A combination of factors such as late planting or low temperatures can produce late maturity for summer grain crops and therefore, increase the probability that those crops could be exposed to freezing temperatures before they reach maturity. In some locations of western Kansas, a hard freeze has already occurred in October. Where a freeze has occurred, the level of yield reductions, if any, will depend on the absolute value of the minimum temperature and the duration of the freezing conditions. The minimum temperature needed to injure a plant is not the same for all crops, but in general for summer grain crops, temperatures below 30 degrees F could produce yield reductions. Producers will need to evaluate the potential yield damage to make a decision if the crop should be harvested for grain or livestock feed.
Corn
Corn is affected when minimum air temperatures are below 32 degrees F for a period from four to five hours. The damage can be similar if the temperatures drop below 29 degrees F for a short period of time. In practical terms, corn is rarely damaged by a fall freeze in Kansas because it normally reaches physiological maturity before the first fall freeze.
Occasionally, however, corn in parts of Kansas will be late enough to be damaged by a freeze. If the ear shank is affected (below 29 degrees F), the mobilization of sugars from the leaf and stem is interrupted; thus the grain filling is stopped and the plant will mature without completing filling all kernels. Frost damage will vary according to crop growth stage when this phenomenon occurs. A frost at milk stage (R3), when grains contain “white” fluid and the moisture is around 80 percent, will definitely reduce yield -- but the final reduction will depend on whether the frost damaged only the leaves or the ear shank. If the plant is damaged by freeze at this stage, ensiling the plant will result in reduced silage quality (with probably high nitrate levels) but will not affect the final dry mass. If the crop is ensiling close to maturity, greater dry mass losses are expected. A similar concept applies if the frost affected corn at the dough stage. If the frost occurs at late dent stage, plant moisture will be close to ideal for ensiling.
The estimated grain yield loss due to a freeze at various growth stages of corn development and under diverse degrees of frost severity -- minor (impacting only the leaves) or severe (affecting the ear shank and the entire plant) -- is presented in the table below:
|
Stage of Corn Grain Development at Time of Freeze |
Estimated Yield Loss (%) |
|
|
Leaf Damage Only |
Ear Shank |
|
|
Dough – kernel is filled with a pasty substance |
35 |
58 |
|
Dent – most kernels dented in at the top |
11 to 26 |
22 to 45 |
|
Late-Dent – kernels dented, and drying down |
0-6 |
1-12 |
|
Physiological Maturity – black layer is visible at the base of the kernel; kernel weight is at a maximum, grain moisture is approximately 30-35% |
0 |
0 |
Grain Sorghum
Sorghum grain weight is not reduced by temperatures of 32 degrees F. However, as the air temperatures decrease below freezing, the amount of damage to a sorghum plant may increase and seed weight decrease, depending on the stage of grain development at the time of the freeze. Maximum damage occurs when plants are exposed for 2 hours or more at a temperature of 28 degrees F or lower at the soft dough stage or before. Damage will not be as severe if plants are exposed for less than 2 hours.
