A handbbok on Weed Control in Rice.pdf

Reducing weeds in dry seeded rice
culture is possible by practicing a stale
seedbed. After land preparation,
weeds are allowed to emerge follow-
ing rain or irrigation, then destroyed
by shallow cultivation or application
of a nonresidual contact herbicide. The
herbicide should be applied or cultiva-
tion done when most of the weeds
have reached the 2- to 5-leaf stage. Rice
is then seeded into the weed-free field.
Planting method
Rice seeds are broadcast seeded into
dry or moist soil and covered by
harrowing, or drilled 3-5 cm deep into
the soil. High seeding rates can sup-
press weeds, but the cost of seeds
should be considered against other
direct control measures available
because there is no yield advantage in
increasing seed rates above 100 kg/ha.
Cultivar
Both short- and intermediate-statured
cultivars are used for rice broadcast or
drilled into dry soil. For broadcast
seeded rice, the cultivar used should
be stiff-strawed to avoid severe
lodging.
Water management
Good water management is important
in controlling weeds in broadcast or
drill seeded flooded rice. After dry
seeding, the soil may be intermittently
flooded and drained to allow for rice
emergence. The water level is then
increased gradually for a few weeks
until a continuous flood of 5-cm depth
is maintained.
Table 6.3 Herbicides suitable for use in broadcast or drill seeded, dry-sown irrigated rice.
Herbicide
Bentazon
Butachlor
Butralin
2,4-D or MCPA
Molinate
Oxadiazon
Pendimethalin
Piperophos +
dimethametryn
Propanil
Thiobencarb
Rate
(kg ai/ha)
2.0
1.5-2.0
2.0
0.5-1.0
3.0-5.0
0.75-1.0
2.0
0.75-1.25
3.0-4.0
3.0
Hand weeding
Interrow mechanical weeding is not
possible in broadcast seeded rice. Even
when seeds are drilled, the interrow
spacing is so narrow that only hand or
hoe weeding is possible. In this rice
culture, two to three-timely hand
weedings are sufficient to ensure
optimum yields. The soil disturbance
involved, however, can cause as much
damage to rice as to weeds. The first
weeding may be done between 14 and
21 d, depending on weed growth,
followed by subsequent weedings
when necessary.
Comments and source of information
Apply as a postemergence herbicide
to control broadleaf weeds and
sedges. Apply when weeds have
germinated but are still small. Water
level may be lowered to expose weeds
24 h. Raise water level after treatment
(COPR 1976).
Apply as preemergence spray 0-3 d
after sowing (DAS) to control annual
grasses and sedges (IRRI 1977).
Apply as preemergence spray 2-3 DAS
to control annual grasses (IRRI 1977).
Apply 3-4 wk after seeding to control
annual broadleaf weeds and sedges
(COPR 1976).
Apply from presowing to early post-
emergence to control grassy weeds
(Smith 1971).
Preemergence application should be
2-3 DAS (IRRI 1977).
Apply at preemergence of rice
(IRRI 1977).
Apply early postemergence to control
annual weeds (Green and Ebner
1972).
Apply postemergence at the 2- to 3-leaf
stages to control grasses and broad-
leaf weeds (COPR 1976).
Apply preemergence immediately
after covering the seeds with soil but
before the first irrigation or ram.
Irrigate 3-5 d after application
(IRRI 1979).
Herbicides
The effects of herbicides are similar for
broadcast seeded or drilled dry-sown
rice and wet-sown flooded rice.
Because of water management prob-
lems and difficulties in hand weeding,
herbicides are particularly important
in this rice culture. Covering the seeds
with soil after drill or broadcast
seeding increases the tolerance of rice
for herbicides but decreases a rice
seedling’s flooding tolerance.
Butachlor, molinate, oxadiazon,
propanil, and thiobencarb are used as
Irrigated rice 79