HERBICIDES in Asian rice - IRRI books - International Rice ...

niques, including hand weeding, may be the best approach from agronomic, economic,
and environmental points of view (Akobundu 1987, De Datta 1981). This
approach can be broadened to include biological controls and other technologies that
favor a crop’s resistance to relatively benign herbicides and that improve the competitiveness
of rice over weeds in ricefield ecosystems.
Experience with integrated pest management (IPM) targeted primarily at insect
control in Indonesia, Malaysia, and the Philippines underscores the potential yield
gains that can come from understanding the dynamic interactions between the economics
and ecology of cultivated rice systems (Teng 1994). In all cases where IPM
practices have been broadly substituted for prophylactic control of pests using insecticides,
rice yields have risen rather than fallen (FAO 1988, IRRI 1992, Rola and
Pingali 1993). The adoption of IPM programs sets a precedent for exploiting such
ecological principles as predator-prey relationships in preference to short-term technological
fixes. Like IPM, IWM implies a movement away from a component-based
system to a knowledge-based system of pest control.
IWM involves both cultural and technology-driven methods of weed control (De
Datta and Baltazar 1996). The most widely used cultural methods include hand weeding,
hoeing, and interrow cultivation, with limited use of herbicides. These often are
complemented by other cultural controls, such as good tillage and land preparation,
careful timing of fertilizer applications (so that the nutrients go to the crop and not the
weeds), flooding to suppress weed growth, and crop rotation. The extent to which
each method is used depends on farm-specific agronomic conditions (for example,
the rice culture used and the dominant weed types), local factor market conditions
(including the availability and cost of labor and machinery), and regional ecological
and agroclimatic conditions (such as rainfall patterns and water availability for flooding).
Most rice farmers in Asia who use these cultural methods already practice IWM
to some extent. The tendency to perceive herbicides as a cure-all for weeds, however,
has increased with rising labor and management costs and the increasing availability
of herbicides in the market (Moody 1996b).
An alternative method to control weeds in rice is beginning to appear-the use of
biological agents, including insects, plant pathogens, and herbivores. Cother (1996)
indicates that an inundative approach using plant pathogens, especially fungal pathogens
(mycoherbicides), is currently the most promising biological control method for
restricting the growth of rice weeds. The inundative approach essentially introduces a
new pathogen or augments natural background populations of pathogens that kill or
severely limit the growth of weeds. The inundative approach can be used to control
almost any weed if a naturally occurring organism with sufficient destructive attributes
can be identified. Grasses are among the most difficult targets for biological control
because pathogens that attack grass weeds tend to have very broad host ranges that
often include the cereal crop being protected. Even so, progress is now being made in
identifying a number of specific plant pathogen control agents for grasses (Gohbara
and Yamaguchi 1992, Smith 1992, Watson 1993), and the potential range of candidate
Herbicide use in Asian rice production 17