A handbbok on Weed Control in Rice.pdf

Chapter 10
Management of some
difficult weeds in rice
Many weeds can be controlled effec-
tively only by using a combination of
methods. Using only one weed control
method leads to a buildup of weed
problems. This chapter gives informa-
tion on how intractable rice weeds can
be managed through an integrated
approach.
Scirpus maritimus
Scirpus maritimus, a perennial sedge
that spreads by tubers, is widespread
in lowland rice in several countries in
Asia, Europe, and temperate climate
USA (see page 000). S. maritimus is a
very competitive weed: it produces
numerous tubers, has fast shoot
growth, is able to emerge through
fields with standing water, and has
rapid nutrient uptake. Season-long
competition from S. maritimus can
reduce rice yields 60-100%. Its tubers
and buds can remain dormant in the
soil, making this weed difficult to
eradicate. S. maritimus is most
competitive from its early growth
stages to 80 d after germination. The
S. maritimus -free period required in
rice is the first 4 wk.
Cultural control
Cultural control of S. maritimus
involves tillage, crop rotation, and
water management.
Depth and type of primary cultiva-
tion greatly affect the S. maritimus
population. Shallow cultivation and
zero tillage encourage emergence of
tubers retained on the soil surface,
resulting in rapid population buildup.
Deep plowing buries the tubers and
results in growth of fewer seedlings.
The practice of zero tillage leads to
the buildup of a S. maritimus popula-
tion, but minimum tillage can be as
effective as conventional tillage in
limiting its growth. The weed persists
under continuously wet conditions but
diminishes dramatically with time
under continuously dry conditions. A
year of rotation of an upland crop with
lowland rice will reduce the preva-
lence of S. maritimus.
Hand weeding
Hand weeding is effective in control-
ling S. maritimus. The rice crop should
be kept weed-free for at least the first
4 wk. This makes several hand weed-
ings necessary, because S. maritimus
tubers germinate within 5 d after
harrowing and grow rapidly.
Herbicides
Herbicides effective against
S. maritimus in transplanted and direct
seeded flooded rice include bentazon,
fenoxaprop, propanil, 2,4-D, and
bensulfuron. 2,4-D at 0.5 kg ai/ha is
best applied when S. maritimus has
6-8 leaves. 2,4-D applied preemergence
does not control the weed, although it
may reduce the stand of annual
grasses. Bentazon at 1-2 kg ai/ha
should be applied at the 6- to 8-leaf
stage (about 25 d after sowing [DASI).
Bensulfuron at 50 g ai/ ha applied at
6-8 DAS or DT (2- to 3-leaf stage of the
weed) effectively controls S. maritimus
and annual weeds.
Integrated control
Integration of all workable weed
control practices can provide effective
and economical control of S. maritimus.
Such integration should begin by
creating an environment favorable to
rice growth but unfavorable to weed
growth. This includes using well-
adapted, high-yielding rice cultivars;
appropriate fertilizers; good manage-
ment; and crop rotation. Weed control
efficiency is improved by integrating
the use of herbicides and hand weed-
ing. The rotational crops used depend
on the region; maize, sorghum, and
soybean are becoming increasingly
important.
Difficult weeds 99