. Sedges are also huge in number and emerge even in the late stage of crop growth. Sedge weeds are difficult to control with chemicals and hand pulling. Among sedges, Cyperus, Fimbristylis, Scirpus species are most problematic weeds. They grow with seeds as well as through vegetative plant organs. Summer ploughing gives good results for the control of these weeds. Problem of broad leaved weed is comparatively less and can be controlled by manual weeding. The common broad leaved weeds are Commelina benghalensis, Casulia axillaris, Celosia, Amaranthus, Ludwigia octovalvis, Eclipta, Ageratum conyzoides, Trianthema portulacastrum.

Low land weeds     Up land weeds
Grasses     Grasses
Echinochloa colona L.     Echinochloa colona L.
Echinochloa crus-galli L.     Digitaria sanguinalis L.
Echinochloa glaberscens L.     Eleusine indica L
Leersia hexandra Sw.     Imperata cylindrical L.
Paspalum distichum L.     Cynodon dactylone L.
      
Sedges     Sedges
Cyperus diffomis L.     Cyperus rotundus L.
Cyperus iria L.     Broad leaved weeds
Fimbristylis miliaris L.     Amaranthus spinosus L.
Scirpus maritimus L.     Amaranthus virdis L.
Broad leaved weeds     Ageratum conyzoides L.
Commelina benghalensis L.     Commelina benghalensis L.
Monochoria vaginalis (Burm.f.) Presl     Euphorbia hirta L.
Ludwigia octovalvis Raven.     Portulaca oleracea L.
Ageratum conyzoides L     Deep water weeds
 Eichornia crassipes Mart.
 Ipomoea aquatica Forssk.
 Ipomoea reptans L.


An identification of weeds is essential for a rice farmer. The common features of different weed plants are picturised in the following slides, depicted from web site of International Rice Research Institute (IRRI), Manila, Philippines.

An identification of weeds is essential for a rice farmer. The common features of different weed plants are picturised in the web site of International Rice Research Institute (IRRI), Manila, Philippines.

Manual weed Control:
Manual weed control includes burning, hand pulling, and mechanical hand weeding. These manual methods are oldest and labor intensive but highly effective. In many cases these are the farmers' only means of controlling weds in rice. Several hand tools are principal weed control means in many developing countries. But manual methods are slow and tedious. Hand weeding may be ineffective for rice mimics (I. rugosum) and deep-rooted perennial weeds thus repeated weeding is necessary.

Hand pulling:
Hand pulling controls weed seedlings growing near and between rice plants where implements are difficult to use. Frequent hand pulling is necessary for weed control. Hand pulling is most labor intensive. It is not effective in dry soil. Manual weeding needs 2-3 weddings (20, 40 and 60 days after transplanting). In transplanted rice, only two weddings are essentially needed. Manual weeding has become difficult due to labour shortage and also involves more cost. Weeding using hand tools is common in almost all rice growing areas. Hand tools such as the hoe, narrow spade (Kudali), Swiss hoe, knife, machete, and pointed sticks are primarily used to remove weeds. Weeds within the rows need to remove by hand. Hand pulling or uprooting always left the roots of weeds and re-generate, therefore careful weeding is to be done.

Burning:
Burning is common under the slash-and-burn system of land preparation. It kills weeds seeds and seedlings, gets rid of unwanted vegetation, and reduces the amount of weed seeds near the soil surface.

Mechanical weeding: Weeding by machine involves the use of hand-pushed or power operated weeders, which is feasible only where rice is planted in straight rows. However, hand weeder such as conventional push-pull rotary weeder and cono-weeder are not effective if land is too dry or in standing water. The pushed weeders are most commonly used in many tropical Asian countries. To achieve the best results in transplanted rice, a weeder should be run in both directions, at right angles to each other. Mechanical weeding needs to be supplemented by hand pulling of weeds that are close to the rice plant.

Integrated Weed Management in Rice: Weed management is most important aspect in rice production. Without effective weed management successful rice production is not possible. Most weeds are endogenous in nature - they remain mainly in the field, reproduce within or close to the rice field and can build up from one rice crop to the next, if not controlled. Management of such weeds requires a strategic approach to keep them at acceptable levels from year to year.

There is increasing trend of herbicide use in rice. Dependence on herbicides for rice weed control seems increasing. But resilience of weed populations under intensive herbicide use, buildup of weed species tolerant of the control methods used, and increasing public concern about indiscriminate herbicide use and its effects on the environment and human health have led to widespread appreciation of integrated weed management concept. Integrated weed management is the rational use of direct and indirect control methods to provide cost-effective weed control. Cost effective integrated weed management practices should be consistent and compatible with other rice production practices. The primary aim for weed control is to optimize profits and minimize losses.

A variety of weeds control techniques are available. No single weed control method will give continuous and effective control. In fact, no single method will effectively control weeds in all situations. Weeds vary in their growth habits and life cycles. Therefore, weed control can be achieved through direct methods used within systems of indirect methods such as tillage, water control, planting method, and even fertility management. Farmers adapt several techniques for weed management in rice. In addition to the indirect control methods rice farmers use some direct weed control including hand weeding, mechanical weeding and herbicides.

Integrated weed control techniques includes all types of controlling measures such as preventive control, mechanical and physical control, cultural control, biological control and chemical control. Chemicals pollute the environment, their application should be restricted only up to an essential limit, and all these efforts should be employed for controlling weeds.

In paddy fields allow to grow weeds by applying irrigation water if there is no rain occurred. Plough the field twice at an interval of 20 days before transplanting so that the weeds grown are to be incorporated well into the soil so that they should fully be decomposed which not only control the weeds but also enhance the fertility of soil. Field should be puddled nicely at transplanting it controls weed effectively. In addition, use the nursery weed free. Apply well decomposed FYM and Compost. Keep the water level up to 5 cm so that weed growth can be suppressed. Adopt line transplanting. Closer transplanting checks the weed growth, so optimize the plant population 45 to 50 hills per square meter. Shallow transplanting of 2-3 cm deep having 2-3 seedlings per hill should be maintained. Promote early vigorous plant growth by applying fertilizer in a balance and an adequate amount  Weeds are plants that interfere with the growth of desirable plants and that are unusually persistent and pernicious. They negatively impact human activities and as such are undesirable. Weeds are universal problem and important constraints of rice production. Weeds interfere with rice plant growth by competing for one or more growth-limiting resources, such as light, nutrients, and water. Allelopathy (chemical produced by living or decaying weed plant tissues) may also adversely affect the growth of neighboring rice plants.

Biological weed control: Biological weed control is the use of natural enemies (bio-control agents) to reduce weed populations to economically acceptable levels. Several biological control agents, such as insects, mites and fungi used successfully to control rice weeds in experimental situations. A thick mat of Azolla can suppress weed in rice fields.

Inter-cultural operations:

Gap-filling:

In transplanted rice some plats may die due to sun heat during summer or transplanting of rootless seedlings. Fill the gaps within two week with same variety seedlings. Increase the number of seedlings per hill if transplanting delayed or do closer transplanting. Weed out the field timely. use Khurpi or Kutila for hoeing is also an effective method by which weeds are removed with roots and upper layer of soil may be disturbed to break the soil capillary. Spread Weeds on soil surface after removing mud or soil from roots. It adds nutrients and works as mulch.