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Rice
Origins and spread
Rice (Oryza sativa) is the most important cereal crop in the developing world and is the staple food
of over half the world's population. It is generally considered a semi-aquatic annual grass plant.
About 20 species of the genus Oryza are recognized, but nearly all cultivated rice is O. sativa L. A
small amount of Oryza glaberrima, a perennial species, is grown in Africa. So-called "wild rice"
(Zizania aquatica), grown in the Great Lakes region of the United States, is more closely related to
oats than to rice. The general consensus is that rice domestication occurred independently in China,
India and Indonesia, thereby giving rise to three races of rice: sinica (also known as japonica), indica
and javanica (also known as bulu in Indonesia).
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In 1988, China was the principal rice producer (35 percent) followed by India (22 percent),
Indonesia (8.5 percent), Bangladesh (4.7 percent), Thailand (4.3 percent) and Viet Nam (3.4
percent). Of the major rice producers only Pakistan, the United States and Egypt had 100
percent irrigated rice land (IRRI, 1991 a). Non-irrigated rice cultivation predominates in many
countries, such as Thailand and Brazil.
Adaptation
In Asia, cultivars with resistance to aluminum toxicity and with tolerance to submergence by flood
water, high salinity and cool temperatures at the seedling or ripening stage have been developed. In
Africa, cultivars with tolerance to iron toxicity and heat constraints have also been developed and
cultivated. Rice is now grown in over 100 countries on every continent except Antarctica, extending
from 50° north latitude to 40° south latitude and from sea level to an altitude of 3 000 m. Rainfall is
the major limiting factor in rice production. Paddy requires 1500 to 2000mm while upland requires
750mm.
Temperature
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Average 20 – 38 oC
Optimum 30-35 oC
Minimum 11-12 oC
It doesn’t survive freezing
Soils – heavy alluvial soils are more suitable for flooding. pH for dry soil 5.5-6.5
A race (in this context) is a genetically and often geographically distinct mating group within a
species.
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1. Japonica Rice
Japonica is one of the three major eco-geographical races of O. sativa (O. sativa japonica), the other
ones are Indica and Javanica.
It is a group of rice varieties from northern and eastern China grown extensively in some areas of the
world. It is found in the cooler zones of the subtropics and in the temperate zones.
It has narrow, dark green leaves, medium-height tillers and short to intermediate plant stature. It is
medium tillering and response to high management levels such as Nitrogen fertilisation.
2.Javanica
Designation for the bulu and gundil varieties of Indonesia and many upland rices. They belong to the
Japonica race of O. sativa. Javanica are rice varieties with broad, stiff, light green leaves. It is lowtillering and has a tall plant stature.
3. Indica
Indica rice is the other of the two major eco-geographical races of Oryza sativa (O. sativa indica). It is
the major type of rice grown in the tropics and subtropics. They grow mostly in the Philippines, India,
Pakistan, Java, Sri Lanka, Indonesia, central and southern China, and African countries. Indica has
broad to narrow, light green leaves and tall to intermediate plant stature (except for the semidwarf). Indica plants tiller profusely. They adapted to infertile zones, low yielding and have poor
response to N fertilisation.
Cultivation systems
Basically rice is grown under three main systems:
1. Lowland rice – continuously grown under flooding (paddy rice).
2. Upland cultivation/irrigated – where rice is grown under high rainfall or is irrigated. It
accounts for 55 % of world production.
3. Deep water or floating rice - Water is 1 to 5 meters deep and is supplied by rivers, lakes or
tides in river mouth deltas. Water depth may exceed 5 meters in some parts of Bangladesh,
as well as in the Mekong, Kariba dam (Tongas) and Niger deltas. Seeds are broadcasted in
ploughed fields, normally unbunded, in regions where the water level rises quickly after the
beginning of the monsoon. Traditional long tiller and few sprout varieties are cultivated. The
plant elongates and floats as the floodwater advances, thus its name - "floating rice". The
panicle is able to adjust by 25mm/day.
