Unusually cool weather through much of August and early September in the central High Plains is sparking concern that crops may not reach maturity before the first freeze.
"Most fields will probably reach maturity before the first freeze, but dry-down could be a problem," said Kansas State University agronomist Kraig Roozeboom, adding that the first freeze would be when temperatures across a region drop to a growth-halting 28 degrees F rather than when the mercury dips to 32 F in scattered areas.
Development and maturation of most summer crops in Kansas are driven by temperature, said Roozeboom, who is a cropping systems specialist with K-State Research and Extension. Corn and sorghum are especially dependent on temperature, while soybean and cotton flowering and maturation depend on a combination of day length and temperature.
Formulas widely used to describe the relationship between temperature and crop growth use the heat unit concept, he said. Growing degree units (GDU) are calculated for each crop based on the sensitivity of that crop to high or low temperatures.
The GDU formula for corn is [(Daytime high + Nighttime low)/2] - 50. In this formula, Roozeboom said, the high is capped at 86 degrees F. and the low has a floor of 50 F. He compared the GDU accumulation for Aug. 1 to Sept. 9 in various locations around Kansas this year to the normal GDU accumulation for those dates and the normal GDU accumulation from Sept. 10 until the date with a 50 percent chance of a 28 degree F frost for each location. Aug. 1 was chosen because the Kansas Agricultural Statistics Service reported that 94 percent of the corn crop had silked by Aug. 3.
"Most of the state accumulated significantly fewer GDU in August and early September this year than normal," Roozeboom said. "A mid-season corn hybrid needs approximately 1,300 GDU from silking until maturity (black layer)." His calculations indicated that medium maturity corn hybrids should reach black layer before frost in most of the state, assuming they had silked sometime before Aug. 1.
For example, Hutchinson had 893 GDUs Agu. 1-Sept. 9 this year compared with the usual 1,111. The normal corn GDU Sept. 10 to 50 percent frost date there is 715, so the estimated total GDU accumulation from August 1 to the 50 percent frost date is 1,608.
Later planted or full-season, irrigated corn in western and north central Kansas may have difficulty maturing if it had not silked by Aug. 1 or even by the second half of July. Colby, in northwest Kansas for instance, had 752 GDUs Aug. 1-Sept. 9 this year compared with 922 normally. The normal corn GDU Sept. 10 to 50 percent frost date is 414, making a projected total of 1,166 GDU from August 1 to the 50 percent frost date. Scandia in north central Kansas had 797 GDUs Aug. 1-Sept. 9 this year compared with the average 1,028 GDUs. The normal corn GDU Sept. 10 to 50 percent frost date is 567 for a projected total of 1,364 GDU.
"Even if the crop reaches black layer before frost, cool temperatures may slow dry-down and delay harvest," he said. "Neither is very attractive given the high cost of drying grain and the increased possibility of lodging the longer the corn stands in the field."
He is encouraging producers to consider their options for harvesting higher moisture corn or to perhaps to look for markets for silage if their corn is unlikely to mature before frost.
Grain sorghum, the agronomist said, also develops in response to temperature, but the relationship between sorghum development and temperature is not as clear. Sorghum development before heading can be slowed in response to moisture deficit regardless of temperature. Temperature does, however, drive grain maturation from the half-bloom stage of development until the black layer stage.
"Most sorghum hybrids need about 1,500 GDU from half bloom to physiological maturity," Roozeboom said.
Sorghum GDUs are figured according to the formula: [(Daytime high + Nighttime low)/2] - 42.
Roozeboom´s calculations for various locations in Kansas indicated that sorghum in western Kansas will likely not reach maturity before frost if it had not bloomed by the middle of August. In addition, if sorghum had not bloomed by Sept. 1 in north central and northeast Kansas, it may not reach maturity before frost.
"These projections are based on normal temperatures from here on out," he said. "If temperatures continue their cool trend, the likelihood of maturing is even less than projected."
More information on the subject is available at K-State Research and Extension county and district offices and on the Web in the publication AF-162, Probability of Sorghum Maturing Before Freeze.
Further information on corn and sorghum maturation and the first freeze is also available in a K-State agronomy newsletter on the Web: http://www.agronomy.k-state.edu/  and click on Extension; Latest Agronomy eUpdates; eUpdate 9/12/08.