Unusually cool weather through much of August and early September in the Midwest is sparking concern that crops may not reach maturity before the first freeze.
"Most fields will probably reach maturity before the first freeze, but the dry-down period could be a problem," says Kraig Roozeboom, Kansas State University agronomist, adding that the first freeze is when temperatures across a region drop to a growth-halting 28 F rather than when the mercury dips to 32 F in scattered areas.

Development and maturation of most summer crops are driven by temperature, says Roozeboom, an Extenstion cropping systems specialist with Kansas State. 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 notes. Growing-degree units are calculated for each crop based on the crop's sensitivity to high or low temperatures.
The GDU formula for corn is [(Daytime high + Nighttime low)/2] - 50. In this formula, Roozeboom says, the high is capped at 86 F. and the low has a floor of 50 F. For example, in Kansas he compared this year's GDU accumulation for Aug. 1 to Sept. 9, in various locations 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 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 notes. "A mid-season corn hybrid needs approximately 1,300 GDU from silking until maturity (black layer)." His calculations showed that medium-maturity corn hybrids should reach black layer before frost in most of the state, assuming silking occurred sometime before Aug. 1.  
Offering a specific example, Hutchinson had 893 GDUs Agu. 1 to 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 Aug. 1 to the 50-percent-frost date is 1,608. 
"Even if the crop reaches black layer before frost, cool temperatures may slow dry-down and delay harvest," he adds. "Neither is very attractive given the high cost of drying grain and the increased potential for lodging the longer the corn stands in the field." 
Grain sorghum 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 says.
Sorghum GDUs are figured according to the formula: [(Daytime high + Nighttime low)/2] - 42.
Roozeboom´s calculations for various locations in Kansas showed 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 says. "If temperatures continue a cool trend, the likelihood of maturing is even less than projected." 
More information on corn and sorghum maturation and the first reeze, click here.

Source: Kansas State University