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Study links rice yield to temperature

It was fortuitous instead of planned: 12 years of rice yield data collected next to a weather station. Until recently, no one thought to see if there was any correlation between the two. When the checking did occur, researchers at the International Rice Research Institute (IRRI) in the Philippines found some intriguing — and disturbing — trends.

They found average daytime temperatures (which, since 1979, increased by 0.35 degrees Celsius) had little effect on yield. However, a strong correlation was found between rising nighttime temperatures (which increased 1.1 degrees C over a quarter century) and declining yields. In the Philippine test fields, when temperatures rose a single degree, rice yields suffered a 10 percent loss.

This is alarming, says agronomist Kenneth Cassman, because rice is the staple diet for the world's developing nations. Any drop in yields, even slight, is problematic for most developing nations' burgeoning populations: rice accounts for 80 percent of the total calories consumed by 2.7 billion Asians — half the world's population.

Cassman, who recently co-authored a paper on the study's findings, was head of IRRI's Division of Agronomy and Plant Physiology from 1991 to 1996. The fieldwork in the paper, recently published by the National Academy of Sciences journal, began in 1992.

Cassman, now based at the University of Nebraska, recently spoke to Delta Farm Press about the study's warnings on feeding the world's population, much-needed research and how that research should be initiated. Among his comments:

DFP: Can you explain how the study began and what you were trying to accomplish?

CASSMAN: We were examining how to grow rice at its genetic yield potential consistently. When you grow a crop at its genetic yield potential, the only factors affecting yield are temperature and the amount of incoming solar radiation. You've eliminated yield losses from pests, diseases, and weeds, you've eliminated all nutrient deficiencies or excesses, and you have an optimal water regimen.

Once we identified how to do this, we began annual studies (about 40 miles to the southeast of Manila). The same variety was used from year to year.

More recently (a colleague) was examining climate data at the site because a weather station close by has maintained detailed, high-quality data since 1979. There appeared to be a statistically significant warming trend. So we put the two together to see how rice yields since 1992 have related to temperature and whether or not there were trends in that relationship.

When we did that for the Philippine dry season — which is typical of many humid, tropical environments where rice is grown in Asia — some things jumped out.

In the Philippines, two crops of rice are grown yearly: one in the wet season, one in the dry. The dry season has the highest yield potential. It also has the greatest increase in temperature.

In examining yield and temperature, we found there wasn't a very good relationship when examined against the mean maximum daytime temperature during the growing season. However, there was a highly significant and negative correlation between yield and the minimum nighttime temperature during the growing season. That means that as nighttime temperatures rose, yields dropped.

When we did the mathematical estimation of what that relationship was, we found rice yields decreased by 10 percent for every 1degree Celsius (or 1.8 degree F) in the mean nighttime minimum temperature.

DFP: How significant is this is terms of world population growth and the amount of land needed to grow enough food?

CASSMAN: The first point to make is that the tropical and sub-tropical areas typical of the research site presently contribute a majority of the global rice supply. Considerably more than 50 percent of global rice production occurs in lowland tropics in Asia.

While it's difficult to extrapolate from our site, it does suggest that rice in many of these areas is grown in a climate where the temperature regime is already at the upper end of the optimal range. That means even small temp increases — like those seen at the site — are more likely to cause a yield reduction.

Answering what this will mean to the global rice supply is difficult because science and agriculture are always moving forward. New technology and varieties are always emerging.

If you think about it, since rice varieties have been selected year-in and year-out in these warming environments, we may already have varieties more resistant to higher temps. We don't know.

However, with what we know now, it's clear we need to give more attention to temperatures and rice yield.

DFP: What does this mean for U.S. rice growing in more temperate climates? Could this temperature increase actually be good for U.S. rice farmers from a yield standpoint? Our bubble won't burst as quickly as it could in tropical zones, right?

CASSMAN: Let me go back to the global issue. The point to be made is that projections for rice demand suggest we'll have to sustain increases in total rice production of about 1 percent annually. That's a compound rate of return, incidentally, since it's a percentage increase.

