A handbbok on Weed Control in Rice.pdf

Chapter 6
Weed control in irrigated rice
About 77 million ha of rice (53% of the
world’s rice area) are partially or fully
irrigated throughout the growing
season (IRRI 1988b). In South and
Southeast Asia, irrigated rice com-
prises 33% of the rice-growing area; in
temperate Asia, most ricelands are
irrigated. In Europe, Australia, Egypt,
Pakistan, and USA, ricelands are
entirely irrigated.
Irrigated rice is classified into four
culture groups according to the crop
establishment technique used.
Transplanted in puddled soil.
Direct seeded on puddled soil
(broadcast or drill seeded using
pregerminated seed).
Direct seeded on dry soil (broadcast
or drill seeded using nongermi-
nated seed.
Water seeded.
Transplanted in
puddled soil
Among irrigated rice cultures, trans-
planted rice has the lowest potential
loss to weeds, because of the head start
rice seedlings have over weeds and
because of the weed control effects of
floodwater. Despite these advantages,
uncontrolled weeds can reduce rice
yields by an average 48%, through
competition for light and nutrients.
Weed problems
The weeds common in transplanted
rice (Monochoria vaginalis, Echinochloa
crus-galli, Cyperus difformis, Cyperus
iria, and Scirpus maritimus) are in
general highly competitive. They have
discontinuous germination and rapid
growth and are adapted to aquatic
conditions. Weeds grow and infest an
irrigated field if optimum water depth
is not maintained. In poorly flooded
ricefields, most semiaquatic lowland
rice weeds can germinate and survive.
Stand establishment method
Twenty- to 30-d-old rice seedlings are
normally transplanted into a puddled
soil. In the irrigated rice-growing areas
of Asia, seedlings are raised in wet
bed, dapog, or dry bed nurseries. In
the wet bed method, pregerminated
seeds are broadcast uniformly on a
raised bed of puddled soil. Seedlings
are ready for transplanting 20-25 d
after sowing. In the dry bed method,
seedlings are grown similarly, but the
soil is not puddled and drainage is
provided.
Weeds in rice seedling nurseries can
cause the complete failure of the
nursery. Nurseries used to raise rice
seedlings should be kept weed free to
prevent transplanting grassy look-
alike weeds along with the rice seed-
lings. Transplanted weeds are highly
competitive and extremely difficult to
control by hand weeding or by
selective herbicides.
Because nursery areas are small
(about 21 × 21 m will provide enough
seedlings for 1 ha rice) and seedling
establishment takes only 20-25 d,
controlling weeds is easy. Propanil,
thiobencarb, butachlor, quinclorac,
bensulfuron, pretilachlor + fenclorim,
and pendimethalin give good weed
control in rice seedling nurseries.
Doubling the seed rate, hand
weeding, or removing large weed
seedlings from rice seedling bundles
resulted in less than 50% control of
weeds. Careful examination of each
plant to ensure that most weeds are
removed from the seedling bundles is
laborious, time-consuming, and more
expensive than herbicide treatment
(Moody et al 1988).
Land preparation
Mechanical land preparation should
provide a weed-free field to allow
optimal early rice growth. The initial
plowing buries weeds and crop
stubble from the previous crop.
Puddling uproots weeds that grow
after plowing and buries them in the
layers of mud. The field is leveled after
puddling to eliminate inadequately
flooded areas that are ideal for the
growth and development of difficult-
to-kill semiaquatic weeds.
Irrigated rice 73