Dr Achim Dobberman speaks.
Nine years ago, I published one of the first papers that tried to provide a critical analysis of the biological principles underlying the System of Rice Intensification (SRI) and its potential to improve rice production.
My curiosity at that time was aroused by some huge rice yields that had been reported in the SRI literature, with some farmers in Madagascar apparently having harvested rice crops with a grain yield of 15.0 to 23.4 t/ha. Naturally, I wondered whether that might actually be possible, and, if so, what one might be able to learn from that. My analysis concluded that it didn't seem to be possible under the given circumstances. A common response to that article was to since then label me as one of the "opponents" of SRI. Nothing could be further from the truth. I believe in evidence-based science for finding the right, sustainable development solutions and I also believe that good agronomy is going to be a key for sustainable rice production, in terms of yield, profitability, resource efficiency, and ecological resilience. I am a curiosity-driven person and I have no vested interest in favoring one opinion (or technology) over another. I don't get paid for that.
About 250 scientific articles on SRI have been published between 2004 and now, engaging numerous researchers and development workers on both sides of the debate. I have followed this with a lot of interest, building up my own collection of publications and reports and making my own field observations. From what I have read and seen with my own eyes, I cannot say that I have been able to unearth much new to me, but I recognize that many more farmers have tried SRI and that it has become a movement with many supporters. On the other hand, an increasing number of published studies have also questioned whether there is much new to it in terms of the management principles and practices proposed as well as in terms of its field performance. Even more has been talked and written about SRI in the press, on the Web, in social media, and in many meetings worldwide. Interestingly, some of that overall discussion has even entered the spheres of sociology, anthropology, and philosophy. It has also become a popular topic in the development community and among donors and politicians because everyone is keen to find solutions to the complex problems and constraints faced by smallholder farmers worldwide.
Naturally, I was intrigued when reading John Vidal's article, published on February 16 in The Guardian, about another new world rice yield record: 22.4 t/ha achieved by Mr. Sumant Kumar, a farmer in Darveshpura, Bihar, India. Whenever such a new record is announced, it triggers a cascade of subsequent media stories and opinion pieces that largely repeat the same content without verifying the facts or without adding new information to the discussion. Judge for yourself by reading, for example, the articles written by Tom Laskawy or Beth Hoffman. Such stories also spawn numerous Internet discussion threads in which details are further dissected or opinions are fortified. Here is just one example: Many of the people writing about this particular subject have not seen that record crop in the field and they may also not be very experienced in growing crops or doing research on them. I don't blame them for that because we all have different professions, but I would like to ask for more fact checking and a little bit more caution when writing about areas that are outside a person's area of knowledge and expertise.
I also cannot claim to have seen the record crops of Mr. Sumant Kumar and his fellow farmers in Darveshpura with my own eyes, but I have seen many rice crops throughout the world, including in the highest-yielding growing areas. I have also grown a few crops on my own and I have done a lot of my own research in farmers' fields all over the world, with varying success. Here are a few observations on the recent story from Bihar:
The precedent for Mr. Vidal's Guardian article was an article published in the Indian magazine Agriculture Today in June 2012, by three people who promoted SRI in that particular district in Bihar. This article describes in detail how this particular rice crop was grown in 2011. There are some striking inconsistencies between that article and the picture painted in The Guardian article.
Mr. Vidal quotes a farmer as one growing 15 hectares of rice and vegetables, whereas in Agriculture Today the farmers are said to have landholdings of 2 to 2.8 ha. Whatever it is, these are hardly small farmers in Bihar, where a more common landholding size is 0.5 ha or less.
The Guardian article suggests that Mr. Kumar and other farmers achieved something beyond what can be achieved by scientists and GMO companies, and that they used only farmyard manure and no herbicides. The much more detailed article in Agriculture Today shows almost the opposite. No GM rice is grown anywhere yet. However, all five record farmers grew commercial rice hybrids from Bayer or Syngenta; their seed was fungicide-treated (carbendazim); they used intensive tillage (two deep plowings plus two puddling operations); they applied green manure, farmyard manure, mineral fertilizer (N, P, K), and other nutrient input products (poultry manure, vermicompost, phosphorus-solubilizing bacteria, micronutrient foliar spray of zinc sulfate); and some also used herbicide (2,4-D) for additional weed control. These are in fact very intensive input management practices, perhaps not something that many poor, small farmers could afford.
How can one "verify" the yield of a crop after it had been harvested?
How is it possible that all of a sudden other farmers in the same village or in a neighboring village were also able to break the world record for potato and the Indian yield record for wheat?
I was quoted with a personal statement in The Guardian article (and re-quoted subsequently by many other outlets). Interestingly, I have never spoken with Mr. Vidal. Instead, the words ascribed to me in his article appear to have been extracted from a phone interview I gave to World Bank staff in 2008, which you can findhere. You will notice that I spoke at great length about SRI (for about 10 minutes), explaining a lot more than what was quoted by Mr. Vidal and others. Journalistic standards should be followed. It would have been nice to see at least a reference to the full interview or to talk with me in person.
Let me now turn to the question of whether known thermodynamic and biological laws could actually explain the stated yield record of 22.4 t/ha paddy. Those who wish can read in more detail what needs to be considered when trying to understand such a number.
What are the upper limits to yield?
In the Agriculture Today article, we learn that the fresh yield was 22.4 t/ha paddy, which, after drying, amounted to 20.16 t grain/ha. The drying probably resulted in a residual moisture content of 12–14%. We also know that any modern rice variety or hybrid will have a harvest index (ratio of grain weight to total biomass weight) of 45–55%. So, roughly speaking, that record crop must have produced about 40 tons of dry biomass per hectare during a life span of 154 days from sowing in the nursery to harvest.
