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Deciding factors of Yield Disparity in Rice


We have observed earlier, in media discussions, that the majority of resource personnel were critical about modern rice varieties in Sri Lanka. The issues brought up against were Genetic Modification, Glycemic Index, Protein Status, Cooking and Eating Quality, Nutritional and Medicinal Properties, to mention a few. There appears to be a knowledge gap, as the critics were non-agricultural professionals; some of them were incapable of even sorting out weedy rice from real ones.

Recently, (Vidusara, Page 10, Oct 28, 2020), a scientist highlighted many traditional varieties of superior quality rice, but without citation of any scientific or experimental evidence for his claims. Among them was Kuruluthuda, a traditional rice variety, highlighted for its aphrodisiac qualities, availability of essential fatty acids, proteins, vitamins and Magnesium with no quantifications, and also its ability to regulate blood cholesterol. This is fantastic, but this variety needs some clarification at this point. Kuruluthuda reported here was red pericarped, 3.5 month variety. If so, it can be cultivated in both Yala and Maha seasons. The variety identified as Kuruluthuda in the list of pureline selections of the Department of Agriculture (Rhind, 1948) was white pericarped, 5 to 6 month photosensitive and can be grown only in Maha season. Prof. M. F. Chandraratne too reported photosensitivity of Kuruluthuda in his text book on Rice Breeding. If so, are we referring in both these instances to the same variety or two different varieties?

And now, there is a new trend in criticism of local rice research, for not delivering rice yields in par with countries like Australia, Japan, China, etc. In this instance, undisclosed technological gaps are highlighted for yield disparity. We, as rice scientists in the country, are left in the dark under these circumstances, as it appears the critics are overstepping their professional boundaries to invade the rice sector.

In China, the majority of rice cultivars are hybrids exploiting F1 hybrid vigor. Hybrid rice is a few steps ahead of us, as we continue research on developing local parental varieties for hybrids, and cross pollination for the F1 seed production procedure. The other countries, Japan, Australia, etc., grow conventionally developed varieties of their own, as in Sri Lanka. However, the disclosure in the text below is to keep critics aware of the biological limits of the tropical environment for any quantum jumps in rice yields, through biotechnological approaches or otherwise.

It is scientifically accepted that the performance of any crop species (genotype) results from its interaction with the environment it is exposed to. Rice is no exception to this phenomenon. The crop environment is composed of biotic (pests and diseases) and abiotic (soil and climate) components. These are basic facts that one should be familiar with, before being critical of paddy cultivation in Sri Lanka.

Sri Lanka is located within the tropical belt of the northern hemisphere, between latitudes 5º55′ and 9º49′. The countries being compared, Australia, Japan, China, etc., are in the temperate zone, and are blessed with soil and climatic factors conducive for rice cultivation.

Irrespective of the parent material involved in the genesis, the soils in Sri Lanka are leached by heavy monsoon rains, and therefore less fertile; particularly the rice soils are subjected to intensive and continuous double cropping, without a resting or fallow period for replenishment. Also, the consistent soil microbial activity, caused by high temperature regimes in the tropical belt, decomposes the organic content rapidly, affecting physical, chemical and biological properties of soil, especially the Cation Exchange Capacity (CEC). As a result, the rice farmer in tropics, Sri Lanka in particular, will have to use both organic and inorganic manure regularly, to sustain good CEC and soil fertility for high productivity.

Soils in the temperate zone are fertile, rich in organic content and CEC due to slow microbial activity resulting from low temperature regimes. In addition, the Japanese paddy fields are provided with subsoil drainage facilities to improve soil productivity. The Australian rice soils are rich in native fertility, and sometimes application of nutrients P and K is not needed for rice production; also the adoption of strict plant quarantine measures keeps the country free of many rice pests and diseases. The pest and disease cycles are usually interrupted in the winter, due to low temperature and/or unavailability of alternate host plants. These ground situations cannot be ignored in a fair comparison of rice productivity in the two different regions.

The other major factor determining crop productivity in the two zones is the difference in photoperiod which involves photosynthesis; i.e. the net-assimilation rate after allowing respiratory losses. In modern rice varieties, the plant canopy structure is designed to improve photosynthetic efficiency, while containing respiratory losses.

We experience short and long day photoperiods regularly within each year (except on the equator) depending on the latitude concerned. The longest day (June 21) in the Northern hemisphere is the shortest day in the Southern, and the shortest day in the Northern (December, 21) is the longest day in the Southern hemisphere. These are basic, but important facts, ignored in the comparison of potential yields among different regions.

