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

WEED COMMUNITY DYNAMICS
IN RICEFIELDS
K. Moody
Since the introduction of organic herbicides, research on weeds in rice has concentrated
primarily on developing and using herbicides to control weeds, to minimize
their economic impact on a rice crop. Over the last three decades, this emphasis has
resulted in significant advances in herbicide technology and widespread adoption of
herbicides. Yet weeds continue to pose problems in ricefields. Some intractable weeds
cannot be contained; herbicide use over time invokes floristic shifts, often to species
or ecotypes inherently more difficult to kill; increasing costs are impacting the profitability
of herbicide use.
As research became more and more preoccupied with weed control, research
into weed ecology was relegated primarily to the category of academic interest. Now,
whether because of a belief that increased understanding of biological mechanisms
will lead to better control or because of heightened academic curiosity, research on
weed ecology is increasing (Cousens and Medd 1994).
At first glance, a rice ecosystem may appear to be a simple system made up
primarily of rice plants. Closer examination, however, reveals a highly diverse complex
of organisms. This diversity occurs in spite of the drastic interventions undertaken
to prepare a ricefield for planting. A ricefield can be colonized by terrestrial,
semiaquatic, or aquatic plants, depending on the type of rice culture and on the season
(Fernando et al 1980). In rainfed rice, all three communities will occupy the same
field at different times of the year (Moody 1983). During cultivation, a ricefield is
subjected to flooding, plowing, and turning up of the substrate, followed by a period
of rice monoculture (Fernando et al 1980). This period of monoculture is usually
short because the field will be rapidly invaded by weeds (Moody 1983).
With knowledge about the population dynamics of a weed, it is possible to determine
the factors that govern weed abundance, to define the conditions and times most
suitable for imposing control measures, and to predict the weed’s response to various
control measures and cropping programs. Knowing how weeds interact in different