A handbbok on Weed Control in Rice.pdf

Chapter 8
Weed control in upland rice
Upland rice, also known as dryland or
pluvial rice, is grown on rainfed,
naturally well-drained soils. Strictly
defined, upland ricefields are not
bunded and no surface water accumulates.
About 13% (18.8 million ha) of the
world's rice area is upland (IRRI
1988b). About 11.9 million ha is in
Asia, 4.5 million ha in Latin America,
and 2.2 million ha in Africa. It is the
dominant rice culture in Latin America
and West Africa.
Upland rice is grown under a wide
range of management practices that
vary from shifting cultivation-as
practiced in Malaysia, Philippines,
Peru, and West Africa-to the mechanized
cultivation practiced in Brazil.
Most of the world’s upland rice is
grown on poor soils in areas with
uncertain rainfall by small farmers
using traditional, low-input technology.
Weed problems
Weeds rank second to drought stress
in reducing upland rice grain yields
and quality (Sankaran and De Datta
1985). Yield losses caused by uncon-
trolled weeds in upland rice are about
96%.
Upland rice, like all upland crops, is
planted in moist soil, that, in general,
does not retain moisture beyond field
capacity. Water is supplied by rains
during the growing season. Optimum
temperature, sufficient aeration, and
ideal moisture for weed germination
and growth exist at planting time. This
enables weeds to germinate earlier and
grow more vigorously than the rice
crop.
Weed competition is more intense
in upland rice than in irrigated and
rainfed lowland rice because upland
fields do not have standing water to
suppress weed growth. Some weeds in
upland rice can withstand drought
better than rice because their roots
penetrate deeper into the soil to tap
moisture. Poor rice germination due to
drought results in excessive weed
growth, especially if semidwarf
varieties are grown.
Because weeds in upland rice
germinate throughout the season,
dense weed growth may reoccur after
hand weeding or after the residual
effects of herbicides have worn off.
A mixture of annuals and peren-
nials, and grasses and broadleaf
weeds, intensifies the competitive
effects of weeds in upland rice. C4
weeds, which have higher water-use
efficiency than rice, prevail (Ampong-
Nyarko and De Datta 1989). Common
upland weeds include Cyperus rotun-
dus, Echinochloa colona, Eleusine indica,
Rottboellia
cochinchinensis, Cynodon dactylon,
Digitaria sanguinalis, Imperata cylindrica,
Amaranthus spinosus, Commelina
benghalensis, Trianthema portulacastrum,
Ageratum conyzoides, Portulaca oleracea,
and Euphorbia hirta.
Land preparation
Land preparation for upland rice
varies greatly among regions. In West
Africa, where shifting cultivation is
common, slash-and-burn is practiced
with hand tools. In South Asia, upland
fields are plowed by bullocks and in
Southeast Asia, by water buffalo.
Deep plowing (25 cm or deeper)
moist soil at the end of the rainy
season is recommended for upland
rice grown in the West African savan-
nahs (FAO 1976). Deep plowing and
subsoiling conserve soil moisture in
the rainy season and enhance root
growth and extraction of soil moisture
from deeper soil layers. It also will
bury weed seeds deep enough to pre-
vent them from emerging. Subsequent
tillage operations must be shallow.
Upland rice 91