Normally a hard freeze in late October or early November in Kansas comes too late to hurt row crop yields. But this year, a significant acreage of grain sorghum and soybeans may not have reached maturity at the time of the first hard freeze, said Kraig Roozeboom, Kansas State University Research and Extension crop production specialist.

A smaller proportion of the corn acreage also may be susceptible to yield losses from the hard freeze, Roozeboom added.

"This is due to a combination of late planting and relatively cool summer and fall temperatures," he said. The potential for yield loss on immature crops is different for grain sorghum, corn, soybeans, and sunflowers. He explained some of the differences:

* Grain sorghum - Sorghum grain weight is not reduced by temperatures of 32 degrees F. However, as the air temperatures fall below freezing, sorghum yields may be reduced, depending on the stage of grain development at the time of the freeze. Maximum damage occurs when plants are exposed for two hours or more to a temperature of 28 degrees or lower; damage is less severe if plants are exposed for less than two hours.

At the soft dough stage of grain sorghum, the estimated yield loss from freeze injury is 38 to 52 percent. At the hard dough stage, yields may be reduced from two to 27 percent, depending on how far along the grain development is. By the time the grain is physiologically mature, a freeze causes no yield loss to grain sorghum.

* Corn - Corn leaves are not seriously damaged at temperatures near 32 degrees. Damage to corn leaves begins to occur at temperatures below 29 degrees. An extremely hard freeze, 28 degrees or lower, can result in damage to the ear shank. Ear shank damage prevents the plant from moving any stored sugars into the developing grain.

At the dough stage of corn, the estimated yield loss from freeze injury is 35 percent if there is only leaf damage; and 58 percent if the ear shank is also damaged. At the full dent stage, damage to leaves will cause an estimated 11 to 26 percent yield loss, while damage to the ear shanks will cause about 22 to 45 percent yield loss. By the time corn is in late dent, freeze damage to leaves can cause an estimated zero to six percent yield loss; and ear shank damage may result in one to 12 percent yield loss. If the corn grain is physiologically mature, a hard freeze will not directly reduce grain yields.

* Soybeans - Soybean leaves are not damaged until air temperatures reach 26 degrees. If the leaves are damaged close to the stem, then the stage of grain development will determine the amount of yield loss.

At the "beginning seed development" stage, soybean yield losses from freeze injury can range from 51 to 65 percent. At "full seed development," when the seed fills the pod cavities and leaves begin to turn yellow, a hard freeze can cause an estimated 24 to 37 percent yield loss. At the stage of "beginning maturity" (R7), where one pod on the main stem has reached mature color and half the leaves are yellow, yield losses may range from zero to 11 percent. By the time of "full maturity," when 95 percent of pods are mature color and the leaves have dropped, a hard freeze will cause no yield loss.

* Sunflowers - Sunflowers are most susceptible to frost during the bud and pollination stages (growth stages R4 & R5). At these stages, temperatures of 30 degrees or less cause poor pollination due to anther and stigma damage. Once the sunflower plant has reached the R7 stage, approximately 10 to 14 days after petal drydown, a sunflower plant can withstand temperatures as low as 25 degrees with little damage.

More information, including calculating heat units and expected yield losses from freezes at different levels of heat unit accumulation in corn and sorghum and information about harvest options after a freeze, is available in the Extension publication MF-2234 "Fall Freeze Damage in Summer Grain Crops" at county and district K-State Research and Extension offices and online at http://www.oznet.ksu.edu/library/crpsl2/MF2234.PDF.