Stuttgart, Arkansas, USA
August 10, 2011
Sunny summer days are usually good for rice farmers. They fuel the photosynthesis that converts sunlight and carbon dioxide into the starches that are making plump kernels in the plants on about 1.4 million acres of Arkansas farmland in 2011. Arkansas grows about half of the U.S. rice crop.
But if the heat of the day lingers into the night, it can hinder starch development by reducing the activity of a key enzyme in the process, says University of Arkansas Division of Agriculture plant physiologist Paul Counce who is based at the division's Rice Research and Extension Center near Stuttgart.
In other words, high night temperatures can reduce rice yield and kernel quality. It happened last year and may be happening this year on many farms.
Head rice yield, which is the yield of whole kernels after removing the hull and bran, was the lowest on record in all Delta rice-growing states in 2010, according to the U.S. Department of Agriculture.
The same scenario may be shaping up for many growers again this summer, said Counce and Terry Siebenmorgen, director of the Division of Agriculture's Rice Processing Program. Siebenmorgen is based in the food science department on the University of Arkansas campus in Fayetteville.
The problem occurs when the daily low temperature remains above 75 degrees Fahrenheit for several days during the grain filling stage, according to research in the Rice Processing Program by Amogh Ambardekar, Sarah Lanning, Counce and Siebenmorgen.
"Unfortunately, the daily lows for this summer are closely tracking those in 2010," Siebenmorgen said. "And much of the rice in Arkansas is now (as of Aug. 8) in the grain filling stage."
Daily lows in central Arkansas from mid July to mid August in 2010 and, so far, in 2011, were mostly above 75 degrees Fahrenheit, well above the temperatures recorded in 2009 and the 43-year average, which is in the low 70s.
Temperatures moderated Aug. 9 with a low of 69 degrees Fahrenheit at the Stuttgart weather station. Therefore, fields of later maturing rice may suffer less damage if temperatures during grain filling moderate, Counce said.
"Every year is different, and the damage we saw last year may not be as pronounced this year. But through Aug. 8, the low temperatures were almost exactly the same as the year before," Counce said.
In addition to lower milling yield, heat-stressed kernels may be chalky, or opaque, rather than translucent, which affects the functionality of kernels for cereal products and other uses, Siebenmorgen said.
Counce developed the definitive growth model for rice that shows stages of plant development through vegetative and reproductive growth. Plants fill rice grains with starch during the reproductive stages of R6 - R9, which is projected to be from about July 20 to Aug. 9 this year in central Arkansas, he said.
"Variation in grain quality from year to year is one of our long-term research topics," Siebenmorgen said. The apparent effect of record high night temperatures in 2010 was consistent with growth chamber studies in which plants had been grown under simulated high night heat conditions, he said.
In two rice varieties tested over the years, grain quality has not suffered as severely from high night temperatures, Siebenmorgen said. Unfortunately, the varieties -- Bengal and Cypress -- have lower yield potential and other less desirable production traits compared to other varieties.
However, the fact that a genetic trait for tolerance of high night heat exists provides hope that the trait can be bred into improved commercial varieties and hybrids, Siebenmorgen said.
Division of Agriculture plant pathologist Kenneth Korth, based in Fayetteville, is working to identify the gene or genes associated with tolerance to high night temperatures. The Arkansas Rice Research and Promotion Board fund Korth's project and other research in the Rice Processing Program.
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