Research indicates that grain sorghum exposed to air temperatures of 28 degrees F or lower is unable to continue filling grain from carbohydrates stored in the stem or remaining leaves. Basically, the flow of sugars from the plant to the grain is interrupted. If the freeze occurs when the crop has reached hard dough (grain endosperm with a chalky consistency), close when the grain start to show color, the plant has close to 2/3 of the final yield attained at maturity. Thus, freezing temperatures at this particular stage can produce yield reductions of about 25-30 percent or less. The estimated yield loss at various stages of sorghum grain development when the freeze occurs is in the chart below:
|
Stage of Sorghum Grain Development at Time of Freeze |
Estimated Yield Loss (%) |
|
Soft-Dough – kernel is filled with a soft pasty substance |
38 to 52 |
|
Hard-Dough – kernel endosperm has a chalky consistency |
2 to 27 |
|
Physiological Maturity – black spot evident at point of kernel attachment; kernel weight is at a maximum, grain moisture is approximately 30% |
0 |
Soybeans
Soybean tops are damaged at temperatures in the range of 30 to 32 degrees F. The colder the temperatures, the greater the extent of damage to leaves. Wisconsin data reported that 80% of soybean leaves were damaged at 26 degrees F for five minutes. These numbers are generalizations as freeze tolerance may be slightly higher in thick stands and narrower rows. Wisconsin research showed that when frost occurred at or before R6 (seed fills the pod cavity at one of the four uppermost nodes), yields were reduced. The most sensitive growth stage for economical yield loss was found to occur at the R5 (seed 1/8-inch long at one of four uppermost nodes) growth stage. Yield reductions are a result of a reduction in number of beans per plant and reduced bean size. The maturity of freeze-damaged soybeans is never delayed, but can be accelerated.
After a freeze has occurred, visual inspection of the plants will determine to what extent leaf damage has occurred. Check for leaf burn into the middle of the canopy. If little or no leaf damage has occurred or if leaf damage is confined to the upper or outer leaves, then the soybean plants were probably not exposed to cold enough temperatures to damage the plant and reduce yields. If the leaves are damaged close to the stem, then the amount of leaf damage and stage of grain development will determine the amount of yield loss.
Indeterminate soybean varieties may have seed at two or more different stages of development on the same plant. For example, a plant may have pods beginning to mature (R7) at the mid to lower nodes and may also have pods at full seed (R6) at the top nodes. Such a plant may be designated as being at stage R6.5 rather than only R6 or R7. Greater yield losses (from 65 to 50 percent) will occur if the freeze takes place at the beginning of seed development (R5-R5.5), which can be associated with an extremely late-planted soybean crop. The estimated yield loss at various stages of soybean development when the freeze occurs is in the chart below:
|
Stage of Soybean Grain Development at Time of Freeze |
Soybean Growth Stage |
Estimated Yield Loss (%) |
|
Beginning Seed – seed 1/8th inch long in pod at one of 4 uppermost nodes |
R5.0 |
65.4 |
|
|
R5.5 |
51.0 |
|
Full Seed – seed fills pod cavity at one of 4 uppermost nodes, leaves beginning to yellow |
R6.0 |
37.1 |
|
|
R6.5 |
23.9 |
|
Beginning Maturity – 1 pod on main stem has reached mature color, 50% of leaves yellow |
R7.0 |
11.4 |
|
|
R7.5 |
0.0 |
|
Full Maturity – 95% of pods are mature color, leaves have dropped |
R8.0 |
0.0 |
Soybean seeds that are soft and green at the time of the freeze will shrivel, but those in pods that have turned yellow may still mature normally. Clearly green soybeans are undesirable because of low seed quality and yield, but also have poor end-use qualities.
Sunflowers
Sunflower is most sensitive to frost during bud and pollination time (between R4 and R5 stages). Temperatures below 30 degrees F could cause damage to both anthers and stigmas of the disk flowers, which will result in very poor pollination. Open flowers are more susceptible to the effects of the frost than the unopened flowers. At the end of the season, freeze effects can be seen as undeveloped seeds in a circular pattern. 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 F with little damage. Still if a hard freeze does occur, the seeds in the center of the head (the last to pollinate, from outside to the center) could be affected.
Summary
For more information, including a detailed discussion of calculating heat units and expected yield losses from freezes at different levels of heat unit accumulation for summer grain crops, and information about harvest options after a freeze, see Extension publication MF-2234 “Fall Freeze Damage in Summer Grain Crops” http://www.ksre.ksu.edu/bookstore/pubs/mf2234.pdf
Ignacio Ciampitti, Cropping Systems and Crop Production Specialist
ciampitti@ksu.edu
Kraig Roozeboom, Cropping Systems Agronomist
kraig@ksu.edu
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