If you want to know how your crop is handing heat stress, take its crop canopy temperature. It may be a much more reliable way to monitor what’s going on inside the plant, according to research by a USDA-ARS researcher based in Lubbock, Texas.

 Canopy temperature is a direct measure of the energy being released by the plant, said James Mahan, with the Plant Stress Laboratory in Lubbock, speaking at the National Alliance of Independent Crop Consultants annual meeting in Reno, Nev. “If we can monitor canopy temperature continuously, it gives us a very sensitive measure of water leaving the plant. That information gives us continuous information on water status, water use and how a plant is functioning metabolically. Right now, we don’t know how to translate that into terms that are important for agricultural management. We know there is a story there. We just don’t know how to put it together yet.”

Researchers do know that there can be significant differences between air temperature and canopy temperature.

 “Under irrigation, you can have a 105 degree air temperature and a cotton canopy chugging away at 85 degrees all day long,” said Mahon.

“As we begin to cut water back, those canopy temperatures begin to rise. This can become important if we’re basing management decisions and modeling crop growth on air temperature, and we have different irrigation levels in the fields. The highly-irrigated field in the same air temperature environment will be cooler and not experience the temperature stress that the air temperature would suggest.”

Conversely, a dryland crop can actually accumulate more heat units than the air temperature would suggest.

The concept of canopy temperature as a tool for management of water has been around for a while, noted Mahan. “But up to now, the temperature sensing devices being used were expensive and cumbersome.

According to Mahan, newly developed temperature sensing devices used in the research are part of a relatively low-cost infrared thermometry system that is battery operated, wireless and capable of sending data to the Web. The sensors are being produced by a company called Smartfield.

“The devices are relatively easy to use. They are placed on a pole in the field and moved or adjusted weekly to keep the canopy in the field in view of the device. The batteries will last a season, so there is very little in-field maintenance.

“The primary drawback is that somebody has to gather that data electronically from the field, put it onto a computer, then shoot it back to the producer so he can look at it.”

At the stress lab, the sensors allow researchers to monitor canopy temperature in 15 minute intervals over an entire season. That’s about 10,000 measurements a season.

Mahan believes this intense monitoring of canopy temperature can be a much more reliable way to determine heat unit accumulation, such as DD-60s in cotton.

“Historically, we have based heat units on air temperature. When we have calculated heat units based upon the canopy temperature, we found that in a single year, irrigation can produce a range of heat unit accumulation as broad as a decade of climate-induced variation. So the differences can be significant. When we use canopy- based temperature heat units, we can account for some variability and some effects that we haven’t been able to account for before.”

Mahan’s research indicated that an application of one tenth of an inch of water a day can cool canopy temperature below air temperature. “If we increase the water application by 0.04 of an inch per day, suddenly there is a much more significant cooling of the canopy temperature . As we increase water amounts, the canopy temperatures become much more stable.”

Canopy temperature sensors could be good water management tools whenever new drought-resistant technologies are introduced, according to Mahan. “The technology may require that to get the most of the drought tolerance, the crop must be managed at 85 percent of a full irrigation. If you over water over that 85 percent, you don’t get an economic return because your yield isn’t going to go up, your water costs are going to be the same, and you’re out the extra pennies you paid for the seed. If you hit that 85 percent sweet spot, you save water, yield isn’t dinged and you pay for the technology.

“On the other hand, if you go below 75 percent of a full irrigation, then the transgene mechanism falls apart. You can no longer protect the plant. How you are going to make sure that you have the level of irrigation management needed to stay in business with this new technology?”

Herbicide efficacy can also be better predicted by canopy temperature, added Mahan.