Frost brought the 2011 corn growing season to an end this week for many North Dakota producers.
"Corn in many areas had not reached physiological maturity, which could lead to storage problems if it isn't dried and ensiled properly," cautions J.W. Schroeder, North Dakota State University Extension Service dairy specialist.
Corn is killed when temperatures are near 32 F for a few hours and near 28 F for a few minutes. Corn can be damaged when temperatures are slightly above 32 F and conditions are optimum (clear skies, low humidity, no wind) for rapid heat loss from the leaves.
At temperatures between 32 to 40 F, 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 the plant, 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. Frost symptoms are water-soaked leaves that eventually turn brown.
Because distinguishing living from dead tissue immediately after a frost is difficult, delay your assessment five to seven days.
Here are some characteristics of frost-damaged corn grain:
* Small, misshapen, soft kernels
* Undeveloped starch structure, pithy kernels
* Test weights progressively below 52 pounds per bushel, depending on maturity
* Average protein (7.5 to 8 percent) in corn heavier than 45 pounds per bushel, lower protein in corn lighter than 45 pounds per bushel
* High breakage susceptibility; many fines generated in handling
* Lower digestibility compared with normal corn, especially for test weights below 45 pounds per bushel
* Little or no increase in test weight after drying
* Variable amino acid levels
* Moisture meters generally read low in immature corn; surface drying of kernels leads to deceptively low (by 1 to 2 percent) moisture readings on dried corn
These effects are progressive and have the least impact on corn closer to maturity.
"Farmers will need to manage frost-damaged corn silage and grain in fields that were harder hit by frost," Schroeder says.
Corn silage should be harvested at the appropriate moisture content for the type of silo in which it will be stored. In general, more moisture is required for good packing in storage structures that allow easy diffusion of air, such as bunkers.
If corn is frosted prior to 50 percent kernel milk, the corn's 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 corn silage frosted prior to the dent stage, the moisture content will be too high for successful ensiling. The silage 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 the 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 pounds of dry material per ton of silage to reduce silage moisture 1 percentage unit.
The maturity may be uneven in many corn fields. When harve sting a corn 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 corn 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 percent kernel milk and finishing it by 25 percent kernel milk. Today, many hybrids grown 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.
For more information on harvesting frost-damaged corn, visit http://www.ag.ndsu.edu/pubs/ansci/dairy/as1256.pdf.