Why is it a problem? Rice is the staple diet of the Bhutanese people. Weeds are one of the constraints to rice production. In a weed survey conducted in the rice growing areas of western Bhutan, at least 19 different species were found infesting transplanted rice (Dorji et al., 2013). Broadleaf weeds dominated among weed types followed by sedges and grasses. In areas where shochum (P. distinctus) is the dominant weed, reported yield loss is about 37%.
Where and when is it a problem? Weeds are a problem in all rice growing areas within Bhutan. The dominant species differs with cultivation methods and climate zone. For instance, shochum is a temperate weed found mainly between 1,200 and 2,500 m asl. Echinochloa colona is a dominant weed in areas where upland rice is practiced.
Major weeds of rice
Broad leaved plants are easily identified, but grasses and sedges can be harder to differentiate. The information below provides identification features.
- Leaves are normally wide, veins branch out in different directions.
- Young seedlings (dicots) have two leaves.
- Stems are often branching.
- Perennial only.
- Leaves are narrow, arranged in sets of 3.
- Stems are solid, triangular in cross section, and lack nodes.
- Rhizomes are often modified for food storage and propagation.
- Annual, biennial or perennial.
- Leaves are narrow, arranged in sets of 2.
- Stems are rounded or flattened and are hollow.
Lifecycle: Weeds can be annual, biennial and perennial. Annuals only live for one year and are propagated through seeds. Biennials live for two years, producing seed before they die. Perennials live more than two years and can often reproduce through both seeds and vegetative parts.
Dispersal: Weeds are spread from field to field by humans, birds, animals, machinery, wind and water. While dispersal through animals, birds, water and wind can be difficult to control, use of weed-free planting material, and cleaning machinery and tools after use, can help to reduce the dispersal of weeds and vegetative propagules.
When can damage be expected? The critical period is when competition between crops and weeds is greatest for nutrition, light, space and moisture. For transplanted rice, the critical period is from 20 to about 50 days after transplanting.
Weed management should focus on minimising competition between paddy and weeds during the critical period (20 to about 50 days after transplanting). Weeds in rice fields can be managed through a combination of mechanical weeding, hand weeding and flooding. Pre-emergence weedicide can be applied to control grasses and sedges soon after transplantation. Shochum is the main aquatic weed and requires special management attention.
- Initial land preparation, usually a dry ploughing some time prior to transplanting.
- Flooding and wet-ploughing immediately before transplanting.
- In larger terraces mechanical weeding is also feasible for post-planting weed control provided the crop has been planted in rows. Rotary weeders can then be pushed between the two rows, greatly reducing labour requirements.
- Flooding is by far the most important method of weed control in rice fields. Most dryland weeds are completely suppressed by submergence. This leaves only the truly aquatic species to deal with.
- For best results it is essential that the paddy fields are kept continuously flooded to a depth of at least 3 cm. However, this may have to be modified when managing shochum and some insect pests and diseases.
- Fields need to be as level as possible so that no dry or deeply flooded spots occur.
- This flooding regime encourages shochum which will need to be managed using other means.
- Hand weeding is by far the predominant method in Bhutan.
- First weeding should be done within three weeks of transplantation.
- Second weeding should be done four weeks after first weeding.
- If necessary, and especially in the case of serious infestation by shochum, a third hand weeding is recommended.
- When hand weeding shochum the rhizomes can remain up to 20 cm in the soil. They are difficult to remove so additional weeding and other management approaches may be needed.
- Over-reliance on chemicals will hasten the development of herbicide resistance. Minimise the use of chemicals through the use of additional management methods such as hand weeding.
- Butachlor is a pre-emergence herbicide applied @ 10-12 kg/ac, 2-3 days after transplanting to control grasses and some sedges.
- While applying butachlor, fields should be drained of water but soil must still be saturated. Fields should be re-flooded 24 hrs hours after application of butachlor.
- Gloves should be worn when broadcasting the granules.
- Butachlor does not control shochum and other broadleaved weeds.
For shochum, ethoxysulfuron is being tested on-station and on farmers’ fields. The preliminary evaluation showed good results.
None currently available.
Image acknowledgements: NPPC; http://ipm.ucanr.edu/TOOLS/TURF/PESTS/brdgrsed.html