This article is very technical and its reading can be complex. Do not be discouraged because the article contains the scientific explanation for Sri’s success.
How does the SRI work?
The concept of synergy seems to help explain how SRI works so well. In this context, synergy means that the practices used in the SRI interact in a positive and enhanced way so that the whole is more than the sum of its parts. Each of the management practices used in the SRI makes a positive difference in performance, but the real potential of the SRI is only seen when the practices are used together.
When used together, SRI practices result in a different structure of the rice plant from what results when traditional methods are followed. Rice plants under the SRI have many more tillers, greater root development, and more grains per panicle. To tiller, plants need to have sufficient root growth to support new growth above ground. But roots need a certain soil, water, nutrients, temperature and space conditions for growth. The roots also need the energy of photosynthesis that occurs in the tillers and leaves above the ground. Thus, roots and shoots depend on each other. In addition, when growing conditions are optimized, there is a positive relationship between the number of tillers per plant, the number of tillers that become fertile (panicles), and the number of grains per talle.
The SRI fields look lamentable for a month or more after transplanting because the plants are so thin, small and very spaced. In the first month, the plants prepare to tiller. During the second month, the prolific tillering begins. In the third month, the ground seems to « explode » with the rapid growth of the tillers. To understand why, you need to understand the concept of phyllochrons, a concept that applies to members of the grass family, including cereals such as rice, wheat and barley.
A phyllochron is the time between the emergence of a plant (a set of tillers, leaves and roots that emerges from the base of the plant) and the emergence of the next one. The length of phyllochrons is determined by temperature, but it is also affected by such things as the length of the day, humidity, soil quality, exposure to light and air, and the availability of nutrients.
Under good conditions, phyllochrons in rice last 5-7 days, although they may be shorter at higher temperatures. Under very good conditions, the vegetative growth phase of the rice plant can last as long as 12 phyllochrons before the plant begins panicle initiation and begins its breeding phase. This is possible when the biological growth rate is accelerated, so that many growth intervals are completed before panicle initiation.
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The increase in the number of tillers that can be produced by the rice plant in successive phyllochrons (from De Laulanie 1993). The first and last tillers send back more tillers, which send back even more tillers. At the end of the series, the plant’s growth becomes exponential rather than additive.
On the other hand, under poor conditions, phyllochrons last longer, and few of them will be completed before the flowering phase. Here is the most important point: only a few tillers come out during the first phyllochrons (and not at all during the second and third phyllochrons), but during each successive phyllochron after the third, each already increasing till brings out a new till from its base (with a latency time of a phyllochron before this process starts). During the latter part of the vegetative growth period, with ideal growing conditions, the production of tillers by the plant becomes exponential rather than additive. (It corresponds to what is known as the Fibonacci series in biology.) Instead of a « maximum time limit » for the production of tillers being reached some time before panicle initiation (PI), as is the case with standard cultivation practices, with SRI, PI and maximum tillers production coincide.
For this reason, it is preferable to transplant the seedlings during the second or third phyllochron, so as not to disturb the rapid growth that begins during the fourth phyllochron. Seedling roots are traumatized when they are exposed to the sun and dry out, when they are immersed in an airless environment, and when the nourishing roots, coming from the first root, are lost or damaged during late transplanting. This trauma slows down their subsequent growth, and phyllochrons that are completed before PI are not as numerous. Many transplanting methods delay plant growth by one or two weeks and also slow down subsequent growth. For maximum tillering, plants need to complete as many phyllochrons as possible during their vegetative phase. If the plants are three or four weeks old when they are transplanted, the most important (late) phyllochrons will never be reached when the talle growth is multiplied.
Contrary to popular expectations, more tillering does not mean less panicle formation or grain filling. With SRI, there is no negative correlation between the number of tillers produced and the number of grains produced by each fertile slope. All components of yield – tillering, panicle formation, and grain filling can increase under favourable growing conditions.
It seems too good to be true. What’s the catch?
The SRI requires more work per hectare than traditional rice cultivation methods. When farmers are not used to transplanting tiny seedlings with fairly accurate spacing and planting depth, this can take twice as long at the beginning. However, once farmers are accustomed and skilled with the technique, transplanting takes less time because there are fewer plants to transplant.
With SRI, more time is spent carefully applying water than when fields are kept flooded all the time. This means that fields must first be built with appropriate irrigation systems that allow water to be « put on » and « removed from » the land at regular intervals. Most rice fields are not set up in this way (i.e. they have been designed to retain the maximum amount of water), so some field reconstruction may be necessary before launching SRI production systems.
