World Grain - November 2014 - 84

RICE QUARTERLY

Temperature impact
on milling quality
T

he effect of high temperature on rice production outcomes has been greatly underestimated in the past,
because the focus has been on paddy yield without accounting for the impact on milling quality outcomes, which
ultimately determine marketable rice yield and market value.
Researchers from the University of Arkansas and the
International Rice Research Institute said in a recent study there
are dramatic losses from reduced paddy yield and increased
percentages of chalky and broken kernels, which decrease the
quantity and market value of milled rice.
The researchers used genotype specific rice yield and milling quality data on six common rice varieties from Arkansas
combined with on-site, half-hourly and daily temperature
observations to show a nonlinear effect of high-temperature
stress exposure on yield and milling quality.
A 1 degree C increase in average growing season temperature reduces paddy yield by 6.2%, total milled rice yield by
7.1% to 8%, head rice yield by 9% to 13.8% and total milling
revenue by 8.1% to 11%, across genotypes, researchers found.
Some estimates show paddy yield reductions of up to
10% across the major rice-producing regions of South and
Southeast Asia due to rising temperatures. But the study
by Nathaniel B. Lyman, Krishna S. V. Jagadish, L. Lanier
Nalley, Bruce L. Dixon and Terry Siebenmorgen, show that
the 10% figure underestimates the economic implications of
climate change for rice producers. This potentially threatens
future food security for global rice producers and consumers.
CLIMATE CHANGE
Global demand for rice is expected to increase 35% by
2035, requiring an additional 116 million tonnes of milled
rice. Meeting the demand for rice will require improvements
in rice production, which is becoming a challenge given the
pace at which the climate is changing, researchers said.
Current models project mean global surface temperatures
between 1.8 degrees C and 4 degrees C over the 21st century,
rising at a rate of 0.2 degrees C per decade. Recent analyses
of climate change's effects on rice output have estimated the
impact of high temperatures on paddy rice yield and milling
quality outcomes, which ultimately determine rice yield.
However, the estimation of high temperature impacts on yield
or milling quality independent of the other outcomes leads to an
underestimation of food security and economic implications because yield and milling quality jointly determine edible rice yield.
84

by Susan Reidy

Study shows climate change could
have impact on rice production
outcomes, threatening food security
Researchers said there is a void in studies on the overall market revenue losses associated with increasing temperatures.
Production shocks in any given stage of rice production affect
value creation in later stages. Producer revenue depends on the
sale of paddy rice and miller revenue depends on the sale of milled
rice and byproducts to domestic and international markets.
High temperature stress can decrease the quantity of paddy
rice available for milling and change the distribution and quality of milling outcomes, researchers said. Milling factors affected by temperature include chalkiness, kernel strength, protein content, amylose content and amylopectin chain length.
The outcomes most important to global human nutrition are
the quantities of intact and broken kernels that remain after
hulling and milling paddy rice. Broken kernels sell for approximately 60% of the value of head rice in the U.S. and
often for less in Asian markets.
Chalkiness reduces the value of head rice kernels and decreases the ratio of head to broken rice produced during the
milling process.
Research tried to fill the gap in data by modeling paddy yield
and milling quality as temporal functions of cumulative exposure to day temperatures above 33 degrees C and night temperatures above 22 degrees C during sensitive growth windows.
They estimated high temperature effects on paddy yield, milled
rice yield (MRY), head rice yield (HRY), broken rice yield, and
chalk content (CHK) given 1, 2 and 4 degree C increases in growing season air temperatures. These estimates were then used to
calculate changes in the quantity and value of paddy and milled
rice for producers and millers across temperature scenarios.
DATA COLLECTION
Data from six locations over four years were used to estimate
the yield and milling quality in the models. Three randomized
plots of each cultivar were planted in each location-year combination and cultivated under standardized management practices
November 2014 / World Grain / www.World-Grain.com

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World Grain - November 2014

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