Rice is considered as the most important food crop in developing countries and the crucial source of employment in rural areas. In the tropics, it is largely grown on small family farms that are usually less than 4 per hectare, and in Asia. Throughout these smallholder systems, weeds are one of the major biological constraints to production.
Rice is grown throughout the tropics in rainfed uplands, seasonally deep flooded areas, in rainfed and irrigated lowlands. Upland rice is the cultivation of "rainfed" rice on well drained, non-irrigated fields. The crop is grown in Asia, Latin America and Africa.
However, rice grown on seasonally deep flooded areas, "deep water or floating rice", is crucial in several areas in West Africa and Asia, often along the courses of major rivers. It is sown before the flood waters rise and is also able to survive the subsequent deep flooding.
Lowland rice describes crops where the fields are inundated for at least part of the time between establishment and harvest. It may depend on rainfall or irrigation.
Dynamics of Weed in Rice:
Weeds are one of the major biological threats to higher rice productivity worldwide. Many cultural, biological, physical and chemical practices affect the composition and intensity of weeds in rice fields. Though generally, weeds can be controlled through herbicide. But, chemical weed control is not a sustainable option in a long run. Different agronomic practices like, the use of tolerant cultivars, adjusting sowing time, tillage permutations and plant geometry can reduce the weed pressure in rice. Integrated approaches for weed management, emphasizing on the combination of management practices and scientific knowledge, may also reduce the economic costs and improve weed control owing to the complexity of the weed community.
Here we will explain the role of planting geometry and herbicides as weed management strategies in rice and different methods to control weeds in rice.
Weed management should be practiced in specific stages of rice production:
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During land preparation
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In the nursery
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During early crop growth
During Land Preparation:
Control of weeds during land preparation is important to reduce the amount of weed pressure in the field. Land preparation should begin 3−4 weeks before planting. Ploughing destroys weeds and remaining stubble from the previous crop. Weeds should be allowed to grow before the next cultivation. Also, a level field helps retain a constant water level that controls weeds.
For Wet Seeded Rice
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Plow and harrow various times before planting. Depending on weed population, three or more operations may be done.
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Allow weeds to emerge for at least 2 weeks then kill by another shallow tillage. It reduces the number of weed seeds in the soil, and greatly reduces weeds for the subsequent crop.
For Dry Seeded Rice
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Allow weeds to emerge within 1−2 weeks, and then kill them with either a non-selective herbicide or by light cultivation.
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Spray herbicides, and perform manual and/or mechanical weeding.
Controlling Weeds in the Nursery:
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Prepare land two weeks before seeding.
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While using soil mix for nursery beds, you must make sure the soil is clean and free of weed seeds.
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If there are weed seedlings in the nursery bed, then separate them from rice seedlings during pulling and bundling to avoid planting weeds.
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Apply pre-emergence herbicide 2−3 days.
Why weed control in rice is important?
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Weed control is crucial to prevent losses in yield and production costs, and to preserve good grain quality.
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Weeds decrease yields by direct competition for sunlight, nutrients, and water
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It increase production costs e.g., higher labor or input costs
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It reduces grain quality and price
How to Control Weeds in Rice?
Weed Management in Nursery:
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Apply any one of the pre-emergence herbicides like, Pretilachlor + safener 0.3kg per hectare, on 3rd or 4th day after sowing to control weeds in the lowland nursery.
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Then, apply a thin film of water and allow it to disappear.
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Avoid drainage of water as it will control germinating weeds.
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Butachlor 2.0 l per hectare (or) Pendimethalin 2.5 l per hectare (or) Anilophos 1.25 l per hectare.
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Herbicides must be applied on 8 DAS with thin layer of water in the field.
Weed Management Main field:
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Use rotary weeder from 15 DAT at 10 days interval. It will save labour for weeding, aerates the soil and root zone, prolongs the root activity, and will also improve the grain filling by efficient translocation and ultimately the grain yield.
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Cultural practices such as dual cropping of rice-azolla, and rice-green manure reduces the weed infestation to a greater extent.
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Summer ploughing and cultivation of irrigated dry crops in post-rainy periods will reduce the weed infestation.
