Weed Management

• Weeds are unwanted and undesirable plants which interfere with the utilization of land and water resources and thus adversely affect paddy yield.
• They can also be referred as plants out of place.
• Weeds are an important factor in the management of all land and water resources, but its effect is greatest on paddy cultivation.
• Weeds intensify the pest and disease problem by serving as alternate host
• Reduce the efficiency of harvesting
• Reduce the land value
• Problems of water contamination
• Reduce the yield and quality of paddy by competing for nutrients, water and sunlight.

           In Transplanted rice                        :  10-15% reduction in yield 
           In Upland direct seeded rice           :  35-45% reduction in yield
           In Direct seeded on puddle land     :  20-25% reduction in yield

• Echinocloa crusgalli  ssp. Hispidula (Barn yard grass)
• Echinocloa colonum L. Link ( Jungle rice)
• Panicum  repense L (Torpedo grass)
• Paspalum  conjugatum Berg ( Hilo grass)
• Digitaria  sanguinalis L. scop (Large crab grass)
• Eleucine  indica L. Gaertn (Goose grass)
• Dactyloctenium  aegyptium L. Beauv. (Crowfoot grass)
• Imperata  cylindrica L. Beauv. (Cogon grass)
• Cynodon  dactylon L. (Bermuda grass)
• Oryza rufipogon (brown beard rice)
• Saccolepis interrupta (polla)
• Isachne miliacea (chovverippullu, naringa)
• Echinochloa stagnina (Hippo grass)

• Cyperus difformis L (Common sedge)
• Cyperus iria L. (Umbrella sedge)
• Fimbristylis milacea L.Vahl (Poonkorai)
• Scirpus maritimus L. ( Bull rush)
• Cyperus rotandus L. (Purple nut sedge)

• Monochoria vaginalis (Oval-leafed pondweed)
• Ludwigia parviflora L. Roxb
• Marselia quadrifolia L. ( Marsilea)
• Amaranthus  spinosus L. Spiny amaranth
• Ageratum   conyzoides L. Goat weed
• Celosia argentia L. ( Cox comb)
• Commelina  benghalensis L. Day flower
• Eclipta alba (False Daisy)
• Portulaca  oleracia L.Common purslane
• Trianthema  portulacastrum L ( Horse purslane)
• Ludwigia perennis (neergrampu)
• Limnocharis flava (nagappola)
• Ammania baccifera (nellicheera)

•  Salvinia molesta (African payal)
• Marsilea quadrifolia (naalilakodian)
• Azolla pinnata (azola)

• Chara spp. (chandi)
• Spirogyra spp. (payal)

• The critical period of weed competition is the shortest time span during the crop growth when weeding results in highest economic returns.
• The crop yield level obtained by weeding during this period is almost similar to that obtained by the full seasons weed free conditions.
• The critical period of weed competition is approximately 1/3rd of the duration of the crop.
            

• Weed control is the process of limiting infestation of the weed plant so that crops can be grown profitably.
• Weed management includes prevention, eradication and control by regulated use, restricting invasion, suppression of growth, prevention of seed production and complete destruction.

• Use of weed free seeds.
• Use of weed free seed bed.
• Use clean tools and machinery.
• Clean irrigation canals and bunds
• Prevent entry of water-carried vegetative propagules of weeds that are perennial.

Eradication methods

Eradication can be done by

• Destroying the species at the initial stage of introduction.
• Degenerating the buried dormant but viable seeds by flooding

Cultural methods Preparatory cultivation - Incorporates weeds into soil, often to a depth of 10 to 15 cm to destroy the existing weed growth. Flooding and intensive puddling is an added advantage in lowland rice weed management.  Cultural practices like dual cropping of rice-azolla and rice-green manure reduces the weed infestation to a greater extent. Summer ploughing and cultivation of irrigated dry crops during post-rainy periods reduces the weed infestation. Soil compaction to a bulk density around 1.8 g cc-1 was very effective for minimizing weed infestation in direct seeded lowland rice. Planting method - transplanting is primarily done for good weed control. Select cultivars which are having greater seedling vigor, leaf area development, early height growth rates, and tillering ability. Adopt greater plant densities to allow the crop canopy to close sooner, reducing weed germination and growth.

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Manual weed control

• Weeds are removed and collected from crop fields by hand. The collected weeds are piled on bunds or in case of certain weeds, taken home to feed animals.
• Manual weeding is the most widely used method of weed control in rice which is, however, difficult, time consuming and often costly. 