Weeding takes longer if there is no standing water. However, yields can be increased several times due to the increase in soil aeration, which results from weeding with the hand-held rotary hoe. The additional harvest will be more than profitable for the additional expense of weeding.
Initially, the SRI can take 50 to 100% more work (and more skilled and demanding labour), but over time this amount is reduced. Experienced SRI farmers say it may even require less work once the techniques are mastered and the confidence acquired. Since yields can be two, three and even four times higher than current practices, yields on both labour and land are much higher, justifying the larger investment.
Some farmers are sceptical about the benefits of SRI. It gives the impression of being a magical phenomenon at first, but there are good scientific reasons for explaining each part of the process. These farmers need to be encouraged to try the methods in a small area, to secure benefits and start learning skills on a small scale.
Planting and weeding are the most labour-intensive aspect of SRI at first. Many families are constrained by the amount of work available, whether within the household or through hiring. If someone does not have enough labour available to plant and care for all rice fields using the SRI, he or she can cultivate just part of the land with rice using the SRI methods, obtaining higher yields for both work and land. Then other crops can be planted on the rest of the land when labour is available.
Is SRI sustainable? How can you get such high yields?
Scientists are not sure, and many are very sceptical, about how these high yields can be achieved on soil as poor as the one found in Madagascar. Fortunately, it has been found that SRI methods are the basis for many improved yields in other countries (China, India, Indonesia, Philippines, Sri Lanka and Bangladesh). So we know that this is not a method that has a success limited to a single country.
Systematic research by plant and soil scientists is underway. Here are some proposed explanations, for which there is some basis in the scientific literature:
Biological nitrogen fixation (BNF). Free bacteria and other microbes around the rice roots can fix nitrogen for plants. The presence of these bacteria has been established for sugar cane, which is a member of the grass family with rice. Where nitrogen fertilizer had not been applied (since this suppresses the production of the nitrogen enzyme required for BNF), the microbial action fixed 150-200 kg of nitrogen per hectare for sugarcane. However, less nitrogen fixation occurs when chemical fertilizers have previously been applied. It is known that about 80% of bacteria in and around rice roots have the ability to fix nitrogen, but this potential will not be realized where inorganic nitrogen has been applied, or probably in anaerobic and waterlogged soils.
Other research suggests that plants can grow very well with very low concentrations of nutrients, as long as these nutrients are provided in a uniform and consistent manner over time. We know that compost provides a low and stable supply of nutrients.
Plants with extensive root growth have better access to all nutrients in the soil. Extensive root growth can occur when the roots of young seedlings have a lot of space and oxygen, and when water and nutrients are so scarce that the roots have to « go get some ». These extended roots may be able to extract more balanced nutrients from the soil, including some rare but necessary micronutrients.
Skeptics have pointed out that SRI can be labour-intensive and requires careful water management. However, work to demonstrate the benefits of the SRI has continued. In the July/September 2001 issue on Appropriate Technology (Volume 28, No. 3), Norman Uphoff described an experiment conducted in Madagascar by Jean de Dieu Rajaonarison and his advisor, Professor Robert Randiamiharisoa, at the Faculty of Agriculture (ESSA) of the University of Antananarivo. Two rice varieties – a high-yielding variety and a traditional local variety – were compared. Both showed the same response patterns. Uphoff wrote:
SRI practices compared to conventional methods are: transplanting age (8 days vs. 16 days), number of plants per mound (1 vs. 3), water management (aerated soil vs. flooded soils) and fertilization – compost vs. NPK (16 – 22-11) vs. no fertilization.
The high-yielding variety produced 2.4 times more rice with SRI practices than conventional methods. The local variety gave 2.8 times more. These results can be analyzed in several ways to determine the degree to which each practice has contributed to differences in performance, all things being equal, under these particular soil, climate and other conditions.
For these particular variety and growth conditions, planting young plants contributed the most to the situation – a supplement of 1.35t / ha. Proper water management, using a minimum of water and keeping the soil well drained and aerated, was the second important factor that added 0.85t / ha. Transplanting the seedlings one by one added 0.46 t/ha. The use of compost increased the yield by 0.27 t/ha over what was obtained on average using NPK fertilizer.
This represents a total yield increase of nearly 3 t/ha, but when the four practices were used together, yields increased by 4 t/ha. This shows an interaction or synergistic effect of more than 1 t / ha. It is therefore in the farmer’s interest to use all SRI practices instead of choosing.
There is still much to study and learn about SRI, but scientists are beginning to take an interest as higher performance reports multiply. SRI should not be considered as a technology to be applied mechanically, but may be considered as a technology to be applied mechanically.