I explained in my paper in 2004 that, even by applying the most favorable assumptions about the amount of available sunshine (photosynthetically active radiation) and its conversion into biomass and grain yield, the maximum rice yield possible by using the C3 photosynthesis mechanism is about 18 t/ha in a cooler environment with 150–160 days' growth duration and high solar radiation. We may think that since then some new plant types may have been developed in certain countries (e.g., super hybrids in China) that could increase that theoretical potential further, to perhaps 19 t/ha. However, that is not what the farmers in Bihar grew and their climate doesn't match that of a cooler, high-radiation environment either.
In Bihar, the rice crop was planted in early July, at the start of the monsoon season. In the rice-growing season, daily solar radiation is usually in the 15–20 MJ/m2/day range and minimum temperatures (during the night) rarely drop below 20–23 °C. I have no reason to believe that this would have been any different in the SRI fields in Darveshpura. Based on what we know about the climate and the maximum photosynthetic efficiency of a C3 rice crop, the maximum yield one could obtain at such a site and with perfect management is probably in the 10–12 t/ha range for an excellent hybrid such as the one grown by these farmers. To get anywhere near the dry matter production and grain yields reported, one would need to have a rice crop equipped with a different photosynthesis engine, such as the one in so-called C4-photosynthesis crops (e.g., maize or sugarcane). We're working on that for rice, but it doesn't exist yet. Those who wish to gain a more detailed understanding of the thermodynamic and biological limits of crop yield should read Jeff Amthor's excellent paper on that.
Could the amount of nutrients available explain such a high yield?
We must also ask the question of whether the amounts of nutrient available could actually support such a high rice yield. We know that plant organs have a certain biochemical composition and that this is not something that would be hugely different under SRI management. Thousands of data sets collected for rice show that, on average, for each ton of grain yield produced, the crop needs to take up 15–20 kg nitrogen, 2.5–3.0 kg phosphorus, and 15–20 kg potassium. Assuming averages of each and multiplying those by the reported yield of 20.16 t after drying results in a total crop nutrient uptake of about 350 kg N, 60 kg P, and 350 kg K per hectare. It could have been somewhat less or more. The point here is that these are indeed very large nutrient amounts that must have come from somewhere.
Based on the description given in the Agriculture Today paper, I estimated rough total nutrient inputs of about 55 kg N, 20 kg P, and 20 kg K from the mineral fertilizer applied. Not all of that ends up in the plant, of course. Assuming very high uptake efficiencies of these nutrients, these mineral fertilizer sources could account for about 35 kg N (approx. 10% of total plant uptake), 10 kg P (approx. 15%), and 15 kg K (less than 5%). Where did the rest come from? Using optimistic assumptions about the nutrient content in organic materials and their conversion into actual uptake by the rice crop grown, the Sesbania green manure, 6 tons farmyard manure, 400 kg poultry manure, 100 kg vermicompost, and 40 kg P-solubilizing bacteria compounds applied to the record field (on a 1 ha basis) may have altogether accounted for another 75 kg N, 15 kg P, and 35 kg K of the total crop nutrient uptake.
That leaves at least 220 kg N, 35 kg P, and 295 kg K unaccounted for, which must have come from the soil and a few other indigenous sources such as rainfall and irrigation water. Particularly for nitrogen, that exceeds by far what a normal rice soil could supply. In fact, such a champion soil would actually produce more than 10–12 t/ha grain without any of the external inputs applied by the farmers, which is something one could of course test easily. But, even if the yield as reported were true, growing a crop with such high net nutrient extraction would actually be a very unsustainable solution because soon we would exhaust that soil.
In summary, given what we know about the upper limits of nutrient supply from soils in that region, I conclude again that the actual yield may have been in the 10–12 t/ha range at best. Japanese researchers studying SRI fields in Madagascar have shown a clear linear relationship between nutrient supply (enhanced by deep plowing as also done in Bihar) and rice yield, irrespective of other SRI management practices, but also not exceeding yields of about 10 t/ha.
I'm sure Mr. Sumant Kumar and the other young farmers in Darveshpura grew an excellent crop and for that they must be congratulated. Based on my analysis of the climatic-genetic yield potential and of the amounts of nutrients available, I conclude that the yield may have been more like 10–12 t/ha. Even that would be a tremendous achievement in a state where the average rice yield is little more than 2 t/ha. We should, however, move beyond unproductive discussions of record yield. The apparent attraction of SRI and similar agroecological movements is that they seem to suggest that large yield increases could be achieved by the poorest farmers of the world just by paying a little more attention to managing their fields: no need for biotech, no need for chemical inputs, no need for big machines, no dependence on multinational companies, etc. Indirectly, that seems to imply that somehow dozens of millions of rice farmers must have been negligent of their fields for decades. Sorry, but I simply cannot believe that because these farmers depend on their land for making a living.
Why haven't more rice farmers all over the world adopted SRI if it is indeed that easy to raise yields to unprecedented levels? SRI has been promoted for about 30 years now and its supporters estimate that some 4 to 5 million farmers in perhaps about 50 countries around the world use it in whole or in part, and that it has led to large yield increases. I have not seen any broad-scale, independently conducted adoption and impact study to verify these numbers. Moreover, we must also keep in mind that there are probably about 200 million small rice farmers in the world (nobody knows the exact number). Many of them have been using agronomic practices for a long time that are not unlike some of the things being stated as unique to SRI, but they choose carefully what they adopt to meet the requirements of their particular cropping and socioeconomic environment.
Brown Mang
No comments:
Post a Comment