Grain types of rice
a)There are five types of grain types in rice:
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1.
2.
3.
4.
Short rice – mostly the Japonica type. Grains less than 5 mm
Medium rice – the grain size is between 5 – 6mm
Long grain – Mostly Indica with grain size 6-7mm
Extra long - >7mm
b) Endosperm types
1. Waxy/glutenous is soft and has amylopectin type of starch. It contains no amylase and when
cooked, tends to be sticky.
2. Non-waxy/non-glutenous is hard and starchy and has high content of amylase and tends to cook
dry and fluffy.
Plant characteristics of rice
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Its a short day annual
Its a facultative hydrophyte (can grow in water logged conditions)
It has a shallow root system of 20 – 25 cm under flooded situations
Plant height is between 50 and 150 cm and can get to 5m under floating systems.
It is mainly self pollinated
Tillering begins at 3 weeks and lasts 3-4 weeks to produce 4-5 tillers/plant
Seedbed preparation
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For upland rice plough 10-20 cm
Seedbed conditions largely similar to other small grains
In wet systems, the land systems is levelled and then puddled (creating an impermeable
layer at a depth of 25 cm to create a permanent bed of water). Once puddled, create bunds
to hold water in the bed
Planting
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Rice can be planted by direct seeding (60-80 kg/ha). It is quite feasible to pre-germinate seed
for use (soaking in water). Depth 50-6cm or less
Have choices to transplant but there is no yield advantage. On the other hand it is labour
intensive. It however, requires less water but requires better timing in the establishment of
the crop. Seedlings are transplanted at 15 cm and spacing for long season cultivars is 20-25
cm, 10-15 cm for short season cultivars. Plant 2-6 seedlings/planting station.
Fertiliser
The most important nutrients in rice production is N and P. Experiments done on absorption of N
show that absorption is low in rice with only 30-40 % of basal fertilisers being absorbed while only
50-60% is absorbed for top dressing fertiliser. Factors affecting fertiliser efficiency include:
i)
Soil type
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ii)
iii)
iv)
v)
vi)
vii)
viii)
ix)
Season in which rice is grown
Time of planting
Water management used in the production of rice
Weed control
Insects and diseases
Cropping sequence
Time of fertiliser application
Type of fertiliser
In general, urea is the most preferred fertiliser. It is readily absorbed by the crop and is quicly
hydrolysed in the soil. Urea is a quick acting fertiliser for rice. In the early phases of growth rice will
prefer N in the form of Ammonia (NH+4) form. In the latter stages of growth source of N is less critical
as both and NO3- are taken up. Optimum N for the Indica type of rice is 30-40 kg N/ha, for the
Japonica type its 100-150 kg N/ha while for the Javanica type is 75-120 kg/ha.
In an experiment carried out in Chiredzi the following results were found out:
Kg N
Yield t/ha
100
6.9
300
7.9
Probability
<0.01
Ammonium sulphate is best source of N in wet soils because of reduced nitrification but urea is also
a popular source in wet soils.
Rates
Nutrient removal for a crop of 3360 Kg (grain) + 3360 (straw) at experiments done ait IRRI is 54 kg
N/ha, 26 Kg P/ha and 46 kg K/ha.
Methods of application of fertiliser
1.
2.
3.
4.
5.
Broad casting
Drilling
Deep placement
Foliar application
Fertigation
Choice of placement method depends on:
1) Fertiliser type
2) Rice culture
3) Soil type
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Deep placement is placing fertiliser at 5-10cm soil depth. Its superior to most fertiliser placements
in rice. Basal fertiliser is applied before or at planting and top dressing is applied just after shooting,
flowering and at full heading. In the Lowveld it translates to 20DAS, 65 DAS and 78 DAS. On clay, its
possible to make one application. Its an advantage to split on lighter textured soil.
Irrigation Management
Water is the most important single factor affecting rice production. Management of water is very
important where rice is grown under irrigation.
There are two production systems:
1) Continuous flooding (paddy)
2) Intermittent irrigation- upland scheduled irrigation.
