Why is it a problem? Potato is one of the main cash crops in Bhutan. Weeds are a severe constraint in potato production, and require considerable resources to manage. Investigations comparing three weed management treatments across a wide range of locations reported average yields of 6.7, 10.7 and 11.6 MT/ha when no weeding was done, when farmers under took weeding, and when there were no weeds, respectively (ARC, 1991). Based on these results, weed competition is limiting yields.
Where and when is it a problem? Weeds are problem in all the major potato growing areas in Bhutan. Depending on the climatic conditions, different weeds cause problems. In Western and Central regions, Galium aparine and Digitaria ciliaris cause most problems, while in the Eastern region, Fagopyrum dibotrysis is the main problem.
Broadleaved 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 potato, the critical period is about 22 to 66 days after emergence.
Weed management should focus on minimising competition between potatoes and weeds during the critical period (about 22 to 66 days after emergence). Weeds can be managed through good pre-planting preparation and hand-weeding at the right times. Post-emergence herbicide can also be used.
- Methods of mechanized planting, weeding, earthing up, and harvesting have been successfully introduced in Bumthang in the 1970s (Kuensel 1981). However, these methods have never been transferred to other regions.
- Initial land preparation is needed, usually a dry ploughing some time ahead of planting.
- Plant potatoes in lines for better weed management and convenience for rotary weeders.
- Hand weeding is by far the predominant method in Bhutan.
- First weeding should be done within four weeks of planting.
- Second weeding should be four weeks later.
- Metribuzin 70% WP is the only herbicide available for use in potato in Bhutan. It is a selective systemic herbicide that is absorbed predominantly by the roots, but also by leaves. It is suitable for pre and post emergence control of most annual grasses and broad-leaved weeds
- Apply Metribuzin (at 1gram per litre) when potatoes are at the 7-8 leaf stage.
- Protective gear, including gloves should be worn while spraying herbicides.
- Do not spray when it is windy.
- Butachlor must not be used in potato as it is has only been approved for use as a selective herbicide in rice by the manufacturer.
- Glyphosate should not be used at harvest time to kill weeds growing through senescing potato plants because no research has been done on such practices.
Weed management by small holder potato producers in Bhutan. (Roder et al., 2006).
Image acknowledgements: NPPC.