Discussion of the system of rice intensification (SRI) is unfortunate because it implies SRI merits serious consideration. SRI does not deserve such attention. A multinational team has shown from both theoretical evaluations and a number of experimental tests that SRI offers no yield advantage. Significantly, these results by Sheehy et al. were published in Field Crops Research (2004), an international journal that requires anonymous reviews. Their research used the classical scientific approach of assessing a concept’s consistency with existing facts and knowledge, and conducting critical experimental investigations with appropriate controls and statistical tests.
Three components of the SRI strategy run directly counter to well-established principles for high crop growth. These principles were developed over many years of careful testing and scrutiny by scientists worldwide, and they have stood the test of time.
First, SRI uses very low plant densities. Energy for crop growth results from intercepted sunlight, and the amount of light intercepted translates directly into plant growth. High plant density enhances light interception, growth and yield. SRI suffers from poor light interception because of low plant densities.
Second, SRI replaces paddy flooding by simply maintaining “moist” soil conditions. The physiology and physics of plant water use have been researched for more than 300 years, and the relationship between growth and plant water use is unambiguous. Ample water maximizes rice yields, and flooded paddy fields assure that no water limitations develop.
Third, SRI emphasizes organic nutrient to the exclusion of mineral fertilizer. SRI faces a serious challenge in obtaining sufficient mineral nutrients from organic sources to achieve high yields. Rice grains contain about 0.013 grams of nitrogen per gram of seed (1.3% N). A claimed yield of 15 t/ha requires nitrogen from over 50 t/ha of organic matter. Such a monumental demand for organic matter creates huge challenges in sourcing, handling and managing these materials. Further, the basis for SRI is explained with misinterpreted or fragmentary literature, which is used without a full understanding of the overall processes regulating and influencing plant growth and yield. Crop growth is fundamentally the accumulation of carbon and nitrogen and their partitioning to growing seeds. For example, one erroneous assumption is that shortening the phyllochron (leaf emergence rate) in itself accelerates growth; no such direct link to growth exists.
Another example of misunderstanding is the claim that not flooding the soil overcomes the supposedly negative consequences of aerenchyma (air channels) in rice roots. Aerenchyma are naturally present in rice roots and form both when the roots are flooded and in SRI. Further, aerenchyma form in the root cortex and neither infringe on the vascular tissue nor negatively impact water or nutrient transport.
Regrettably, SRI appears to be only the latest in a family of unconfirmed field observations (UFOs) that have several features in common with their space UFO cousins. While there is an abundance of “sightings,” they are anecdotal and reported by people who have minimum understanding of the basic scientific principles being challenged by such reports. In many cases, mysterious circumstances are invoked to explain the miraculous — for SRI, there are unexplained “synergies” and processes in the rhizosphere (the zone in which plant roots interact with soil microbial populations).
Egregiously, some people who have little or no research experience are able to influence the agricultural research agenda and cause UFO reports to be taken seriously. Such decisions require widely published scientists to produce documented responses, causing losses in time and resources that could otherwise be committed to investigating well-founded hypotheses for true understanding in maintaining and increasing crop productivity.
One lesson to be learned from the SRI experience is that there are no shortcuts to increasing crop yields. The history of crop yield increase tells of decades of hard-won scientific advances in understanding the biology, biochemistry and physics of plant growth and yield. Research requires intensive investigations by those trained to understand the theoretical context of their research and to undertake the critical experiments. Most importantly, results are not accepted until the research is described in an unbiased manner in a scientific journal that relies on anonymous reviews.
It is hoped that the SRI experience will infuse those making funding decisions for agricultural research with renewed skepticism and caution upon the next “sighting” of an agronomic UFO.
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Dr. Sinclair is a plant physiologist in the U.S. Department of Agriculture’s Agricultural Research Service.
