Farmers will need to carefully manage silage and grain corn in fields that were hard hit by frost this week, warns J.W. Schroeder, North Dakota State University Extension dairy specialist.

“Corn in many areas had not reached physiological maturity, which could lead to storage problems if it isn’t dried and ensiled properly,” he says.

Corn is killed when temperatures are near 32°F for a few hours and near 28° for a few minutes, says Schroeder. Corn can be damaged when temperatures are slightly above 32° and conditions are optimum (clear skies, low humidity, no wind) for rapid heat loss from the leaves.

At temperatures between 32° and 40°, damage may be quite variable and strongly influenced by small variations in slope or terrain that affect air drainage and thermal radiation, creating small frost pockets. Corn at the edges of fields and in low-lying areas, and the top leaves on plants, are at greatest risk. Greener corn has more frost resistance than yellowing corn.

Symptoms of frost damage will start to appear about one to two days after a frost. Because distinguishing living from dead tissue immediately after a frost is difficult, delay your assessment five to seven days.

Corn silage should be harvested at the appropriate moisture content for the type of silo in which it will be stored. If the crop was frosted prior to 50% kernel milk, its moisture content may be too high to be ensiled properly. However, during the dry-down period, dry-matter yield will decrease due to leaf loss, plant lodging and ear droppage. Thus, a trade-off exists between moisture and yield.

For silage corn frosted prior to the dent stage, the moisture content will be too high for successful ensiling. The crop should be allowed to dry in the field for several days and moisture content should be monitored. For corn frosted during the dent stage, the harvest should begin quickly to prevent yield loss as damaged leaves are shed or break off the plant. Because mold can occur on the ears before corn reaches the desired moisture level, producers may have to begin harvesting immediately.

To help control problems with excess moisture, mix wet silage with ground grain, straw or chopped hay. The rule of thumb is to add about 30 lbs of dry material per ton of silage to reduce silage moisture one percentage unit.

The maturity may be uneven in many fields. When harvesting a field with differing maturity levels, handle field sections separately where possible. In fields where the chopper must move through areas differing in maturity, such as low spots, chop when the majority of the field is at the proper moisture.

The immature spots will be wetter than the rest of the field and moisture might seep in the silo, but as long as the seepage does not leave the silo, nothing is lost. Fermentation should be adequate to preserve the silage. However, corn that is too dry might develop a "hot spot" in which mold can develop, increasing the chances for mycotoxin development.

For many years, corn was harvested for silage at the black layer stage of development. However, more-recent Wisconsin research recommends beginning corn silage harvesting at 50% kernel milk and finishing it by 25% kernel milk. Today, many hybrids have a "stay-green" trait that improves standability by keeping the stalk and leaves green while husk leaves turn brown and open, allowing the ear to dry.

If producers ensile frosted corn at the proper moisture content and follow other steps to provide good-quality silage, nitrate testing should not be necessary.

“The only way to know the actual composition of frosted corn silage is to have it tested by a good analysis lab,” Schroeder says.

Here are some characteristics of frost-damaged corn grain:

  • Small, misshapen, soft kernels.
  • Undeveloped starch structure, pithy kernels.
  • Test weights progressively below 52 lbs/bu, depending on maturity.
  • Average protein (7.5-8%) in corn heavier than 45 lbs/bu, lower protein in corn lighter than 45 lbs/bu.
  • High breakage susceptibility; many fines generated in handling.
  • Lower digestibility compared with normal corn, especially for test weights below 45 lbs/bu.
  • Little or no increase in test weight after drying.
  • Variable amino acid levels.
  • Low moisture-meter readings. Surface drying of kernels leads to deceptively low (by 1-2%) moisture readings on dried corn.

These effects are progressive and have the least impact on corn closer to maturity, says Schroeder. For more information on harvesting frost-damaged corn, visit http://www.ag.ndsu.edu/pubs/ansci/dairy/as1256.pdf.