By Fred Miller, Science Editor
Arkansas Agricultural Experiment Station

Arkansas cotton producers averaged 914 pounds per acre in 2003, breaking the previous record of 877 pounds per acre set in 1994. But the state's notorious year to year variability in cotton yields mean nothing is certain for 2004.

Now, University of Arkansas Division of Agriculture researcher Derrick Oosterhuis is convinced he knows what's behind the yield variability of Arkansas cotton. It's the weather. "Arkansas had a mild summer last year," Oosterhuis said. "Cotton comes from hot climates, but it really doesn't like heat all that much. It does best between 68 degrees and 86 degrees Fahrenheit, but we're almost always above optimum temperature range during fruit-set, so the cotton's always suffering." Bill Robertson, extension cotton specialist, agrees that mild temperatures helped in 2003. "I give a lot of the credit to boll weevil eradication and new technologies, like Boll Guard and Roundup Ready plants," Robertson said. "But even with improved varieties, Arkansas cotton yields have been running pretty flat." The 10-year average yield is 745 pounds per acre, and the five-year average is only a little better at 755 pounds per acre, Robertson said.

UA cotton physiologist Derrick Oosterhuis and U of A graduate students controlled overnight temperatures of cotton test plots to study the impact of night temperatures on cotton yields.

"Last year's mild temperatures helped us set fruit and keep it," he said. Oosterhuis plotted temperatures and yields over several years and found that yields were always higher in mild years. But what struck him most about this study was that nighttime temperatures seemed to make the biggest differences. "Plant respiration increases with higher temperatures," he said. "When overnight temperatures stay above 74 degrees, respiration causes the plants to work harder and consume carbohydrates that would otherwise be used for plant growth and fruit development." Oosterhuis discovered the importance of overnight temperatures when he compared yields and temperatures in Arkansas with those of Greece, where he lectured and worked during a sabbatical in 2002.

A plot of August temperatures and annual yields shows the correlation between temperature and cotton yields in Arkansas.

"Greece copies our production practices, but typically produces yields averaging about 50 percent higher than ours," he said. "I charted 10-day average temperatures for a whole year and found that daytime temperatures in Greece were virtually the same as Arkansas, but nighttime averages were about 8 degrees cooler."

He added other cotton-producing areas to his study - California, Brazil and Australia - and found the same correlation of similar daytime temperatures to Arkansas, but lower night temperatures.

"All those areas have drier climates than Arkansas," Oosterhuis said. "They have lower humidity, which allows the temperature to fall lower at night, and they all have higher yields than Arkansas.

To test his theory, Oosterhuis and his graduate students set up test plots at the Arkansas Agricultural Research and Extension Center in Fayetteville, which were covered at night and either heated or cooled to specific temperature ranges. Plots with cooler overnight temperatures consistently had higher yields.

"This test supported our hypothesis," he said.

Oosterhuis and Robertson said there's nothing farmers can do about the weather, but understanding the impact of temperature is useful information when managing cotton.

"Manage crops to reach maturity as early as possible," Oosterhuis said. "Control pests and other stresses to avoid delays in setting fruit, and a good water regime will help keep cotton cooler during the day. I think the early stages of fruit-set are more sensitive to heat than the later stages, so setting fruit early, before the worst of the summer heat, is important."

In the meantime, Oosterhuis and his graduate students are doing a lot of research to find out exactly what parts of cotton plant physiology are affected by heat. He is also working with UA cotton breeder Fred Bourland, director of the Northeast Research and Extension Center at Keiser, to learn how to evaluate germplasm and breeding lines for heat tolerance to improve future varieties.

"Water is the most limiting factor for reaching yield potential," Oosterhuis said. "After that, temperature is the most important. Farmers can use irrigation, nutrient management and pest control as tools to get the most from their crops. The only factor outside their control is temperature, but improved heat-tolerant varieties may offer a way to stabilize yields."