Convinced that cotton producers could further improve their yields by minimizing drought stress, Midsouth researchers are developing a high-tech way for growers to better monitor their cotton plants' thirst.

The system, initially developed by plant physiologist Gretchen Sassenrath in collaboration with agricultural engineer Lyle Pringle, uses thermal sensors to measure the temperature of the crop canopy at that location.

Because there has not been a particularly good way for cotton growers to determine when to start irrigating, Sassenrath saw the need to more precisely quantify drought stress in cotton. “We are looking at water use in cotton and are trying to detect that point when a cotton crop begins to be stressed and irrigation needs to begin,” she says.

In Sassenrath's high-tech water sensing system, thermal sensors are attached to a boom, which is then mounted onto a six-wheeled utility vehicle. By adjusting the boom height with a hydraulic cylinder, the sensors can get close enough to the cotton plant to register the exact temperature of an individual leaf or of a one square meter area of a plant's canopy.

“We can now tell when an individual leaf or an individual plant is going into stress, but it is much more difficult to scale up and determine when an entire crop is under stress and is in need of irrigation,” says Sassenrath.

In theory, the system works well. In arid areas of the country, substantial differences often exist between air temperatures and plant temperatures. Because of that difference, the cotton plant is cooler than the air temperature due to the effects of transpiration.

However, Sassenrath's locationat the USDA Agricultural Research Service's Jamie Whitten Delta States Research Center in Stoneville, MS, has somewhat hampered her success using canopy temperature to determine the cotton plant's need for water.

Like Midsouth cotton growers, Midsouth researchers must con-tend with long periods of high humidity combined with frequent cloud cover.

“With high humidity, the cotton plant cannot cool itself below the dew point. At the dew point you have 100% humidity and because no more water can evaporate, the plant can't cool itself,” Sassenrath explains. “Also, when the cotton plant begins fruiting at the end of June or the first part of July the canopy is not yet closed, resulting in an increase in our temperature readings due to soil reflection.

“That first onset of water stress usually happens before canopy closure, and by the time we can clearly see that the crop is under stress, it may be too late to initiate irrigation,” she says. “We are seeing some success with this system, but we are not yet there.”

While stumbling blocks to the high-tech system still exist for Midsouth cotton producers, Sassenrath is confident she and her fellow researchers will soon find a better way for cotton growers to determine when they should initiate irrigation.

Research agricultural engineer Steven Thomson, also located at Jamie Whitten Delta States Research Center, is conducting a concurrent study using an aircraft-mounted thermal imaging camera to visually show the differences in canopy temperature over whole fields. His research, too, is somewhat hampered by soil reflectivity and intermittent cloud cover.

“Soil signatures are much different than the surrounding crop. Before full canopy, the much warmer soil influences the temperatures you obtain from the image,” Thomson says.

The same issues are also there regarding limited canopy cooling by evaporation in periods of high humidity. “So far, it has been difficult to use images for irrigation scheduling, although irrigated vs. non-irrigated fields have shown pronounced differences in canopy temperature. It is quite easy to see differences on a spatial basis,” he says. “To improve the ability of this system to register differences in canopy temperature from day to day, we now collect weather data the instant the airplane flies over the field. Images can then be compensated for varying wind and solar radiation between days.”

Other potential solutions involve the use of broad-range remote sensing and soil sensors. “We have seen some good correlations between soil temperature and yield. The problems we still need to solve relate to early-season soil reflectance before canopy closure, high humidity, and frequent cloud cover. We need to determine what time of day offers optimum solar heating and less humidity to better determine crop temperature,” Sassenrath says.

“The frustration we have with making remote images is the variable sky conditions with frequent, intermittent cloud cover. The clouds interfere directly with capturing the remote images, and also alter the reflectance from the sun, contributing scattered light from the clouds,” she says.

High Mexican Tortilla Prices

Rising tortilla prices in Mexico are due to a supply issue in that country — not increased U.S. ethanol production or U.S. corn prices. The U.S. Grains Council (USGC) and the National Corn Growers Association (NCGA) report that lower corn production in Mexico and the lack of import licenses have caused white corn shortages there.

“While there has been much in the media on this issue, no one in Mexico is pointing fingers at the U.S.,” says Chris Corry, USGC senior director of international operations. “They recognize that this is a supply issue coupled with a political situation in Mexico.”