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

The composition of a ricefield’s plant community will change with the type of
agricultural practices that are applied. Modifying crop management inputs will alter
the competitive environment, and the morphological and physiological traits that confer
plant success will shift-the type of agricultural activity influences the type of weeds
found.
WEED MANAGEMENT
Weed management can be defined as the manipulation of an agroecological environment
to create a situation favorable for crop growth but unfavorable for weed survival.
Continuous application of a particular cropping practice can contribute inadvertently
to a shift in dominance and distribution of rice weeds. In formulating a weed
management program, it is important to systematically manipulate recommended production
practices in synchronization with the current location-specific farming activities
(Ho et al 1994).
Factors affecting composition of weed flora
Better understanding of the mechanisms of interaction between the different species
in an agricultural field population can lead to more effective weed management practices.
Most crop-weed communities, especially those predominantly composed of
annual species, exist in simplified, disturbed habitats. Weeds are well adapted to such
conditions (Gliessman 1989). Adoption of direct seeding, increased herbicide use,
intensified crop rotations, and other modifications of crop production practices alter
the environment. This means the morphological and physiological traits that confer
the success of weeds will shift.
Most plant species are tolerant of a wide range of soil types, and soil factors are
not a major limitation to plant distribution. The primary factors that encourage weed
communities to coexist with rice are habitat, water status of the field, and crop planting
method (Tiwari and Nema 1967). The weed species found often will reflect the
history of agricultural activity in the field, not the soil type.
Broader understanding of the influence of chemical and nonchemical weed control
treatments on weed population dynamics would enable us to predict ecological
changes. This would facilitate adjusting treatments to prevent the weed population
shifts that make control more difficult (Shaw and Jansen 1972).
Landscape position. In the Muda area of Malaysia, the composition of the weed
flora is strongly influenced by landscape position. The higher ponding potential of
the coastal areas that have a higher water table favors growth of aquatic weeds such as
Ipomoea aquatica Forssk., Nymphoides indica (L.) O.K., Blyxa auberti Rich., Hydrilla
verticillata (L.f.) Royle, Ottelia alismoides (L.) Vahl, and Nelumbo nucifera Gaertn.
In the inland areas with lower water ponding potential, a lower water table favors
Cyperus iria L., C. babakan Steud., Scirpus supinus L., and Melochia concutenata L.
(Soo 1972).
Weed community dynamics in ricefields 29