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produce rice on both lowland and upland fields.
In such areas, farmers invest more in their
lowland fields, where greater response to inputs
is usually obtained.
Upland rice is grown in an intensively
managed commercial system in the Brazilian
Cerrado, where it is rotated with soybean and
pasture and often receives supplemental
irrigation. Rice is grown no more often than one
season in three because of the yield decline
associated with continuous or short-rotation
upland rice production. Commercial, high-input
upland rice production with supplemental
irrigation is also emerging in water-short areas of
northern China. These intensively managed
upland crops are now usually referred to as
“aerobic rice” to distinguish them from
traditional low-input rice crops. Areas planted to
aerobic rice in Asia can be expected to increase
because of water shortages in some areas
currently growing irrigated lowland rice.
Productivity
Although accounting for 13% of total world rice
area, upland rice contributes only 4% to total
world rice production. Grain yields average
about 1 t/ha in traditional low-input systems, but
may reach about 2 t/ha in favorable upland
environments such as those found in Mindanao,
Philippines. In the Brazilian state of Mato
Grosso, where upland rice is grown with
supplemental irrigation and high inputs of N,
yields averaged nearly 2.5 t/ha in 1998-99.
Aerobic rice yields of more than 5 t/ha have been
reported in China and Brazil.
Production constraints
Biological and physical constraints to upland rice
yield are numerous. Weeds are the most severe
biological constraint, followed by blast disease
and brown spot. Weeds reduce upland rice grain
yield and quality. Estimates of yield losses
caused by weeds in upland rice range from 30%
to 100%. Small farmers using traditional
production systems cannot afford chemical weed
control, but herbicides are used in commercial
aerobic rice systems in Brazil and China. Stem
borers and rice bugs are the predominant insect
pests. Nematodes can cause yield losses of up to
30%. Rodents are reported in many countries as
a major problem and birds are sometimes serious
pests.
Amount and distribution of rainfall and poor
soil fertility are the most serious physical
constraints. In Asia, the monsoonal rainfall
ranges from 1,500 mm to more than 3,500 mm.
Rainfall is usually erratic during the monsoon
season. In Africa, annual rainfall ranges from
1,200 to 2,000 mm where upland rice is grown.
Rainfall is frequently erratic in semiarid areas of
West Africa. Mean annual rainfall in Latin
America ranges from 1,400 mm in central Brazil
to 4,800 mm in Costa Rica. Dry spells frequently
occur during the growing season in most of these
areas, mainly in northern and central Brazil.
Upland rice soils are predominantly acidic
(pH varies from 4 to 6) and depleted in major
elements. In South and Southeast Asia, more
than half of the upland rice is grown in infertile
soils. In most upland soils, P rather than N is the
most common limiting nutrient. Physiological
disorders result from major and trace element
imbalances. Major toxicities are caused by high
Al and Mn content in strongly acidic soils.
The temperature of most regions of Asia,
Latin America, and Africa is relatively favorable
to upland rice except in some high-altitude areas
in India, Indonesia, Myanmar, Nepal, and
Thailand. Minimum and maximum temperatures
average 20 and 30 °C for Latin America, 20 and
32 °C for South Asia, and 18 and 35 °C for
Africa. The optimum temperature for maximum
rice photosynthesis is 25–30 °C.
Flood-prone rice ecosystem
Physical description
The flood-prone ecosystem has several
environments and incorporates a number of rice
plant types. These rice plants must be adapted to
conditions such as (1) deepwater: submergence
in depths usually exceeding 100 cm and for
durations ranging from >10 d to 5 mo, requiring
the plant to elongate greatly to reach the surface;
floating rice up to 5 m long is included here; (2)
flash flood for periods of longer than 10 d; (3)
salinity caused by tides in extensive low-lying
coastal areas, where plants are subject to daily
tidal submergence; the plants do not elongate
greatly but tillering and tiller survival are at risk;
and (4) problem soils, such as acid-sulfate and
sodic soils, in which the problem is often excess
water, but not necessarily prolonged
submergence. Around 11 million ha of rice lands
worldwide are affected by one or more of these
conditions.
22 Rice almanac