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IInnnnoovvaattiioonn BBrriieeff
No. 2, February 2010
Lost to the weeds – changing practices favor an
old enemy Johnson D, Casimero D, Chauhan B, Janiya J
Weeds have always benefited from people’s efforts to raise crops. Dormant weed seeds from previous
seasons, together with those introduced by water, animals, or people, germinate when the conditions are
favorable. They compete with crops for light and nutrients, and reduce crop quality and yield. Throughout the
tropics, cultural practices, such as crop rotation, flooding, and soil cultivation, are often the primary means to
limit weed growth and favor crops. Rice is a good example.
Losses despite farmers’ efforts across diverse systems
Management systems to meet the challenges of weeds are reflected in the varied rice production systems
worldwide. Rice is grown across agro-ecosystems that range from deep-water systems along the major
rivers in Asia and West Africa to the intensive irrigated systems with two or three crops per year in Asia, and
the upland systems throughout the humid tropics. All these systems are increasingly threatened by weeds.
Losses to weeds tend to be “chronic” in nature rather than sporadic and, as a result, are often
underestimated. In lowland irrigated rice, dry-season losses in farmers’ fields were recently estimated as 12
to 15% in the Philippines and from 6 to 16% in Sulawesi, Indonesia. These were despite the farmers’ usual
practices of land preparation, herbicide application, and hand weeding. Indeed, although losses to weeds are
commonly from 10 to 20% in the lowland systems in Asia, they can be considerably higher where weeds are
not controlled (Rao et al. 2007).
CGIAR Systemwide Program on
Integrated Pest Management (SP-
IPM) is a global partnership that
draws together the diverse IPM
research, knowledge and
expertise of the international
agricultural research centers and
their partners to build synergies
in research outcomes and impacts,
and to respond more effectively
to the needs of farmers in
developing countries.
SP-IPM Technical Innovation
Briefs present, in short, IPM
research findings and innovations
for the management of pests,
diseases and weeds in
agricultural production.
This and other IPM Briefs are
available from
In West Africa, more effective weed management could raise yields by 15% in irrigated areas and by 23% in
rainfed lowland areas (Rodenburg and Johnson 2009). Preventing such losses could raise annual production
by 2.2 million tons; this is equal to approximately half the current imports of rice into sub-Saharan Africa.
Changing practices and problems - new and old
Weeds pose some of the greatest challenges in lowland rice systems – the most productive and sustainable
agricultural systems on the planet. Commonly, farmers transplant rice seedlings and use flooding to
suppress weeds, yet farmers often change their practices. Many areas have seen a shift from transplanted
rice to direct-seeding as a consequence of rising farm labor costs and the availability of selective herbicides.
In countries such as Malaysia, Sri Lanka, Thailand, Vietnam, and the Philippines, the majority of rice areas
are established by direct-seeding. Farmers’ weed management practices have changed so that herbicide
application has become the primary intervention and the “solution” to weed problems. This has led to the
dominance of pernicious grass weeds such as Echinochloa spp., Ischaemum rugosum, and Leptochloa
chinensis in direct-seeded rice areas. Equally, or
perhaps more seriously, in many areas in these
countries, farmers are now battling with weedy rice.
Weedy or feral rice is favored by many of the
cultural practices adopted to support good crop
growth and it tolerates the selective herbicides
used in rice. The difficulties associated with
controlling weedy rice have led some farmers to go
back to transplanting, despite the high associated
costs. In addition, herbicides are commonly
reported to be less effective than they used to be
Weedy rice resembles the crop in its early stages and is very
competitive. – D. Johnson