Why is it a problem? Maize is the second most important cereal crop in Bhutan. Weeds in maize are a serious constraint to increased production and accounts for about 50% of the input costs. Maize is often inter-cropped with other crops, making weed management more difficult; whether hand weeding or using weedicide.
Where and when is it a problem? Weeds are a serious problem in all maize growing areas in Bhutan.
IDENTIFICATION
Broadleaved plants are easily identified, but grasses and sedges can be harder to differentiate. The information below provides identification features.
Broadleaves
- Leaves are normally wide, veins branch out in different directions.
- Young seedlings (dicots) have two leaves.
- Stems are often branching.
Sedges
- 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.
Grasses
- Annual, biennial or perennial.
- Leaves are narrow, arranged in sets of 2.
- Stems are rounded or flattened and are hollow.
BIOLOGY
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 maize, the critical period is from about 2 until 6 weeks after sowing.
MANAGEMENT PRACTICES
Weed management should focus on minimising competition between paddy and weeds during the critical period (about 2 to 6 weeks after sowing). Weeds can be managed through good pre-planting preparation and hand-weeding at the right times. Pre-emergence herbicide can also be used. Weed management approaches will need to be modified if inter-cropping is being practiced.
Mechanical weeding
- Initial land preparation is needed, usually a dry ploughing some time ahead of planting.
- Plant maize in lines for better weed management and convenience for rotary weeders.
- Mechanical weeding is possible only if maize is mono-cropped and sown in line.
Manual weeding
- Hand-weeding is by far the predominant method in Bhutan
- First weeding should be done within 20-25 days of crop emergence.
- Second weeding should be done 40 – 45 days after the first hand weeding
Chemical management
- Metribuzin 70% WP is the only herbicide available for use on maize in Bhutan. Studies in Bhutan have found that its use can halve the labour requirements for weeding.
- Metribuzin (1 gram per litre of water) is applied as a pre-emergence herbicide, 1 – 5 days after sowing maize
- Protective gear, including gloves, should be worn while spraying herbicides.
- Do not spray when it is windy.
OTHER RESOURCES
None currently available.
Version: NPPC 2017. Weeds in maize V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact: nppcsemtokha@gmail.com
Image acknowledgements: NPPC, Chemical weed control in maize (Dorji Wangchuk, maize coordinator, RDC-Wengkhar).
