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

for drought tolerance (Pamplona et al 1990). Dryland rice has a higher acclimation
potential to shade than Rottboellia cochinchinensis.
Another indirect control option being explored is the use of allelopathic cultivars.
Studies in Japan have identified some native japonica cultivars that possess allelopathic
activity against certain rice weeds (Fujii 1992). In small-plot field trials in
the United States, some rice lines have been shown to possess weed suppression bands
against certain aquatic broadleaf weeds (Smith 1992). An offshoot of the development
of allelopathic cultivars could be the transfer of allelopathic traits from rice or
other plants to existing rice cultivars, if these traits are found to be genetically determined.
An initial step in this direction is the identification of allelochemicals; work is
under way in some laboratories in Korea and Japan (Kim 1992). Allelochemicals also
may be developed as botanical herbicides.
Crop rotation
Particular crops are associated with particular weeds. Monocropping on the same
land using the same cultural and chemical weed control practices can lead to the
buildup of dominant weed species or to the development of weed tolerance or resistance
to control measures. To break the weed cycle, flooded rice is rotated with dryland
crops having different growth or cultural requirements, such as maize, soybean,
peanut, mungbean, vegetables, wheat, barley, or pasture. In rainfed areas in the Philippines,
where a rice-based cropping system is common practice, farmers plant vegetables
and legumes for a food and/or cash crop in the dry season and wait for the next
rainy season to plant rice. In Korea and Japan, the choice of rotation crops with rice
depends on the potential market value of the crop (Kim 1992).
Rotating flooded rice with a dryland crop is an effective way to dry out and kill
aquatic weeds. Scirpus maritimus populations, which increased with continuous cropping
of transplanted irrigated rice, decreased when rice was rotated with a dryland
crop (De Datta and Jereza 1977, Bernasor and De Datta 1986). In the United States,
rotating rice with soybean, sorghum, or cotton every 2 yr reduced the red rice population
(Smith 1976).
Rotating rice with a broadleaf crop also enables using herbicides that otherwise
could not be used in rice due to selectivity problems. For example, postemergence
grass herbicides can be used to control grass weeds in soybean without fear of injury
to rice.
Preventive measures
Preventive measures include practices to avoid or minimize weed and weed seed dispersal
and buildup of a field’s weed seed bank. They include using weed-free seeds or
weed seedling-free transplants; using clean, weed-free farm equipment to prevent
transfer of weed-contaminated soil from one farm to another, and maintaining clean
fields, field borders, levees, and irrigation canals during the cropping season and during
fallow periods.
156 De Dana and Baltazar