TRADITIONALLY farmers transplant rice in puddled soils, as they believe that rice requires more water for better growth and good yield. The other reason is that puddling does not allow emergence of weeds.
Weeds are mostly removed from the field manually in traditional method of rice cultivation. But high weed infestation is a major problem in direct-seeded rice (DSR) and causes grain yield …
TRADITIONALLY farmers transplant rice in puddled soils, as they believe that rice requires more water for better growth and good yield. The other reason is that puddling does not allow emergence of weeds.
Weeds are mostly removed from the field manually in traditional method of rice cultivation. But high weed infestation is a major problem in direct-seeded rice (DSR) and causes grain yield losses up to 90 per cent.
For high productivity of a direct-seeded crop, good and effective weed management is essential. Efficient weeding process enhances utilisation of biomass, facilitates proper aeration of the soil, helps in healthy growth of root system, and hence healthy plant. It facilitates growth of new roots which helps uptake additional nutrients resulting in more productive tillers, larger panicle size, increased number of grains per panicle and more grain weight.
Integrated weed management practices include stale seed bed techniques in which weeds are allowed to germinate by giving irrigation and then killed by non-selective herbicides two days before seeding, using mulch and subsequently killed by 2,4-D at 30 DAS, and growing of rice varieties having greater ability to compete with weeds. However, 40-50 per cent reduced weed densities are reported by mulching and rice intercropping.
Effective weed management in DSR depends on several factors, including cultural and chemical methods; efficacy, however, depends mainly on timeliness of operations, particularly during early growth stages of rice. Various mechanical methods are also available for weed control in direct- seeded rice. Scientists at the Department of Crop Physiology, University of Agriculture, Faisalabad, have developed a single row manual weeder which is being tried to control weeds.
For chemical weed control, it is necessary to select the right herbicide depending upon the weed flora, and the herbicide should be applied with proper spray techniques. For efficient weed management in a DSR crop on use of chemical molecules proper useful guidelines followed are given below:
Pre-plant herbicides:
Existing weeds of perennial nature can be eliminated before planting. Existing and newly germinated weeds can be destroyed with a timely and judicious use of glyphosate (systemic herbicide) or paraquat (contact herbicide) or mechanically by 1?2 shallow ploughings (with harrow). It is preferable that the control of perennial weeds and seeding of rice should be done 7?10 days after glyphosate spray. Best results are obtained when weeds are in active growth stages. If weeds are vegetating (not growing), apply light irrigation several days before glyphosate spray. This knocks down all weeds.
These herbicides bind with suspended soil particles and metal surfaces (iron buckets), thereby reducing their efficiency. Non-reactive surfaces such as plastic containers can be used for preparing diluted solutions for sprays. Cattle and small animals should not be allowed to graze in treated fields. Nibbling of leaves sprayed with herbicide adversely affects their translocation to underground parts and thus effective control of weeds.
Pre-emergence herbicides are generally used before the emergence of weeds. Thus, these chemical molecules are applied immediately after the sowing of the crops. Generally, two chemical molecules, pendimethalin and pretilachlor with safener, are used as pre-emergence herbicides. It is effective in the control of annual weeds (grasses and broadleaves), and needs sufficient soil moisture in the surface layer. It should be applied in the evening hours to minimise its photo-degradation.
Pretilachlor with safener is a selective pre-emergence herbicide applied within three days of sowing. It is effective against annual grasses, sedges, and broadleaf weeds. Since without safener pretilachlor causes phytotoxicity in rice, it is advisable to use it with a safener such as Sofit/Erase-N. The herbicide should be used in wet rice fields.
Post-emergence herbicides are used to eliminate weeds after they are up or growing vigorously. To be effective, herbicide molecules must be absorbed through aboveground plant parts (leaves) and, consequently, liquid sprays generally work better than dry granular materials. Almix is post emergence herbicide. It is effective for a longer duration because of its dual action that is, from leaves and roots. Almix controls major annual broadleaf weeds, also suppresses Cyperus rotundus for 1?2 weeks. Sedges generally germinate in full light and therefore mulching helps control them.
Fenoxaprop is effective against grassy weeds, particularly at early growth stages. Repeated application (second dose) of pendimethalin after seeding rice has been reported to control the second flush of grassy weeds in farmers? participatory trials. When the stale-bed technique is used to establish a direct dry-seeded rice crop, pre-plant application of glyphosate followed by the pre-emergence herbicide pendimethalin and post-emergence herbicide azimsulfuron/almix can eliminate weed problems in a DSR crop, including weedy rice.
However, more and integrated research efforts are needed for weed management in direct seeded rice particularly at farmer?s field to disseminate a environment-friendly and resource conservation technology to farmers under the changing socioeconomic conditions.