Pre-Emergence Herbicides
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Use Butachlor 1.25kg/ha or Anilophos 0.4kg/ha as pre-emergence application. Alternatively, pre-emergence application of herbicide mixture viz., Butachlor 0.6kg + 2,4 DEE 0.75kg per hectare, or Anilophos + 2, 4 DEE ‘ready-mix’ at 0.4kg/ha followed by one hand weeding on 30 - 35 DAT will have a broad spectrum of weed control.
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Any herbicide has to be mixed with 50kg of dry sand on the day of application (3 - 4 DAT) and applied uniformly to 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 must not be given.
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Pre- emergence application of pretilachor at 1.0 kg a.i. ha-1 on 3 DAT + weeding with Twin row rotary weeder at 40 DAT
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PE Pyrazosulfuron ethyl 10 % WP @ 150 g ha-1 on 3 DAT + hand weeding (HW) on 45 DAT. PE butachlor 0.75 kg a.i. ha-1 + bensulfuron methyl 50 g ha-1 on 3 DAT + HW on 45 DAT
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PE Oxadiazon 87.5 g a.i. ha-1 to be followed by Post emergence (POE) 2,4-D 1 kg a.i. ha-1 along with hand weeding on 35 DAT.
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PE butachlor 0.75 kg per hectare + bensulfuron methyl 50 g ha-1 on 3 DAT must followed by mechanical weeding on 45 DAT is effective for broad spectrum weed control.
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Crop growth and yield were enhanced by butachlor 1.2 + 2, 4-DEE 1.5 lit ha-1 with 100% inorganic nitrogen.
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Conventional tillage of one dry ploughing and two passes of cage wheel puddling combined with pre-emergence application of butachlor at 1.25 kg ha-1 under lowland situation.
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50kg of dry sand + Butachlor 1.25 kg per hectare
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It must be noted that, if pre-emergence herbicide application is not done, hand weeding has to be done on 15th DAT.
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2,4-D sodium salt (Fernoxone 80% WP) 1.25 kg/ha dissolved in 625 litres with a high volume sprayer, three weeks after transplanting or when the weeds are in 3 - 4 leaf stage.
Early post emergence application of Bispyripac sodium 50 g a.i. ha-1 (2-3 leaf stage of weeds) + Hand weeding on 45 DAT
Dry Seeded Irrigated Un-Puddled Lowland Rice:
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First weeding must be done between 15 and 21 days after germination.
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Second weeding may be done 30 - 45 days after first weeding.
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Use pendimethalin 1.0 kg per hectare on 5 days after sowing or Pretilachlor + safener 0.45kg per hectare on the day of receipt of soaking rain followed by one hand weeding on 30 to 35 days after sowing.
Unweeded Rainfed Rice:
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Use rotary weeder / Cono weeder / power operated two row weeder.
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Move the weeder with forward and backward motion to bury the weeds and aerate the soil at 7-10 days interval from 10-15 days after planting on either direction of the rows and column.
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Manual weeding is also important to remove the weeds closer to rice root zone.
Wet Seeded Puddled Lowland Rice:
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In wet seeded rice, pre-emergence application of pretilachlor 0.75 kg/ha on 8 DAS or pretilachlor + safener at 0.45kg per hectare on 3-4 DAS then followed by one hand weeding on 40 DAS in direct drum seeded rice.
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In wet seeded rice, sowing with drum seeder and cono weeding (manual / power weeder) must be done at 10, 20 and 30 DAS
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In transplanted rice, hand weeding twice on 15 - 20 DAT and 45 DAT will control the weeds effectively (or) Pendimethalin 3.0 lit per hectare at 8 DAT with optimum moisture condition and one hand weeding on 45 DAT.
Land Preparation:
Tillage serves to provide a suitable soil tilth for a seed-bed and control weeds prior to crop establishment. In smallholder systems, practice varies from zero tillage, as in many of the systems of shifting cultivation, to repeated deep cultivation to remove troublesome perennial weeds, such as Oryza longistaminata which has an extensive rhizome system. Shallow tillage is often ineffective in controlling weeds and, regardless of tillage practice, post-emergence weed control is normally necessary.
Flooding:
It is the most effective method of cultural control of weeds in rice. Flooding to a depth of 10 cm prevents germination of most weed seeds and also kills the majority of weed seedlings. Generally, flooding is used in conjunction with other control measures, like herbicides or hand weeding. Though, for successful flooding, water levels should be maintained. Fields must be well levelled to ensure an even depth of water. In many smallholder schemes, limited irrigation water and poor land development can be major constraints to effective weed control.
