A handbbok on Weed Control in Rice.pdf

7.2 Rice growth stages when
herbicides can be applied on wet-seeded
rainfed lowland rice. Bars (—) show
periods when a particular herbicide is
applied. *Timing of herbicide application
is based on weed emergence and growth
stage within the rice growth stage.
Hand weeding
Hand pulling of weeds can be done
in broadcast or drill seeded rice;
mechanical weeding is feasible only
when rice is planted in rows. Two to
three hand weedings at 2-3 wk after
sowing are usually sufficient to
ensure optimum yields.
Herbicides
The use of herbicides for weed
control has proved effective and
economical in direct seeded rainfed
lowland rice. Because weeds and rice
germinate together, the number of
herbicides that can be used safely is
limited. The efficacy of herbicides is
dependent on soil moisture condi-
tions and is markedly reduced by dry
or deep flooding conditions immedi-
ately after herbicide application. In
the tropics, butachlor, thiobencarb,
butralin, and propanil have been
used in direct seeded rice. Table 7.2
and Figure 7.2 provide herbicide
information for this rice culture.
Direct seeded
on dry soil
Direct seeding on dry soil provides
an opportunity to increase cropping
intensity in rainfed lowland rice.
Eliminating puddling shortens land
use time. In this culture, dry seed is
sown directly into moist, nonpuddled
soil at the beginning of the rainy
season. The field may be bunded to
accumulate water as the rainy season
progresses and the crop may end its
cycle under flooding.
Direct seeded dry rice culture is
practiced in Africa, South America,
and in parts of tropical Asia. In
Africa, direct seeded rice is grown in
valley bottoms and on hydromorphic
soils—soils on transitional slopes
between upland soils and valley-
bottom soils, with the groundwater
table in the root zone for most of the
growing period. Direct seeded
rainfed lowland rice is known as aus
cropping in northeastern India and
Bangladesh. In Indonesia, it is called
gogorantja (gogorancah).
Weed problems
Among rainfed lowland rice cultures,
weed problems in direct seeded rice
are more intense and wider in range
than those in transplanted or broad-
cast rice where puddling reduces
weed problems. Rice yield losses
from uncontrolled weeds can be as
high as 74%. Rice-weed competition
for moisture is heavy during the early
growth stage, when there is no
standing water. Dry rice seeds germ-
inate 3-5 d later than pregerminated
rice seeds. Weeds germinate and
establish faster than rice.
The prevailing moist, aerobic
condition for direct seeded rice
encourages the growth of upland,
semiaquatic, and aquatic weeds.
Dominance of any of these weed
communities depends on the availa-
bility and depth of standing water.
Many well-established upland weeds
continue to survive under flooding
later in the crop cycle. Lowland
weeds that are important include
E. crus-galli, I. rugosum, L. chinensis,
C. difformis, F. miliacea, and
S. maritimus.
Rainfed lowland rice 87