In Zimbabwe in S.E. loweveld, rice can be grown successfully with 40% depletion. Stress is most
severe at flowering where you can get a reduction of 40-60%. Yields are generally higher if rice is
under continuous flooding, but WUE is low in this system.
Days to panicle initiation
Days to panicle emergence
Days to flowering
C Flooding
54.6
78.2
82.3
Irrigation
60.5
88.2
92
Maturity
Yields
C Flooding
107
7.6 t/ha
Irrigation
114
7.2 t/ha
NB Significance P<0.01
Rotations
Although rotations are desirable for rice, they can be problematic depending on the systems
selected. On puddle soils, rotations can be problematic especially where the puddle has to be used
again for rice. Popular rotations for rice in China and Japan are rice-rice-wheat, Australia RiceWheat-oats, Zimbabwe Rice-Cotton rotation. In the smallholder sector Rice-maize system.
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Harvesting
Rice will take 35 days from heading to physiological maturity, very early varieties mature by 110 days
or less, early maturing 110-140 days, late 150-180 days and very late >180 days . where paddy rice is
grown, drain beds 2-3 weeks before harvesting. Moisture content at harvest of grain 18-25 %.
Optimum for storage 12.5 %.
Commercial classes when rice is harvested
1) Rough rice-is that rice that is from the field with up to 50 % of hulls removed (rice used as
seed for the next crop).
2) Brown rice- all hulls removed from the rough rice to get brown rice.
3) Milled rice- kernels polished.
Rice products
1.
2.
3.
4.
Grain- eaten boiled or steamed.
Breakfast cereals, either flaked or puffed.
Beer, wines or spirit
Other- livestock feed, fuel from the straws and husks, roofing material and straw mats etc.
Economic importance
90 % of rice found in Asia. Largest areas are 1) India 2) China 3) Bangladesh 4) Thailand 5) Indonesia.
China is the leading producer.
In Zimbabwe
Rice has been around from the 1900s. Traditional systems were based on the hydrophomic systems
on the vleis. There has been very little change in technique. Yields from traditional varieties are 0.5
to 1.5 t/ha. Essential changes came in the 1920s when the Department of Conservation and
Extension (CONNEX now Agritex). Prohibited farmers from growing crops in wetlands. Commercial
production started in the 1960s in the Lowveld area. Production increased from 4 ha in 1960 to 1500
ha in 1970. That increase was accompanied by an increase in yield from between 0.5 t/ha to 4.5 t in
1970. The increase was a result of improved varieties and better agronomic management practices.
By 1970, 4500 t of rice was produced from 20 farmers. After 1970 production declined in the
lowveld leaving one grower in the lowveld with average yield 0f 5t/ha. In 192 ARDA Jotsholo began
rice production on 350 ha. Average yields were between 2.2 and 4.5 t/ha.
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Decline in rice production was a policy issue when Rhodesia decided to import rice from Malawi to
reduce the balance of trade between the two countries. The current government wanted the
country to be self sustainable and produce >20 000t per annum. Potential exist for rice production in
Zimbabwe.
Factors that led to the decline in rice production
1) Production factors
2) Fungal disease called ‘blast’ caused by Piricularia oryza affected rice production in the
lowveld during the 1969/70 season. The then commercial varieties Le Bonnet and Blue Belle,
IR400 were heavily hammered (quality and yield) and a lot of farmers went out of business.
3) There was a trading policy with Malawi that meant importing cheaper rice coupled with the
‘blast’.
4) Policy that restricted wetland production decreased smallholder rice producers.
Research Efforts in Zimbabwe
Started in Chiredzi, Chisumbanje, and Save Valley Experiment Station in the 1950s. Focuss was on
adapted germplasm, time of fertiliser application, fertiliser types, irrigation, disease control,
cropping systems and fertility management.
Researchers who have have worked on rice include Mharapara I., Gata R.N., and Robbie. Mupawose
did PhDs on Rice.