The so-called high potential countries do cultivate only one rice crop a year, and the cropping season is determined when the temperature is conducive and the photoperiod is almost above 13 hr/day. Photoperiod reaches its peak (around 16 hr/day) when the crop is in its reproductive phase; the crop too spends more than four months in the field to mature. In contrast, the poor farmers in Sri Lanka have to cultivate their major rice crop (Maha) when the photoperiod is below 12 hr/day throughout the season (October – February); and their minor crop (Yala ) when the photoperiod is just above 12 hr/day, but never exceeding the maximum of 12 hr and 30 min.. The crop duration in either case is less than four months. Sri Lankan rice crop eternally suffers this disadvantage of photoperiod difference between the temperate and tropical zones. Also, a single day increase of crop duration, within the range of 3.5 – 4.5 months of age, leads to a yield increment of around 0.05 t/ha, even under local climatic conditions.

The facts above (soil fertility, photoperiod and crop duration) explain the yield disparity between Sri Lanka and countries away from the tropical belt. Any critic can evaluate popular Japonicas, Koshi-hikari, Akitakomachi, Reiho etc. or the Australian counterpart; Calrose, Ingra, Blue-bonette, Bluebelle etc. or any other known high potential technology package under the local agro- ecological conditions, and verify how they perform. The results will convince you that it is not the cultivar or technology but the crop environment (Soil and Climate) that is the deciding factor of yield disparity between the two regions; and that your conclusions, potential of variety and/or technological gap, are utterly irrelevant, invalid.

A fair comparison is needed among the countries within the tropical belt, without confounding the effects of soil and climate of other regions, to conclude the claims of low yields in Sri Lanka by these uninvited critics. Also, there is no known single gene solution in biotechnology (genetic transformation), similar to that of Bt or β Carotene (golden rice) gene, leading to a quantum jump in yield potential; rice yield, as in any other crop, is determined by quantitative trait loci (QTLs).

Also, it is important to record that the national average rice yield (year 2020) was 4.85 t/ha. In some stable crop environments, yields of 10 t/ha, approximating the potential of the cultivars, is not uncommon despite overall average performance is low. The inconsistent yield by any genotype within the country is attributed to the effect of specific agro-ecological environments.

Scientists have made futile attempts to change the photosynthetic system of rice, C3 to more efficient C4, with different approaches. There had been reports of rice-sorghum hybridization, with the objective of changing rice to C4 photosynthetic system, by introducing Kranz anatomy with bundle sheath cells carrying chloroplasts. Also, there were some unsuccessful atmospheric N-fixation projects (Azolla-anabaena complex, blue green algae and other soil microbes and Susbania spp.) where the cost factor has overridden the cost of inorganic N. There was also the internationally known SRI (System of Rice Intensification) project in Sri Lanka implemented around two decades ago, but no participant farmers of the project are traceable now. There are many more examples of this nature. These are the realities we have faced already with innovative technologies in rice. We know what is appropriate and what is not. Let the rice researchers work peacefully towards their intended objectives, without being disturbed.

Sri Lankan rice scientists have gained a lot from little more than a century’s old, recorded history of local rice research and field experiences; they understand the farmer’s need very well and appropriateness of technologies they could adopt. It is natural, with the experience behind, that the researchers may disagree with inappropriate, expensive, futile technological innovations. The country had bitter experiences in the past by embarking on projects designed by experts with no local experience, but had spent their youth in green pastures abroad (e.g. Psophocarpus tetragonolobus (Dambala) project).

The Department of Agriculture has competence and capability to decide on seasons (Yala, Maha) and agroecological regions, based on long-term changes in soil and climatic parameters, and they will attend to any changes as and when needed. Naming Yala and Maha seasons may be older than 900 years, but as long as no consistent and significant differences are noticed, the cropping seasons can remain as designated. The major climatic regions and agro-ecological zones were mapped by scientists of high caliber in the past, and their successors are consistently monitoring the changes in respective parameters for necessary amendments.

Many things have happened in the rice sector since the green revolution in the 1960s. We really feel sorry for the poor knowledge of some critics in the field of local rice improvement program, and the ignorance of the fact that the Department of Agriculture initiated and continued to release modern rice varieties in Sri lanka since 1970, with Bg 11-11 as the first improved cultivar. The process is still being continued.

The local rice scientists contributed their best within the available facilities and the limited budgets, and are satisfied with their accomplishments, as the rice production within the country can look after the national requirement.

Source: island.lk