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

deepwater (100–500 cm); tidal; and dry-season
irrigation.
Deepwater rice and floating rice are sown or
transplanted before the floodwaters rise and
flower near the time of maximum water depth.
Deepwater rice varieties are adapted to
maximum water depths of around 100 cm and
most can elongate 2–3 cm/d when flooded.
Floating rice varieties are those that elongate
very rapidly under submergence, sometimes up
to 20 cm per day. They are adapted to rapidly
rising water and very deeply flooded areas.
Tidal rice is cultivated in coastal zones
during the wet season. It can tolerate
submergence caused by tidal fluctuations or flash
floods. It should not elongate stems when
flooded since floodwaters recede within about 2
wk and the plants would fall over. Salt-tolerant
tidal rice is needed where salt water intrudes
from the sea. During the dry season, these lands
are too dry or saline for cropping.
Irrigated rice is grown in flood-prone areas
during the nonflood periods if irrigation is
available. It is called boro rice in Bangladesh and
India. Traditionally, boro rice was cultivated only
in local land depressions, where there was
sufficient residual water in the soil for a crop
during the dry season. However, with improved
irrigation, mainly from tube wells, boro rice is
now cultivated on many floodplains. Although
boro rice replaced some floating rice, there is
now progress in integrating both crops. Floating
rice has also been largely replaced by irrigated
rice in southern Vietnam.
In Bangladesh, deepwater rice farmers have
traditionally grown a wide range of crops after
the floods by seeding or planting onto the straw
of the deepwater rice crop. These crops include
pulses, oilseeds, spices, potato, onion, garlic,
wheat, and barley. Postflood and dry-season
cropping are now widely practiced in
Bangladesh, India, Myanmar, and Vietnam.
Preflood cropping, although not widespread,
includes mungbean, sesame, chili, and sorghum.
These crops, if successful, are harvested before
the main floods arrive. Sometimes jute or maize
is planted along with deepwater rice or just
before the deepwater rice crop is sown. In India,
farmers in many deepwater rice areas have
recently incorporated grain legumes (mainly
mungbean), sesame, maize, and sorghum into
their rotation. They grow these crops as a
mixture with deepwater rice on stored soil water
at the start of the wet season.
Productivity
Wet-season nonirrigated rice yields range from
zero to 4.0 t/ha depending on the season,
location, and rice type. Floating rice yields are
usually low, from about 1.0 to 2.5 t/ha.
Deepwater rice yields higher than floating rice,
sometimes up to 4 t/ha, and generates high
profits because of low production costs.
Submergence-tolerant varieties are grown in
more favorable areas and produce relatively high
yields if flash floods are not severe. Yields of
tidal rice vary widely and crop failure can occur
in salt-affected areas.
Production constraints
Although fertilizer tends to increase nonirrigated
rice yields, the response is often irregular
because of environmental stresses. As a
consequence, fertilizer use is limited and yields
are also low compared with irrigated rice, which
has controlled water supplies and high inputs of
fertilizer and pesticides.
There are many opportunities for increasing
the rice yields of the flood-prone ecosystem. The
yield potential of deepwater rice could be
improved by introducing an appropriate plant
type. Yields of flash-flood and tidal areas could
be increased by incorporating increased tolerance
for submergence, salinity, and acidity. Varieties
with tolerance for cold at the seedling stage and
in the early vegetative stage could increase boro
rice yield. However, yield increases for floating
rice through varietal improvement are more
difficult because survival is the first
consideration.
Enemies and friends
Pests
Rice fields harbor a tremendous diversity of
animals, plants, and microorganisms, some of
which are harmful to the rice crop and many of
which are beneficial. The goal of many scientists
at IRRI and other institutions is to manage rice
pests in ways that are safe, sustainable, and
economical. Emphasis is placed on breeding rice
varieties with resistance to insect pests and diseases
and on minimizing the use of pesticides to
promote natural biological control by beneficial
24 Rice almanac