Maclean et al. - 2002 - Rice almanac source book for the most important e

growth duration (around 100 d), greater yield
stability due to genetic resistance to pests, and, to
a lesser extent, tolerance for some environmental
stresses such as problem soils.
Breeding programs have improved grain
quality and increased grain size and milling
recovery of the modern, high-yielding varieties.
Increasing nutritional value by incorporating
micronutrients such as iron and zinc or betacarotene
(vitamin A) are currently goals of the
breeding program at IRRI.
The irrigated ecosystem benefited greatly
from the development of associated management
practices that allow the semidwarf varieties to
express their yield potential. In partnership with
national agricultural research and extension
systems, international research on the irrigated
rice ecosystem focuses on yield-limiting factors
and technological advances to overcome them.
Research is also conducted on the resilience of
intensively cropped systems, including
identifying factors that contribute to stagnating
or even declining yields.
The objectives of IRRI’s research are to
extend the yield frontier through varietal
improvement, more efficient use of inputs, and
rehabilitation and maintenance of the physical,
biological, and natural resource base. Technologies
to increase the efficiency with which the
rice crop uses inputs should improve rice lands,
reduce dependence on pesticides, and increase
the profitability of rice production.
WARDA emphasizes integrated rice
management options and short-duration, highyielding
cultivars that are weed-competitive and
N-use efficient to close the yield gap, that is, the
difference between actual yields in farmers’
fields and yield potentials of existing rice
varieties.
Rainfed lowland rice ecosystem
Physical description
Rainfed lowland rice grows in bunded fields that
are flooded for at least part of the cropping
season to water depths that exceed 100 cm for no
more than 10 consecutive days. Rainfed
lowlands are characterized by a lack of water
control, with floods and drought being potential
problems. In favorable rainfed lowlands, small
investments in water-control measures can
increase water control considerably. In these
cases, cropping systems are on a continuum
between rainfed and irrigated ecosystems and
available technologies of the irrigated sector
need only minor adaptations. Considerable
adaptations or different technologies are needed
in the rainfed lowlands where risk of drought or
flood is significant. About 34% of the world’s
total rice land or approximately 54 million ha are
rainfed. Adverse climate, poor soils, and a lack
of suitable modern technologies keep farmers
from being able to increase productivity.
The rainfed lowland rice ecosystem can be
divided into four subecosystems: (1) favorable
rainfed lowland, (2) drought-prone, (3)
submergence-prone, and 4) drought- and
submergence-prone. Technologies for the
irrigated rice sector can be applied in the rainfed
lowland ecosystem only where there is no
significant risk of drought or flood. However, the
majority of lands in this ecosystem do face these
risks, requiring different rice varieties and
management strategies from those used in the
irrigated ecosystem.
Another ecosystem that has significant
potential for rainfed rice production is inland
valleys, particularly in Sub-Saharan Africa. This
is discussed in “International rice research and
development” (pages 51-52).
Predominant cropping systems
The rainfed lowlands include areas in which
farmers grow only one crop of rice, although in
some areas farmers grow rice and a postrice
crop, usually on a smaller area. The main postrice
crops are chickpea, mustard, linseed, rice,
and mungbeans, followed to a lesser extent by
vegetables, maize, wheat, and other legumes
such as soybeans and lentils. The choice depends
on water availability and season duration.
Farmers often cultivate rainfed lowland rice at
several toposequence levels such that on one
farm some fields may be drought-prone, while
others may be flood- and submergence-prone in
the same season.
Productivity
The world’s rainfed lowlands offer substantial
potential for increasing rice production.
Population pressure on arable land is low; the
area planted is increasing, but yields remain low.
Farmers have developed a range of practices that
address variability across sites as well as
heterogeneity within local ecosystems.
The rice plant and its ecology 19