A handbbok on Weed Control in Rice.pdf

Land preparation
Land preparation should be done to
provide weed-free conditions at
planting. Land leveling is essential to
improve water control. Large soil
clods must be broken up so that they
do not interfere with rice seedling
emergence. A stale seedbed may be
practiced, but it gives no advantage if
planting is delayed until after the
start of rains.
Planting method
For most direct seeded rainfed
lowland rice, common establishment
methods include broadcasting on a
leveled field, broadcasting over
shallow furrows and passing a spike-
toothed harrow at an angle to concen-
trate seed in rows, drilling, and
dibbling seeds for uniformly spaced
seedlings in hills. In all planting
methods, close spacing will increase
the competitive ability of rice against
weeds. Seed rates of 100-150 kg/ha
are often used, with higher seed rates
where weed problems are expected.
Cultivar
Early-maturing, drought-tolerant rice
cultivars are desirable.
Water management
There may be no standing water in
the early crop growth stages of direct
seeded rainfed lowland rice. Later,
when water from rains has accumu-
lated, water is an important tool in
suppressing weed growth. However,
continuous flooding to a 5-cm depth
is seldom achieved. If weeds establish
because of lack of standing water
early in the season, deep flooding at
10-20 cm is necessary to reduce weed
growth. Where rainfall is not enough
to maintain continuous submergence,
other weed control methods are
essential.
88 Weed control handbook
Table 7.3. Herbicides suitable for use in rice direct seeded on dry soil.
Herbicide
Bentazon
Butachlor
Butralin
2,4-D or MCPA
Oxadiazon
Oxyfluorfen
Pendimethalin
Pretilachlor + antidote
Propanil
Quinclorac
Thiobencarb
Rate
(kg ai/ha) Comments and source of information
1.0-2.0
2.0
2.0
0.5-1.0
0.75-1.0
0.14-0.20
0.75-2.0
0.3-0.4
2.0-4.0
0.3
3.0
Fertilizer
Nitrogen application should be timed
to prevent weed proliferation and to
obtain maximum benefit from the
fertilizer applied. In direct seeded
rainfed lowland rice, application of
fertilizer after thorough weeding
gives maximum benefits.
Hand weeding
Two to three well-timed hand weed-
ings should provide adequate weed
control. An advantage of early hand
weeding is that it requires less time.
Early-season hand weeding, espe-
cially where there is no standing
water, reduces the competition of
Apply postemergence to control
broadleaf weeds and sedges at the 2-
to 10-leaf stages (IRRI 1984).
Apply as preemergence spray 0-3 d
after sowing (DAS) to control annual
grasses and sedges (IRRI 1988).
Apply preemergence 2-3 DAS to
control annual grasses (IRRI 1978).
Spray 3-4 wk after seeding to control
annual broadleaf weeds and sedges
(COPR 1976).
Apply preemergence 6-8 DAS
(IRRI 1978).
Apply preemergence 3-5 DAS
(IRRI 1982).
Apply preemergence (IRRI 1982).
Apply 3 DAS (IRRI 1987).
Apply postemergence at the 2- to 3-leaf
stages to control grasses and broadleaf
weeds. Decrease water level of
flooded fields 24 h before application
to expose weeds (IRRI 1982).
Apply 6-8 DAS (IRRI 1986).
Apply preemergence Immediately
after covering rice seeds with soil,
before rains (IRRI 1982).
weeds for nutrients and moisture.
The first weeding should be done
15-21 d after seeding. Hand weeding
later than this will reduce rice yields.
Mechanical weeders can be used if
seed is drilled or dibbled in straight
rows.
Herbicides
In fields with no standing water,
rainfall after herbicide application is
needed for preemergence herbicides
to be effective. The persistence of
residual preemergence herbicides
under warm, moist, or flooded
conditions may be too low to provide
a weed-free rice crop. Follow-up
hand weeding or postemergence
herbicide application may be
necessary.