Frost Damage to Soybeans

Source: Charles Hurburgh, Rebecca Vittetoe, Meaghan Anderson, Iowa State University

Figure 1. Frost-damaged soybeans with shriveled and discolored leaves. Photo courtesy of Rebecca Vittetoe.

Temperatures fell into the low 30’s and upper 20’s in most of Iowa over the weekend of October 11-13.  Because of the very late planting season, some crops were immature enough to be injured by the freezing temperatures. This ICM Blog will address frost damage concerns to soybeans.

Frost normally forms early in the morning, driven by radiation cooling especially on clear cold nights.  The visual impact of the frost damage is most evident the next day (Figure 1).

Temperatures below 32°F can damage soybean leaves, and temperatures below 30°F for an extended time periods can damage the stems, pods, and seeds. A killing freeze is considered to be 28°F for soybeans.

The severity of frost damage to soybeans depends on the following: how low the temperatures dropped, the duration of the cold temperatures, and the growth stage of the soybeans. Soybean fields with at least one mature pod on the main stem of each plant (R7 growth stage) are likely to be minimally affected by frost. Soybean plants with green pods will be more affected by frost. If the frost just damaged soybean leaves, particularly in the upper canopy, pods and seeds will continue to develop with yield minimally affected.

If the frost was more severe and damaged the stems, pods, and seed, the potential for reduced yield and quality is higher. In addition to reduced yield and quality, severely frost-damaged soybeans will dry down more slowly and be more likely to shatter at harvest. Farm moisture meters and older elevator meters will read about 1.5-2% points low until moisture and color have equalized.  The new 150 mhz meters used by many elevators are more accurate.

Handling frost-damaged soybeans

If you do have frost damage to your soybeans, it is suggested to not sell the soybeans immediately as this is when the soybeans will have the most grain discoloration and the highest moisture. They will appear worse than they will end up after aeration.   Instead, if you do harvest the field shortly after a frost, it is recommended to put the soybeans in an aerated bin. With air and drying, the green discoloration will subside somewhat over time.

Grain grading of frost-damaged soybeans
Green damage (frost) is an Official damage category in the USDA Grades. Green color will result in price discounts, however, greenness must be fairly intense and comprehensive to be classed as damage.  Figure 2 is the USDA statement and example color photo of frost damage.

Processors will be concerned about green soybeans because of the likely lower oil yield and the need to remove the green color from the oil by more intensive refining. While some processors use Official Federal graders, there will likely be problems in grading as local markets may call anything somewhat green “damage”. Federal graders are trained in color differentiation. Five samples or more, with varying degrees of greenness, can be taken to an Official Federal grader to “calibrate”  house grading.  A list of Official grading agencies in Iowa is available online (linked).

Further information is available in the Iowa State University Extension and Outreach publication “Frost Damage to Corn and Soybeans” (PM 1635).

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