A handbbok on Weed Control in Rice.pdf

Chapter 9
Weed control in deepwater
and floating rice
Deepwater and floating rice constitute
about 11% of the world’s rice area
(IRRI 1988b). They are grown in the
deltas, estuaries, and river valleys of
Bangladesh, Cambodia, India,
Indonesia, Myanmar, Thailand, and
Vietnam. In Africa, deepwater and
floating rice-growing areas are found
in the inland Niger River delta in Mali,
in the Niger River delta, Nigeria, and
in inland swamps.
Deepwater and floating
rice cultures
Deepwater rice is grown in areas
where the maximum depth of stand-
ing water exceeds 50 cm for a signifi-
cant period of rice growth. Where
water depth is greater than 100 cm, the
culture is referred to as floating rice or
very deep water rice. Fields are not
bunded and flooding usually occurs
only during the later part of the grow-
ing season. Rice may grow under
drought conditions for 40-60 d before
flooding occurs.
Weed problems
Weeds are a major problem in deep-
water and floating rice because rice
normally is seeded into soils that may
remain dry for 6-12 wk. Fields may
also be flash flooded. The dry soil and
intermittent flash flooding result in a
broad spectrum of weeds-from
upland to semiaquatic and aquatic-
that successively compete with the rice
crop. Weeds adapted to dryland
compete at early crop stages, die upon
flooding, and are succeeded by aquatic
weeds.
Before flooding, deepwater and
floating rice may be infested by
Echinochloa colona, Eleusine indica,
Cyperus rotundus, C. iria, and
C. difformis. These weeds also may
compete with rice during the early
stages of flooding.
Deepwater and floating rice may
suffer severe weed infestations during
the preflood period. Weed problems
are often aggravated by poor stand
establishment due to drought. At the
preflood stage, weeds reduce growth
and tillering of rice plants and can
reduce grain yield as much as 33% (De
Datta and Hoque 1982). Total crop loss
can occur if flooding is below normal.
Minimizing competition from
weeds at early rice growth stages
largely determines how well deep-
water and floating rice will tolerate
stresses caused by flooding at later
stages. Aquatic weeds that occur upon
flooding are less a problem when
stand establishment is good and rice
tillering is adequate than when early
weed competition is severe. In small
cropping areas where farmers wet
seed or transplant deepwater rice,
puddling reduces weed infestation.
Amphibious weeds, such as the
wild rices, Scirpus sp., Sesbania sp., and
Leersia hexandra, can compete with rice
under dryland or flooded conditions.
Eichhornia crassipes, Ipomoea aquatica,
Monochoria vaginalis, and Pistia
stratiotes are found in deepwater rice
after flooding has occurred.
Weeds such as the wild rices
possess flood tolerance and elongation
ability, and grow with rice in rising
floodwater. Therefore, they are able to
cause considerable yield losses. Masses
of E. crassipes are usually introduced to
the ricefield by water currents or
strong winds. When this occurs, E.
crassipes can completely smother the
rice crop within a few days. In West
Africa, the most troublesome weeds
include E. stagnina and E. pyramidalis,
and the wild rices Oryza longistaminata
and O. barthii.
Weed control before
flooding
The deepwater rice cycle has two
distinct phases—before flooding and
after flooding. Weed control methods
can be described best within these
phases. Deepwater rice before flooding
is treated either as rainfed lowland rice
or as upland rice, using the same weed
control methods.
Deepwater and floating rice 95