In Asia (which produces in excess of 90 percent of all rice grown), there isn't much land left to grow rice. Clearly, if temps are rising throughout Asia (our study doesn't answer that question, we studied only the Philippines and several other sites), this is an additional roadblock to meeting the increased demand by raising yields on existing land.

Clearly, if we want to protect natural resources and remaining natural ecosystems, rainforests or wetlands, then we must meet food demand on existing land. Otherwise, production will have to expand to vulnerable areas.

As far as the implications for U.S. farmers, I'm not as familiar with the temperature regimes in the South or California. However, I assume that especially during the grain-filling period, temperatures may not be at the upper end of the optimal range. In those cases, the temperature increases may not be as harmful. Moreover, those higher temperatures may mean we can expand rice production further north if the land and water supply are available.

DFP: To achieve this 1 percent needed annually, what's the best route to take? Would it be to improve genetics or is it better to study cropping systems? What should be done immediately versus the long-term?

CASSMAN: Those are great questions, but the answers won't be very satisfying. There's no silver bullet.

The first observation I'd make is that we're simply under-investing in good, fundamental research on determinates and physiology of yield potential.

In Asia (where indica varieties are utilized), there hasn't been an increase in yield potential since the release of IR8 — the original “miracle rice” — in 1966. Although there's been a major effort to try to identify new rice varieties with higher yield potential, it hasn't been successful. This points to how difficult and large the scientific challenge will be to raise yield potential through genetics. Regardless, we simply must pursue this avenue of research.

Likewise, there's always a gap between genetic potential and what the average farmer is achieving. In Asia, for example, we estimate the genetic yield potential adjusted for climate at around 8 tons per hectare. (Between that and what farmers typically produce there is an) exploitable yield gap of about 2 tons. That gap can be closed by improved management.

Many people believe it isn't important to do new research on rice. That's why there's been little investment. Over time, that means we'll run into this genetic yield ceiling.

DFP: Has there been anyone challenging the global warming side of your argument?

CASSMAN: I haven't heard from them. It's hard to argue with the climate data. A number of sites have documented statistically significant, carefully measured increases in temperature. Global warming is becoming less and less of a controversial issue.

The greater controversy is the cause of temperature increases. Is it from a natural long-term cycle or due to the releases man-made gases into the atmosphere? I'm not a climatologist, so won't try to answer that.

For those that question the warming trends, I think their arguments run up against clear data.

DFP: Regarding research, you've laid out what needs to be done. Is anyone actually taking the bull by the horns and doing it?

CASSMAN: There are small, valiant efforts. And I say valiant because, quite frankly, it just isn't a politically correct area for research. There is far more interest in silver bullets like biotechnology or transgenic crops to solve the problems. While those can help, they won't help much if we don't have a basic understanding of, for instance, the cause of the negative yield response to increasing temperatures. If you don't know what that response is, it will be very difficult to identify genes and manipulate the plants.

The IRRI has some efforts under way in this area. Unfortunately, the efforts aren't well-funded. Additionally, at major U.S. land-grant universities like Texas A&M, the University of Arkansas and LSU, there just isn't enough support for research that seeks to understand yield potential and how to achieve it. It's time to get cracking.

DFP: Is there someone or some organization to appeal to on a global basis?

CASSMAN: No. Other than the strength of the scientific argument, there's nothing. The first step in setting this straight is we must begin to publish papers exposing the magnitude of the problem. My co-authors and I feel this is a step in the right direction.

DFP: What motivates you to do this work?

CASSMAN: In the United States, food is readily available and inexpensive. It's estimated that the average U.S. family only devotes 10 percent of its income to food. In developing nations with huge populations like India, Vietnam or Indonesia, the typical family spends 50 percent of its income on food. And often they're making $1 or $2 per day.

It's clear that if we come up short of producing enough food, those who will suffer are those already working so hard just to get by.

I find it difficult to understand why we're so casual in our approach to food demand. We may not suffer in the United States, but hundreds of millions of people are in jeopardy.

The United States and everyone else needs economic and political stability. Without enough food, we'll have neither. If we don't maintain a stable food supply, it will be a very dismal world we hand off to our children.


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