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Western research examines global warming on world wheat production

Envision these images — behemoth-sized shards of ice falling from icebergs melting in the Arctic Circle, and a golden ripe wheat field in the West dancing in the wind just days before harvest.

While these scenarios seem latitudes apart, Western researchers are studying the tie between predicted global warming (warmer temperatures) and worldwide wheat production. The landmark research is a world first.

U.S. Department of Agriculture-Agricultural Research Service Soil Scientist Bruce Kimball of the U.S. Arid-Land Agricultural Research Center (ALARC) is feverishly involved in the “Hot Serial Cereal Experiment” at the Maricopa Agricultural Center (MAC), Maricopa, Ariz. The word ‘serial’ refers to sequential wheat planting dates in the project.

Launched in March 2007, the five-acre, open-field experiment included six-week planting intervals across 42, 40-foot square plots of Yecora Rojo hard red spring wheat. With five plantings in the ground, the final 2007 planting is planned for December. The cycle will repeat in 2008.

Yecora Rojo, a northern Mexico-developed wheat variety, was selected for the tests, as it's widely grown in the West. Additional wheat varieties could be tested in future studies.

Each research plot is a 10-foot-diameter circle in the middle of the larger 40-foot square planted area. There are three treatments for the March, September, and December plantings: heated plots with hexagonal arrays of infrared heaters, reference plots with similar arrays of dummy heaters to mimic shading without heating, and control plots minus any heater apparatus. Each treatment is replicated three times.

What makes the experiment a world first is utilizing electric infrared heaters to increase wheat plot temperatures.

“We have developed a way to impose a controlled warming treatment in an open field without chambers in a way that provides a uniform heating, and nobody has done that before,” Kimball said.

Specifically, six 1,000-watt electric infrared heaters in the plots are increasing the wheat canopy temperature by 1.5 degrees Celsius (about three degrees Fahrenheit) during the day, and double that at night.

“It's not a very severe temperature treatment but it is what's predicted with global warming in about 50 years,” Kimball said. “We want to predict the impact of global warming on future wheat production. The idea is to capture the knowledge in wheat growth models and then predict the effects of temperature change on wheat growth whether in Arizona, California, Mexico, or Great Britain.”

In test plots, infrared thermometers are keeping tabs on actual plant temperatures. The infrared gear dangles from a spider web of wires above the wheat. Several times a week Kimball uses an instrument called the Exotech to measure reflectance of solar radiation in several wave bands, from which wheat growth can be inferred. Above ground drip tape provides controlled water distribution and more precise measurement.

Wheat growth, development

In the experiment, wheat growth and development will be assessed at three stages: 4-leaf, anthesis, and physiological maturity. Two, one-by-three-foot areas will be sampled per plot at each of the intermediate growth stages, and a square meter (10.7 ft2) will be used for the final harvest, including grain yield.

Ten stems will be removed from the sample for determination of growth stage, plant height, leaf area, and mass of leaves, stems, and kernels. Bulk sample masses will also be determined.

Major growth developments will be assessed visually through frequent field visits, including emergence, first tiller appearance, heading, anthesis, and maturity. The main stem leaf number and plant height will be measured through thrice weekly observations.

Measuring protein content, volume weight, and kernel weight will assess grain quality. Leaf nitrogen content will be determined at the 4-leaf and anthesis stages.

Plant measurements

The overall objectives are to determine the effects of long-term exposure of wheat to a very wide range of air and leaf temperatures due to variation in planting date and infrared heating. Measurements of ecophysiological parameters such as leaf gas exchange, water relations, plus total non-structural carbohydrate (TNC) pool dynamics will be collected.

Measurements at the top middle and bottom of the canopy will occur since large variations in water potential occur with height in the canopy.

Looking forward

One of the expected results of higher temperatures from global warming, Kimball said, could be increased water needs to meet plant requirements in a region of the country already facing shorter water supplies annually. The impact of reduced water amid global warmed-wheat could likely reduce wheat acreage for growers.

“We are expecting under these conditions the grain-filling period will shorten causing slightly lower wheat yields,” Kimball said. “In the colder part of the year, increasing the temperature will improve early wheat development. As the season warms and the plant moves through its maximum growth cycle, we believe the warmer temps would lower crop yield.”

For Arizona and California wheat growers, the value of this experiment is to determine the most efficient time frame for wheat planting under global warming conditions, Kimball said.

“Stay tuned and hopefully we will improve our plant growth models and we will see some better predictions of future productivity of wheat growth.”

Kimball said that the first actual experiment using the infrared heater arrays of his design started in May 2006 on the Tibetan Plateau in China on grazing land. Up until then, he had been testing various configurations trying to perfect the design. A similar experiment looking at the interaction of the heater treatment with elevated CO2 started on grazing land at Cheyenne, Wyoming in spring 2007. Another major experiment using the heater arrays is scheduled to start in spring 2008 on two boreal forest sites in northern Minnesota.

Could any plus result for agriculture from global warming? Increasing worldwide temperatures are expected to increase evaporation from the oceans resulting in more precipitation worldwide. Yet the Intergovernmental Panel for Climate Change predicts the Southwestern U.S. will not realize any moisture benefit, Kimball said.

“The real joker in the deck is exactly where the rain will fall. The latest climate models suggest that the Southwestern U.S. and northern Mexico will experience more severe droughts and less rainfall.”

If temperatures in the Southwest rose 10 degrees F, Kimball said water requirements for a perennial crop like alfalfa could increase by about ten percent. For a crop like wheat, the increased temperature is likely to accelerate development and shorten the growing season, thereby reducing the amount of water used to grow a particular wheat crop.

Overall, Kimball is excited about the Hot Serial Cereal Experiment and the end results.

“The purely scientific aspect of addressing global warming effects on agriculture is exciting. This is something that needs to be done and we've figured out a new technique to attack the problem.”


The ALARC at Maricopa, Ariz. is a joining of the former U.S. Water Conservation and Western Cotton Research Laboratories at Phoenix, Ariz.

Other researchers involved in the temperature experiment include Jeffrey White and Gerard Wall, plant physiologists, ALARC, and Michael Ottman, Extension agronomist, University of Arizona.

The USDA's Agricultural Research Service is funding the experiment.

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