Global warming seen causing wheat yield declines

by Laura Lloyd
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The research focused on what happens to winter-wheat yields if temperatures turn excessively warm during the spring growing season.

WASHINGTON — A new study exploring global warming’s effect on hard red winter wheat production in Kansas has concluded that rising temperatures may hurt wheat yields at a time when world population growth is expected to significantly increase demand for the grain.

Published in the Proceedings of the National Academy of Sciences, “Effect of Warming Temperatures on U.S. Wheat Yields,” was written by agricultural economists Jesse Tack of Mississippi State University, Andrew Barkley of Kansas State University, and Lawton Lanier Nalley of the University of Arkansas. Their focus on wheat, the most widely planted crop in the world, reflected its ongoing importance as a staple crop in many societies as well as its special vulnerability to effects from long-term global warming.

“With the exception of potatoes, wheat is the food crop most affected by climate change, and as such, greater knowledge and understanding of the agronomic relationship between weather outcomes and wheat yields is crucial to our ability to forecast, understand, and respond to potential future increases in temperature,” the study said.

The study pointed to a measurable decrease in yields in a warmer climate and evidence newer hard red winter wheat varieties are less heat-resistant than older varieties. Higher levels of spring rainfall may partially offset warming effects, the study said, and irrigation may be recommended to compensate for insufficient precipitation.

The study did not define what weather conditions need to be in place in Kansas to prove the presence of global warming. Instead, the study reported effects on winter wheat yields at different air temperatures beginning with fall planting, winter dormancy and spring crop development. A total of 268 observed seed varieties were included in the study.

The research especially focused on what happens to winter-wheat yields if temperatures turn excessively warm during the spring growing season. The amount of spring rainfall is a crucial variable, with warming’s impact measured as 11 percentage points larger when seasonal precipitation is low rather than high.

For their research, the study’s authors selected 11 trial locations in Kansas, the top U.S. producer of hard red winter wheat.  Researchers reported collecting “a unique data set that combines Kansas wheat variety field trial outcomes for 1985-2013 with location-specific weather data is used to analyze the effect of weather on wheat yield using regression analysis.”

The study found major weather-related predictors of yield loss in hard red winter wheat were freezing temperatures in the fall and “extreme heat events” in the spring, such as temperatures hitting above 34 degrees Celsius, or above 93 degrees Fahrenheit. Each additional “degree day” — an interpolated measurement used in the study — above 34 degrees Celsius in the spring was associated with a 7.6% yield reduction, largely because of negative effects of high temperatures at crucial stages of crop development such as flowering, when seed set may be affected, or during the grain-filling phase.

Each additional degree day of freezing temperatures in the fall was associated with a 9% yield reduction because more cold weather pushes wheat toward dormancy, shutting down tiller formation and reducing yield. But study results indicated reduced exposure to freezing temperatures in a warming climate was a potentially yield-enhancing benefit, although not sufficient to override the damaging impact of very warm spring temperatures.

“The net effect of warming on yields is negative,” the study said.

“Our findings suggest that there are large negative yield effects at temperatures above 34 degrees Celsius in the spring,” the authors wrote. “Although exposures to 34 degrees Celsius are somewhat rare in most wheat-growing regions, global warming has the potential to increase their frequency.”

The study suggested the need for continued efforts to breed both higher-yielding and more heat-resistant new winter-wheat wheat varieties. But difficulties in accomplishing that goal were acknowledged.

“Our findings provide opportunities for the international wheat breeding community to intensify research efforts to increase resistance to heat stress during focused developmental stages,” the study said. “These efforts could result in net positive warming effects since reduced exposure to freeze was found to be a yield-enhancing benefit of warming. Our results indicate that advancements in heat resistance could come at the expense of higher average yields, and that there is currently limited scope for producer adaptation through alternative variety selection.”

The authors said they saw their research as part of continuing efforts to develop sustainable wheat production across the world.

“Achieving this goal in the face of climate change requires integrated approaches across economic, agronomic, soil, biological, hydrologic, and other scientific disciplines whose research can be guided by the results provided in this study,” the article concluded.
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