Rice is a cereal crop belonging to the grass family and is one of the main sources of carbohydrate for human consumption, particularly in the Far East. The major rice growing countries have always been China, India, Burma, Thailand, Japan, Sri Lanka and Malaya. Over the last 30 years, rice production has expanded in North and South America, and these areas are now major producers of rice. Any world shortage of rice has a major impact on the availability and price on the world market of the other major cereals; wheat, barley and coarse grains.
Rice has a high water requirement, between 750 to 1750 mm, including rainfall and irrigation.
Rice is an aquatic plant and can absorb oxygen from water, hence it being traditionally grown in flooded paddy fields. It can also be grown as upland rice under dryland or irrigation regimes.
Rice plants grow to 1,2m to 1,8m tall and require a long growing season, with high temperatures of 32⁰C – 36⁰C. In summer, average temperatures should be around 28⁰C.
Long periods of sunshine are necessary, especially during the first two months of growth. Fertilization of the rice flower takes place during the night and if the temperature drops below 10⁰C, the development of the stamens is retarded, resulting in sterile flowers.
Soils should be fertile, rich in organic matter and have a high water-holding capacity. A relatively impervious sub-surface layer would be an advantage, to retain moisture. In the case of paddy fields, the soil must be drainable at harvest to allow the access of harvest machinery.
Paddy fields should be level, with walls built strong enough to hold 150 mm to 200 mm of water, and should not be larger than 1 – 2 ha.
2. TYPES OF RICE
Rice belongs to the genus Oryza, and as well as the many wild species, there are two cultivated species: Oryza sativa, which is found in all the rice-growing areas of the world, and Oryza glaserima, which is grown only in tropical West Africa.
Oryza sativa is sub-divided into O. sativa japonica, which is cultivated in Japan, Korea, China and other rice-growing areas in the sub-tropics, and O. sativa indica, cultivated in India and other tropical Asian countries.
Japonica rices give high yields, a good response to high levels of fertilizer; have short, stiff straw and a round coarse grain with a high percentage of rice to un-hulled grain.
Indica rices have a large type of grain, give a lower yield because they cannot stand heavy fertilization, but are hardier and more resistant to unfavourable conditions. Work has been carried out to cross Japonica and Indica to produce a hybrid rice.
The species Oryza sativa has a great number of varieties with different characteristics, among them the following:
- Early maturing varieties with a growing period of less than 80 days compared to late maturing varieties which can take up to 200 days to mature.
- Varieties which can be grown in salt water as opposed to most varieties of rice which require fresh water. This is important where irrigation is being done with brak water.
- Deep water or floating rice which can grow 0,5m per day if flood water is rising at that rate. This rice can reach a height of 3 metres when the water level falls. The long stalks lie along the soil surface, become imbedded in the mud, and sprout from each node in the stem, giving a new crop.
- Upland rice, which grows like other cereals and is dependent on natural rainfall only.
- Lowland or wet rice, which has to be grown under irrigation or in natural swamps where there is standing water during the growing season. World production of wet rice is much greater than that of dryland rice.
- Varieties which produce a grain and remain separated after cooking, as well as gluten varieties which produce a gluey, sticky mass when cooked. These are used for special foods such as pastries and sushi.
3. CULTIVATION AND FERTILIZATION
There are more than one hundred rice varieties, which all have different growing periods, growth heights, size of grain and cooking qualities. Varieties grown in Southern Africa include early varieties (IR 28, La-Bella, Le Bonnet, and Riverside) and late varieties (IR 400, Brazos, Lemont, Star Bonnet and Blue Bonnet).
Highest yields are obtained when planting takes place from August to October for longer maturing varieties and from mid September to the end of November for early varieties, that have a growing period of 3 to 4 months.
Rice requires a fine tilth for sowing, and the land is cultivated in the same way as for other cereals. Seed can be broadcast, or is usually sown with a wheat drill in rows which are 0.3 metres apart. The seed rate is 100 kgs per ha. The crop is sown later in the season to avoid rainy conditions at harvest.
Paddy Rice is cultivated in standing water. Rice is sown in well prepared, damp soil, which is then irrigated and drained to prevent the seed rotting. When the plants are 150 mm tall, water is applied to a level of 25 mm – 50 mm. Water is then gradually increased to 100 mm – 150 mm.
During autumn and winter, the rice fields should be disced and thereafter irrigated, in order to germinate any weed seeds. Prior to planting, shallow cultivation is necessary to kill the germinated weeds.
With upland rice, post-emergence herbicide should be applied when weeds are at the 2 – 5 leaf stage. In paddy rice, most weeds are drowned when water is applied to the rice plants when they are 150 mm tall.
The most important nutrient in the growing of rice is nitrogen and a green manure crop should be included in the rotation to maintain a high level of organic matter. The straw should be left on the land after combining. Where detailed soil analysis is not available, the following rough guide is recommended:
- Nitrogen: 50 – 100 kgs LAN/ha as a top dressing.
- Phosphorous: 200 – 400 kg/ha superphosphate.
- Potassium: 100 – 150 kg/ha muriate of potash.
The time of application of the top-dressing is important for the early maturing varieties, such as Blue Belle. The crop should be top-dressed about 60 days after sowing and for the late-maturing varieties such as Blue Bonnet, it should be done approximately 85 days after sowing.
The seed is planted and irrigated in a similar manner to wheat, by means of overhead or centre pivot irrigation. This should be done in accordance with Class A pan scheduling methods.
The rice is cultivated in standing water, which has the advantages that the crop receives the required amount of water, most weeds are drowned and temperature fluctuations are reduced. The rice is sown in damp, well-prepared soil, which is then thoroughly irrigated and drained to prevent the seed from rotting. When the plants are 150 mm tall, water is applied to a level of 25 mm – 50 mm. As the plants grow, the water level is gradually increased to 100 mm to 150 mm.
Rice suffers from a number of pests and diseases. The main pest is nematodes which can be controlled by using sound rotations. The chief diseases are fungal diseases, notably rice-blast disease which causes the stalks bearing the seed-heads to break off near the top of the stalk. Some fungal diseases can be controlled by dressing the seed with fungicide before sowing. The use of resistant varieties can reduce the effects of rice blast disease.
Yields of 7 tons per hectare of unhulled rice have been obtained from Blue Belle, and 6 t/ha of unhulled rice for Blue Bonnet in Southern Africa.
Approximately 45 – 55 days after the rice has come into ear, it is ripe enough for harvesting. In the case of paddy rice, the grower should time the draining of the field accordingly.
The best quality rice is produced when it is harvested at a moisture content of 20% to 27%.
The best method of harvesting is with a combine. When rice is harvested by hand, it is first cut and tied into sheaves, which are then stoked and then threshed with a wheat thresher after 10 – 14 days. Rice must not be dried in the sun, but rather spread out in a thin layer under shade, where it should be turned several times a day. If rice is dried gradually, cracking is reduced. Rice should be stored at a moisture content of 13% – 14%.