March 20, 2009

Fossil fuel prices were near historical highs in September and October, while grain prices were dropping rapidly. This put producers in a squeeze between declining grain prices and high drying costs. Many producers with on-farm grain drying capabilities decided not to pay the high prices for propane or natural gas to heat the air stream and reduce drying time. Instead they decided to use natural (unheated) air to dry their corn.

Ambient air properties (temperature and humidity) often were not conducive to drying grain and many producers were still running their fans in late November and into December when the air temperature was well below freezing. Going into winter with a grain temperature between 30°F and 40°F is a sound management practice because it slows mold growth and stops insect activity; however, taking grain well below the freezing point is not recommended.

To reduce convection currents in the grain, uniform grain temperatures need to be maintained throughout the bin and aeration needs to be managed so grain mass temperatures approximate the average air temperatures outside the bin.

Take Advantage of Cool, Dry Days to Push Front Through Stored Grain

The amount of time required for an aeration cooling or warming cycle depends on the airflow rate. The time in hours to cool or warm corn can be estimated by dividing 15 by the airflow rate. For example, 150 hours is needed with an airflow rate of 0.1 cubic feet per minute per bushel (cfm/bu) and 75 hours is needed for 0.2 cfm/bu and if the grain is in the drying bin (1 to 1.25 cfm/bu), a temperature front could take as little as 12 to15 hours to pass through the bin. Check grain temperature at several locations to determine when the temperature front has been pushed completely through the grain.

As the air temperature warms in the spring, the grain next to the bin walls will be warmed, but the grain in the middle of the bin is insulated from the outside and stays cold. Air will rise through the warmer grain near the wall and will be replaced by warm, moist springtime air sinking into the grain at the top center of the bin. The moisture in the air that enters the cold grain mass in the center of the bin will condense, forming a wet spot at the top center of the bin. This can lead to crusted, moldy grain in the top center of the bin and increased insect activity.

If you are still holding grain into the warmer spring months and have not warmed it to above 25°F with aeration, you should take advantage of cooler, less humid days to push a warming cycle through the grain. A rule of thumb is air can only hold half as much water when the air temperature is reduced 20 degrees. If you have frozen corn in your bin (e.g. 20°F) and pick a beautiful, warm spring day (e.g., 60°F and 50% relative humidity) to run your aeration fans, the 60°F air will be cooled 40 degrees as it passes through the grain, finally reaching equilibrium with the grain temperature. This will result in air moisture condensing into the grain. When the grain is well below freezing, the condensation could conceivably turn to frost or ice. If enough ice forms, it could prevent air movement through the grain. (This is known as an ice dam in the grain.)

On the other hand, if the ambient air is 35°F with 50% relative humidity, the air temperature will only drop 15°F as it passes through the 20°F corn and condensation should almost be eliminated. Once you have brought the grain temperature up to near or above freezing, you can then aerate with warm humid air. You might get some condensation at first, but it won't create an ice dam.

Tom Dorn
Extension Educator in Lancaster County