A handbbok on Weed Control in Rice.pdf

Land preparation
Land preparation for dry seeded deep-
water and floating rice is similar to
that for upland rice. At planting, the
seed bed should be weed free. Deep
plowing and harrowing are recom-
mended in the dry season if rhizo-
matous perennials are a problem.
Many deepwater and floating rice soils
have a high clay content, and, the only
time to plow them is often when they
are dry and very hard. Cultivation
must start early because the crop,
which draws first on residual moisture
and moisture from occasional show-
ers, must reach the stage where it can
elongate before flooding starts. The
stale seedbed technique can be used to
reduce weed problems, particularly
infestation by wild rices.
Planting method
Broadcast seeding rice into dry soil is
common in deepwater and floating
rices. Transplanting or broadcasting
pregerminated seeds into puddled soil
is also practiced in some areas. If early
flash floods occur, however, trans-
planting involves a greater risk of crop
failure than does broadcast seeding.
The rice seedling grows as an upland
crop for 4-20 wk before flooding
occurs. As the floodwaters rise, deep-
water rice plants elongate to as tall as
6 m and form a dense mat on the
water surface.
Cultivar
Rice cultivars with elongation ability,
photoperiod sensitivity, drought toler-
ance at the seedling stage, and
tolerance for stagnant water conditions
at later growth stages are ideal for
deepwater and floating rice.
96 Weed control handbook
Table 9.1. Herbicides suitable for use in deepwater and floating rice.
Herbicide
Rate
(kg ai/ha)
Bentazon 2.0
Butachlor
Butralin
2,4-D or MCPA
Fluorodifen
Oxadiazon
Piperophos +
dimethametryn
Propanil
Thiobencarb
1.0-2.0
2.0
0.5
2.5-4.0
0.75-1.0
1.0-2.0
3.0
2.0-4.0
Plant population
Because of poor stand establishment,
seeding rates tend to be high-
80-200 kg/ha. The crop produces
nodal tillers at low plant densities,
however, and a high seeding rate does
not necessarily increase final grain
yield. Nevertheless, the high seeding
rate is a good agronomic practice
because a high plant population
allows rice to develop a canopy that
suppresses weed growth.
Crop rotations can help control or
prevent the buildup of difficult-to-
control weeds. In Bangladesh, farmers
on the higher ridges of deepwater rice
areas grow a crop of jute after one or
more deepwater rice crops. The jute
crop suppresses wild rice and other
weeds.
Comments and source of information
Selective control of certain broadleaf
weeds and sedges. Does not control
grasses. Apply postemergence early,
when weeds are at the 4- to 10-leaf
stages. Delayed application allows
weeds to exceed maximum size and
results in inadequate control (COPR
1976).
Effective against grasses and broad-
leaf weeds. Less effective against
perennial sedges (COPR 1976).
Apply preemergence to control annual
grasses (COPR 1976).
Apply as postemergence spray after
rice tillering, 21-30 d after seeding
(DAS) (COPR 1976).
Apply preemergence 0-3 DAS to
control residual annual grasses
(COPR 1976).
Apply 6-8 d after application
(COPR 1976).
Apply 4-6 DAS (COPR 1976).
Apply at 2- to 3-leaf stages of grass
(IRRI 1980).
Apply before 2-leaf stage of weeds and
after 1-leaf stage of rice (COPR 1976).
Fertilizer
Deposits of the silt carried by rivers
during their annual floods provide
high fertility for most deepwater and
floating rice soils. The high soil fertility
improves stand establishment and
elongation ability of rice, but weeds
also grow quickly and compete
aggressively for nutrients, moisture,
and light. Weeds must be controlled to
maximize the nutrient uptake
efficiency of the rice crop.