Sept. 14, 2015

My grandfather liked to say there was nothing wrong with Hooper, except that it had never made up its mind, alternately wanting to be Lubbock, Texas or Duluth, Minn.

After a wet, late (in many areas) spring, parts of Nebraska are facing risks of frost or even a killing freeze, with the highest likelihood in northeast Nebraska. Today, after a cool, mostly wet summer we are facing the possibility of significant crop damage, especially because crop development is somewhat behind schedule.

Corn

The amount of damage to corn from a frost or killing freeze depends largely upon two factors:

• how cold it gets and for how many hours, and 
• the plant's stage of development.

A frost, typically occurring at air temperatures between 36°F and 30°F and lasting only a short period, will likely result in killing the leaves exposed to those temperatures. Its impact is similar to defoliation by hail at the same developmental stage. Table 1 indicates potential yield losses from defoliation alone.  However, the corn stalk (stem) is also a storage organ, and a significant amount of photosynthate is mobilized from the stalk to fill the ears, late in the season. If the leaves are killed, but not the stalk, the plant can still mobilize sugar from the stalk and store it in the ear.

Table 2 is more relevant to our situation.  At least some of our corn crop is between the R5 and R5.5 stages, and likely is facing significant loss from a light frost event that kills the leaves, but not the stalk.  However, a hard freeze, which would kill the stalk, will have a much larger impact.

Beyond potential yield losses, a frost/freeze can cause other issues, even if corn has reached black layer (R6). Corn plants that mature "normally," with green, healthy plants, keep stalk tissue alive. As most stalk-rotting organisms primarily feed on dead tissue, this tends to keep stalk rots at bay. Also, green healthy plants, including the grain and ears, continue to respire. When stalks/leaves die, this moisture loss pathway isn't available and the only way to dry the ears and grain is by evaporation. And in cool, moist weather, evaporation is slow. That can leave us vulnerable to a wet (caused by late development), slow-drying corn crop.

When grain filling is stopped by frost/freeze damage, the grain itself is often impacted. The amount of damage will depend on the corn growth stage when the event occurred. Test weights are often impacted, and kernels tend to be softer and more susceptible to mechanical and drying damage.  Misshapen and broken kernels impact air movement through dryers and storage bins, increasing the potential for losses at those stages as well.  And freeze-damaged, low test weight corn is notorious for fooling moisture testers, often testing 2% drier than it really is, especially with older testing equipment. 

When we know the severity and range of damage from this weekend's frost/freeze, UNL extension will follow up with further information and suggestions.

Soybeans

A killing frost or freeze won't damage soybeans (or most other crops) AFTER physiological maturity has been reached.  However, most soybean fields in eastern Nebraska have not reached that stage.  Typically, you can get a good idea visually of when this stage occurs: most/all of the leaves are yellow and about 60% have dropped off the plants; all the pods are yellow, and the lower ones are turning their mature color—gray or brown. Bean fields frosted/frozen at this stage may have their harvest dates accelerated a few days.

A significant freeze (28°F or colder for a few hours) will kill the whole plant, and any frost will act to defoliate plants, resulting in diminished grain filling for the seeds, especially on the upper half of the plants. 

Maximum dry matter accumulation of soybeans has been reached when:

1. all leaves are yellow and about 60% of the leaves have dropped from the plant;
2. pods are all yellow and more than 50% of the lower pods have turned brown; and
3. beans within the pods have about 60% moisture, show little evidence of green color, and may be shrinking.

Expect yield loss to be proportional to the physiological stage at the time of the event. Beans that are still completely green, with all seeds set and filling (R6 stage) can be expected to suffer significant losses, perhaps in the range of 10%. Late-planted beans (and very long maturity beans) in the R5 stage may suffer up to 50% loss, if the freeze is severe enough to kill the whole plant. However, fields that are nearer to maturity—perhaps with some noticeable "leaf coloring"—will likely have much less damage.

Frosted/frozen soybean plants/fields will turn color within a week or so, but may be slower to lose all their leaves.  Allowing them to stand and "field dry" has been a good practice in the past. When they are harvested, "frozen" beans tend to have near normal development near the bottom of the plant but nearer the tops, green, or even yellow, elongated lima bean like seeds may be seen. Often, these are slow to dry, resulting in a grain mass ranging from very dry seeds to very wet seeds. This may require artificial drying, and the grain mass may "test" drier than it actually is, resulting in potential spoilage in storage.  Selling the "green" beans directly off of the combine may result in significant deductions, as the buyer may need to dry them. As with the corn discussion, after we know the extent of damage from this arctic blast, UNL Extension will follow with more information.

Assessing Damage

How can you recognize frost-damaged soybeans? Watch for these characteristics:

• Green or elongated yellow soybeans that shrink to smaller than normal size after drying.
• Reduced oil content and quality.
• Higher moisture level (by 1% to 2%) than indicated by a moisture meter.
• Slower field dry-down.

Soybeans left standing in the field may lose green color within two weeks of maturity, so allow for field dry-down if possible, even if the plants were only partially frosted. 
Also see the Iowa State Extension Integrated Crop Management News article Soybean Quality Issues in 2009.

Related Story

• How Low Did it Go? A glimpse of snowy fields and a check of lows in western Nebraska.

Tom Hoegemeyer
Professor of Practice, Department of Agronomy and Horticulture