Rice Fact File

Origin—Domesticated in Africa (Oryza glaberrima) and Asia (Oryza sativa). Several centres of origin have been proposed for O. sativa, including India and northern Thailand. Evidence points to the Yangzi Valley in southern China as one site of origin for domesticated rice.

Botany—Rice is a grass (Gramineae) and belongs to the genus Oryza (meaning oriental). Oryza sativa is grown in a wide range of environments from the equatorial tropics to sub tropical mid-latitudes, from lowland paddy fields to high altitude terraces, and from swamps toupland rice fields.

Cultivars—Since the introduction of modern varieties in the 1960s, most paddy rice farmers cultivate short straw, nitrogen (N)-responsive varieties with multiple pest resistance. Local varieties are more common in upland, rainfed, and deep-water rice environments. Improved germplasm for some of these environments is now available.

Harvest part—In upland rice fields, the ripe panicle is removed with a special knife concealed in the palm of the harvester’s hand, and straw is left standing. In paddy rice fields, rice is harvested with a sickle or mechanical harvester, and the panicle together with a portion of the stem is removed. The amount of stem removed depends on the threshing method used and farmer requirement for straw as livestock bedding, fuel or mulch.

Life cycle—The growing season of some traditional varieties is about 260 days, but is between 90 to 110 days for most modern varieties. Shortening the growing season is a key factor in increasing cropping intensity (crops/ha/yr). Crop maturation is extended under conditions where phosphorus (P) or other nutrients are deficient.

Maximum yield—At present, the genetic yield barrier for inbred varieties in irrigated rice systems is about 10 t/ha. Under best management practices in favourable environments, farmers are able to achieve yields of greater than 8 t/ha. To meet future food demand, short duration varieties with a yield potential of 15 t/ha will be required. Some researchers argue that the rice plant’s radiation conversion factor of 2.6 to 2.9 g/MJ (megajoule) may not be sufficient to reach such yields. The possibility of incorporating C4 plant physiological characteristics into rice to increase yield potential is presently under consideration.

Nutrient removal—Nutrient balance is strongly affected by straw management.
Straw contains more than 85 percent of the potassium (K) contained in the above-ground biomass. Thus, much greater amounts of K must be applied to maintain the soil supply where straw is removed from the field. Removal of N and P is mostly associated with grain harvest.

Micronutrient requirements—Rice often requires zinc (Zn) in alkaline soils and soils containing very large concentrations of organic matter. Copper (Cu) is usually required to prevent male sterility in rice grown on peat soils.

Fertilizer nutrient recovery efficiency—In irrigated lowland rice fields with good crop management and grain yields of 5 to 7 t/ha, typical fertilizer recovery efficiencies are 30 to 60 percent for N, 10 to 35 percent for P, and 15 to 65 percent for K. Recovery efficiency for N and K is strongly influenced by splitting and timing of fertilizer applications.

Planting density and canopy management—Optimal planting density depends on the crop establishment method and variety (tillering capacity).
In transplanted rice, a plant spacing of 0.2 x 0.2 m gives 250,000 hills/ha. In direct seeded paddy rice, rates range from 60 to 80 kg seed/ha. In upland rice, seed is dibbled into evenly spaced planting points, and seed rates are lower, ranging from 30 to 35 kg/ha. Excessive early canopy development (seeding/transplanting to early tillering) may result in a very leafy canopy that is more susceptible to pest and disease infestation. Proper splitting and timing of N fertilizer applications are required to produce an optimal canopy without incurring pest and disease damage.

Climatic requirements—In paddy rice, maximum yields are obtained in the dry season, when cloud cover is less and photosynthetic active radiation (PAR) is greater than during the wet season. In irrigated rice, rainfall is not important, provided the irrigation water supply is reliable and sufficient in quantity. In rainfed and upland rice, rainfall is a major yield determinant, particularly in coarse textured soils with poor water retention.

Soil requirements—In upland and rainfed rice, soil structure and fertility are major yield determinants because the amount of mineral fertilizer used is often small. In irrigated rice, soil structure is deliberately destroyed during land preparation. The effect of flooding generally improves nutrient availability and reduces the effects of very alkaline or acid soil conditions on plant growth that occurs under aerobic conditions. In high yielding environments where modern varieties are used, the difference between the soil’s indigenous nutrient supply and crop nutrient demand must be provided in the form of mineral fertilizer.

Source: Better Crop International