Transplanted rice

• Pulling by hand or using tools like hoe, spade or sickle.
• Take up one or two hand weeding between 15-40 days after transplanting

SRI

• Manual hand weeding to remove left out weeds after cono weeding

Rainfed rice

• First weeding can be done between 15 and 21 days after germination.
• Second weeding may be done 30 - 45 days after first weeding

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Mechanical weed control      Mechanical control consists of methods that kill or suppress weeds through physical disruption. Such methods include pulling, digging, disking, ploughing and mowing. Transplanted rice Use of rotary weeder four times from 15 DAT at 10 days interval.  It saves labour for weeding, aerates the soil and root zone, prolongs the root activity, and improves the grain filling though efficient translocation and ultimately the grain yield. Upland dry seeded rice Repeated intercultivation by passing hoe or small blade harrow 2-3 times between 20-45 days after sowing. SRI Using rotary weeder / Cono weeder Moving the weeder with forward and backward motion to bury the weeds and as well to aerate the soil at 7-10 days interval from 10-15 days after planting on either direction of the rows and column. Manual weeding is also essential to remove the weeds closer to rice root zone

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Chemical weed control

     Using chemicals, generally referred as herbicides, for the control of weeds is called chemical weed control.
Herbicide application can provide the most effective and time-efficient method of managing weeds.


Tamil Nadu and Kerala

Transplanted rice:

Pre-emergence herbicides

• Use Butachlor 1.25 kg/ha or Anilophos 0.4 kg/ha, Thiobencarb 50 % EC @ 2 kg/ha (1 kg ai/ha), pendimethalin
  30 % EC @ 4.5 kg/ha (1.5 kg ai /ha), as pre-emergence application.  Alternatively, pre-emergence application of
  herbicide mixture viz., Butachlor 0.6 kg + 2, 4 DEE 0.75 kg/ha, or Anilophos + 2,4 DEE ‘ready-mix’ at 0.4 kg/ha
  followed by one hand weeding on 30-35 DAT will have a broad spectrum of weed control.
• Any herbicide has to be mixed with 50 kg dry sand on the day of application (3-4 DAT) and applied uniformly in the
  field with thin film water on the 3rd DAT.  Water should not be drained for next 2 days from the field (or) fresh irrigation should not be given.

Post – emergence herbicides

• Where broad-leaved weeds and sedges are predominant, apply 2,4-D sodium salt (Fernoxone 80% WP) 1.25 kg/ha dissolved in 625 litres of water with a high volume sprayer, three weeks after transplanting or when the weeds are in 3-4 leaf stage.
  

Wet seeded rice

Spray any of the following herbicides:

• Butachlor 50% EC @ 2.5 kg/ha (1.25 kg ai/ha) or Thiobencarb 50% EC @ 2 kg/ha (1.00 kg ai/ha) 6-9 days after sowing
• Pretilachlor + safener (sofit) 50% EC@ 2.25 kg/ha (0.45 kg ai/ha) 3-5 days after sowing. Give a follow up application of
  2,4-D 3.8% EC @ 3.04 kg/ha (0.8 kg ai/ha) at 20 days after sowing.
• To control Echinochloa sp. spray cyhalofop butyl 10% EW @ 0.8 kg/ha (0.08 kg ai/ha) at 15-18 days after sowing.

Biological weed control

• Use of living organisms viz., insects, disease organisms, herbivorous fish, snails or even competitive plants for the
  control of weeds is called biological control. 
• In biological control method, it is not possible to eradicate weeds but weed population can be reduced.
  Hirschmanniella spinicaudata is a rice root nematode which controls most upland rice weed.
• Bactra verutana - a moth destroys Cyperus rotundus. 
• Ludiwigia parviflora is completely destroyed by Altica cynanea (steel blue beetle)

   
  Integrated weed management (IWM) involves the concept of multiple tactics of weed management, maintenance of weed population below economic injury level and conservation of environment quality.  A successful IWM strategy has the principles of enhancing farmer’s profitability, environmental protection and responsiveness to consumer preference.

Major components of IWM are

• Monitoring weeds, shift in weed flora, appearance of resistant weeds and introduction of new weeds.
• Emphasis on ecological, biological and biotechnological methods for environmental safety. 
• Low cost agronomic technology for weed management in IWM system:                   

Wet seeded rice

• Pre-emergence application of Pretilachlor 0.75 kg/ha or Butachlor 1.0 kg/ha on 8 DAS or Pretilachlor + safener (Soft) at 0.45 kg/ha on 3-4 DAS followed by one hand weeding on 40 DAS.
• Alternatively post-emergence application of Almix 4g/ha + 0.2 % surfactant at 20 DAS followed by one hand weeding on 40 DAS.

Rainfed rice

• Apply pendimethalin 1.0kg/ha on 5 days after sowing or Pretilachlor + safener (Sofit) 0.45kg/ha on the day of receipt of soaking rain followed by one hand weeding on 30 to 35 days after sowing.

   In certain areas like Kuttanad, wild rice has become a menace in recent years. The following agronomic practices can successfully control the infestation of wild rice.

• Coat dry seeds with 20% calcium peroxide using 4% PVA solution as an adhesive and then broadcast in the field with 10-15 cm column of standing water.

• Maintain the water level for 10-12 days to prevent germination of wild rice.

• Drain the field and apply N and K fertilizer as per recommendation for rapid growth of the rice seedlings.

• Trampling salvinia in situ in the wet lands a week before transplanting will control the weed and add to soil fertility.

• For chemical control of salvinia, spray paraquat 42% TP @ 3.15 kg/ha (0.75 kg ai/ha).