A handbbok on Weed Control in Rice.pdf

Cyperus rotundus
C. rotundus, a perennial sedge, is a
persistent, aggressive weed wherever
upland rice is grown. It has been
reported to infest fields in tropical
areas of Africa, Asia, and Latin Amer-
ica (see page 29). Under intensive
cultivation, it becomes a serious weed.
C. rotundus has an extensive under-
ground system of basal bulbs, roots,
rhizomes, and tubers, which permit
rapid and vigorous vegetative propa-
gation. Buds in a tuber and tubers
within a chain exhibit apical domi-
nance. That apical dominance can be
broken during cultivation by severing
any tuber from the chain. This stimu-
lates dormant tubers to sprout.
C. rotundus germinates before, or
simultaneously with, upland rice and
competes for nutrients and moisture,
causing yield reductions as high as
50%.
Cultural control
Adequate fertilizer, optimum plant
density, and optimum time of planting
are good cultural practices for
managing C. rotundus.
Hand weeding
C. rotundus emerges with the rice crop
and outgrows it during the early
growth stages. Because of the rapid
regeneration of C. rotundus, hand
weeding has to be done at frequent
intervals to effectively prevent the
smothering of rice by the weed. Hand
weeding should continue until the rice
canopy closes-when shade will
suppress C. rotundus growth.
102 Weed control handbook
Herbicides
Glyphosate applied preplanting is best
to control C. rotundus. Other herbicides
that can be used are bentazon at 2.0-
3.0 kg ai/ha and 2,4-D at 0.5 kg ai/ha
applied postemergence, 3 wk after
seeding. These selectively kill
C. rotundus, but control is temporary.
Integrated control
Management practices, such as apply-
ing adequate fertilizer after weed
control, optimum plant density, and
optimum time of planting to avoid
drought, combined with hand
weeding or herbicides, will reduce
C. rotundus and maximize upland rice
yields.
Combinations of tillage and
chemical methods have proven more
effective in controlling C. rotundus
than tillage or herbicides alone. In an
integrated control scheme, C. rotundus
is allowed to grow as long as possible
(at least 3-4 wk) after plowing and
harrowing have stimulated dormant
buds to sprout. Then, systemic
herbicides, such as glyphosate, that
have no residual soil activity are
applied. Rice is planted within a week
without further land preparation. This
will provide season-long control of
C. rotundus and, if continued over
several seasons, may eliminate the
weed altogether.
lmperata cylindrica
Imperata cylindrica, a perennial
rhizomatous grass, is a major weed in
tropical Africa, Australia, South Asia,
South America, and the Pacific islands
(see page 35). It grows under a wide
range of ecological conditions ranging
from long dry spells to waterlogging.
I. cylindrica produces extensive
rhizomes, which readily regenerate. It
has a high tillering capacity and
establishes quickly to cover a large
area. Tillering is encouraged by
burning, cutting, or grazing.
Apart from competing directly with
upland rice for nutrients, moisture,
and light, I. cylindrica limits upland
rice production by reducing the total
land area available for cropping; lands
badly infested by the weed are
abandoned.
Cultural control
Rhizomes of I. cylindrica are suscep-
tible to partial desiccation even in the
wet season. Effective control is
obtained when the weed is frag-
mented by plowing to depths of
15-20 cm at the beginning of the dry
season, when the fragments have time
to dry before the onset of rain. This
dry season plowing should be
repeated every year until I. cylindrica is
controlled. Tractor-drawn implements
are needed for adequate plowing. The
economics of repeated plowing need
to be examined.
Hand weeding
Hand weeding has to be done at
frequent intervals for good control of
I. cylindrica.