If cotton — being a desert plant — loves hot, dry temperatures, then Mid-South farmers should have a lot of happy plants these days, right? Well, not necessarily.
The idea that cotton needs hot weather to thrive may pre-date Eli Whitney's invention of the cotton gin, but that doesn't mean it's any truer today than it was 200 years ago, says Bill Pettigrew.
Studies show that the 100-degree-plus temperatures that plastered many Delta cotton fields mid-July to late-July may not have been the best thing that could have happened for the region's crop.
“One of the more common misconceptions is that it never gets too hot for cotton,” says Pettigrew, a plant physiologist with USDA's Agricultural Research Service. “It's a native desert crop that is supposed to be able to take the high temperatures. Well, it may be a desert crop, but high temperatures can hurt it just like they do other plants.”
A speaker at the Delta Research and Extension Center's Agronomic Crops Field Day, Pettigrew said other myths have arisen about cotton's supposed affinity for hot, dry weather. Among those:
It's not the heat, but the humidity that causes the problems.
The nighttime temperatures are what's important for cotton.
All DD-60s are created equal.
“Cotton has an internal mechanism for cooling itself through evapotranspiration to maintain its core temperature within a certain range,” he said. “It's like us when we're sweating — the more humid it is outside, the slower it is for that sweat to evaporate.
“When you combine the higher temperatures with the higher humidity, it's like putting a double-whammy on these cotton plants.”
High nighttime temperatures can cause problems for the cotton plant, but so can high daytime temperatures, Pettigrew noted. “When we start seeing temperatures above 95 degrees, plants begin to scale back photosynthesis.”
Another myth, that all DD-60s are created equal, isn't so, either. “DD-60s have been a wonderful tool for us to understand how plants grow and when certain developmental stages should occur,” he said. “It's been a monumental improvement over regular time intervals.
“But the basic premise is that you have a base temperature for cotton growth, and that does not hold when temperatures drop below 60 degrees. However, DD-60s don't add anything for cutoff on the high end. Essentially, when you're getting above 95 to 100 degrees, the plant is not doing as much growing as it was when it was under 90 degrees.”
Pettigrew said scientists can conduct studies in the greenhouse or growth chambers in which they can readily control temperatures and gauge how plants react to higher and lower readings.
“But it's been my experience that plants that have been grown in growth chambers or greenhouses really don't look or act anything like those that have been grown in the field,” he noted.
For that reason, researchers with the USDA-ARS' Crop Genetics and Production Research Unit at Stoneville conducted field tests with two cotton varieties — Sure-Grow 125 and Sure-Grow 125 BR, beginning in 2002.
The researchers compared the growth of the varieties at two temperature treatments. One was simply the ambient air temperature in the plots on a given day. In the second, researchers used 20-foot heating bats to raise the temperatures in selected plots by 2 to 3 degrees.
“If the ambient air temperature on a certain day was 90 degrees F, then the temperature would be 93 degrees in the higher temperature plot,” said Pettigrew. “At night, if the ambient air temperature was 70 degrees, it would be 73 degrees in the higher temperature plots.”
The scientists began the test during the first week of July each year and ran it through Labor Day.
They were able to detect few growth and development differences in the two temperature regimes, said Pettigrew. They also found that flowering rates were similar for the plants in the treatments.
But the higher temperature plots did produce slightly lower nodes above white bloom counts than in the normal temperature plots. And the high temperatures appeared to accelerate maturity by about three days.
“That doesn't seem like a lot, but it could have an impact in some years,” he noted. “We also saw an impact on yield in 2003 and 2004. In 2002, we were trying to get organized, and we have discounted the data for that year.”
In 2003, the researchers harvested an average of 1,281 pounds of lint per acre in the normal temperature plots vs. 1,202 pounds in the high temperature. In 2004, yields averaged 1,324 pounds of lint per acre in the normal temperature plots or 13 percent more than the 1,146 pounds in the high temperature areas.
The yields in the 2005 test were not significantly different (1,111 in the normal temperature plots vs. 1,117 in the high temperature plots).
“We believe that temperatures were hotter in 2005 than in 2004,” he said. “The thermostats in the plots cut off at 100 degree F, but we know that temperatures went higher in both plots. Thus, yields dropped across the board.”
Researchers also found that bolls were smaller in the high temperature plots than in the normal temperature areas, significantly in 2004 when the average weight for the normal was 4.37 grams compared to 4.11 grams in the higher temperature plots.
“I think the real story is in the lower boll size,” he said. “We saw no change in micronaire, but the higher temperature plots produced about a 3 percent increase in strength. We don't know why we're getting an increase in strength.”
The researchers have continued the tests with more varieties at Stoneville this year and have added test locations in Arkansas and at the USDA-ARS research facilities in Phoenix.
“We know we're doing damage control,” said Pettigrew, “because there is little farmers can do to control the weather. We are trying to determine if some varieties are more heat-tolerant than others. But early planting and managing for earliness to try to avoid the higher temperatures in late July and August may be about the best growers can do.”