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Weeds and their control

Common weeds in rice fields of Kerala are:

Grasses:

  • Oryza rufipogon (varinellu)
  • Echinochloa crusgalli (kavada)
  • E. colona (kavada)
  • E. stagnina (kavada)
  • Saccolepis interrupta (polla)
  • Isachne miliacea (chovverippullu, naringa).

Sedges:

  • Cyperus iria, (manjakora, chengoal)
  • C. difformis (thalekkattan)
  • Fimbristylis miliacea (mungai)

Broad leaved weeds:

  • Monochoria vaginalis (neelolppalam)
  • Ludwigia perennis (neergrampu)
  • Limnocharis flava (nagappola)
  • Ammania baccifera (nellicheera)

Algae:

  • Chava spp. (chandi), Spirogyra spp. (payal)

Ferns:

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

Control

Keep the rice fields free from weeds up to 45 days either by hand weeding or by use of herbicides. The recommendation for the use of herbicides in different system of rice culture are given below. 

A. Dry seeded rice [upland and lowland (semi-dry) rice]

Spray any of the following pre-emergent herbicides:

  • butachlor @ 1.25 kg ai/ha
  • oxyfluorfen @ 0.15 kg ai/ha
  • pendimethalin @ 1.50 kg ai/ha
  • pretilachlor @ 0.75 kg ai/ha on the same day of seeding or within six days of seeding.

B. Wet seeded rice (direct seeding with sprouted seeds under puddled conditions)

Spray any of the following herbicides:

  • Butachlor @ 1.25 kg ai/ha 6-9 days after sowing;
  • Pretilachlor + safener (sofit) @ 0.45 kg ai/ha 3-5 days after sowing

Give a follow up application of 2,4-D @ 0.8 kg ai/ha at 20 days after sowing.

  • Pretilachlor @0.45kgai/ha at 3-4 DAS and one light hand weeding at 28 DAS for wet sown rice in Kole land. 

Control of Echinochloa sp.- spray cyhalofop butyl @ 0.08 kg ai/ha at 15-18 days after sowing.

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

  1. 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.
  2. Maintain the water level for 10-12 days to prevent germination of wild rice.
  3. Drain the field and apply N and K fertilizer as per recommendation for rapid growth of the rice seedlings.
  4. Where yeranda (common teal) is a problem, bird scaring should be arranged till the water is drained.

C. Transplanted rice

Apply any of the following herbicides at 0-6 days after transplanting.

  • pendimethalin @ 1.5 kg ai/ha
  • butachlor @1.25 kg ai/ha

Pendimethalin @ 1.5 kg ai/ha, butachlor @1.25 kg ai/ha. For controlling broad leaved weeds and sedges 2, 4-D sodium salt may be applied @ 1 kg ai/ha on 25 DAT. Pre emergence application of 2, 4-D sodium @ 0.8 kg ai/ha on 5 DAT can control all the three groups of weeds viz. grasses, sedges and broad leaved weeds.

Wherever fields are levelled and water management could be effectively done 2, 4-D can be mixed with 10kg urea/ha and broadcast on 20 DAS/DAT. This would save the spraying charges.

Apply 2,4-D Na salt as pre-emergence broad-spectrum herbicide @ 0.8 kg ai /ha at 5 days after transplanting. Spray on moist soil surface with a fan nozzle to give blanket coverage. Use 300-400 litres of spray fluid.

Almix 20WP(Chlorimuron ethyl 10% +Metsulfuron methyl 10%@ 4g ai ha-1+ 0.2% surfactant at 20-25 DAS/DAT is equally effective as 2,4-D for controlling sedges and broad leaved weeds in rice fields. This herbicide is effective for controlling Marsilia quadrifolia also. Almix can be given as follow-up application to cyhalofop butyl used for the control of grass weeds with a minimum of one day gap between the applications.

Management of Salvinia Molesta (African Payal)

Trampling Salvinia insitu in the wet lands a week before transplanting will control the weed and add to soil fertility. Herbicides should be applied in areas where protected drinking water supply is available.

Release of Cyrtobagous salviniae weevils is found effective for the control of salvinia. Even one pair of weevil is sufficient for establishment in a locality. But for practical consideration 50 to 100 weevils are recommended for release in a particular area. When collection of weevil is not possible, about one kg of infested salvinia can be used as the inoculum. Release may preferably be made whenever tender salvinia growth is available. If the plants are very old, they may be removed mechanically to promote re-growth and then weevils are to be released. Almost 100 per cent control of the weed will be obtained in a span of 12-18 months. The rate of natural dispersal of the weevil is rather slow and hence it is desirable that the infested weed mats are redistributed at periodic intervals. In canals used for navigation, the rate of spread of the weevil is found to be quite adequate.

Precaution while using herbicides

  1. Apply herbicides at the recommended dose and time.
  2. Drain the field before herbicide application.
  3. Re-flood after 48 hours to prevent further weed germination when post emergent herbicides are used. For pre-emergent herbicides, wait for a week before re-flooding the field.
  4. Use herbicide nozzle (flood jet / flood fan) for herbicide application.
  5. Move at uniform speed when applying herbicides.
  6. Spray without gaps and overlapping.
  7. Use 300-400 litres of water per hectare for spraying the herbicide.

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