March 5, 2010

March is upon us. Soon average air temperatures will climb into the 40s and 50s and grain held in the bin above safe storage moisture content (15% for corn) will be in danger of spoiling.

Start watching now for the first symptoms of heating in the grain. These will likely appear next to the bin wall, especially on the sunny side of the bin where it will be warmer, or at the top center of the bin.

When moisture content is above 16.5% and grain temperature is above 40ºF, you can expect some level of mold growth in the grain. As fungal organisms metabolize the starches in the grain, heat is generated. The rate of fungal growth accelerates with higher grain temperatures. This can lead to a runaway situation with rapidly deteriorating corn.

Our cold Nebraska winters can be a blessing as well as a curse. Nebraska farmers were able to hold high moisture corn from December through February because they could keep the grain cold. I advised people to shoot for a grain temperature 30ºF (plus or minus 5 degrees). At these temperatures, the fungi that cause grain mold are almost completely dormant. In winter the mantra is: "If you can't get your grain dry, at least get it cold."

With warmer temperatures, the one liner recommendation changes to "If you can't keep the grain cold, then you must get it dry." The target moisture content for corn sold by June 1 is 15%. Corn held into the summer should be dried down to 14% moisture.

Airflow values as small as 0.2 cfm/bu are adequate for pushing temperature fronts through the grain, as mentioned in a February CropWatch article. To estimate the number of hours needed to move a temperature front through a bin of grain, divide 15 by the airflow rate in cfm/bu.

Grain Drying 101

Drying occurs in a fairly narrow horizontal zone in the bin and moves through the grain in the direction of airflow. When the corn nearest the aeration fan comes into equilibrium moisture with the air, no further drying occurs. This defines the bottom edge of the drying zone.

Within the drying zone, moisture migrates to the surface of the kernels and evaporates into the passing airstream. The air picks up more moisture as it moves through the grain mass until the grain moisture and the humidity in the air come into equilibrium again. This defines the top of the drying zone. Beyond this point, the air simply passes through the remaining grain and exhausts from the bin.

The minimum airflow I recommend for drying grain with either natural or heated air is 1.0 cfm/bu. More airflow is better. You should not try to make up for inadequate airflow by adding heat. If your airflow is inadequate, remove some of the corn from the bin and re-level the top. This will reduce the static pressure the fan must overcome and increase the airflow the fan can produce. Pushing more air (cfm) through fewer bushels results in higher airflow rates per bushel (cfm/bu). Often, reducing grain depth only a few feet can result in recommended airflow rates. For example, assume a 36-foot diameter drying bin is equipped with a 9.5 horsepower, 24-inch axial flow fan. At 18 feet of grain depth the airflow is 0.84 cfm/bu. Reducing the grain depth to 16 feet increases the airflow to 0.97 cfm/bu; reducing it further to 15 feet increases the airflow rate to 1.1 cfm/bu. When you have at least the minimum of 1.0 cfm/bu airflow, you can safely add heat to reduce time to dry the corn.

Continue Monitoring for Heating

Don't forget to continue monitoring stored grain twice a month for signs of heating. Air takes the path of least resistance through grain. Pockets of wetter grain could be buried in the grain mass. This is usually due to an accumulation of fines or a frost dam which is impeding airflow.

As I said in February, if you have a grain temperature probe, take the grain temperature near the bin wall about every 20 feet around the outside of the bin and a couple of places near the middle of the bin. Let the probe stay in place for 7 to 10 minutes before taking each reading. If there is more than a 10 degree difference in temperature between the highest and lowest readings in the bin, run the aeration fan long enough to push a temperature front through the entire grain mass. (The rule of thumb is: To estimate the hours needed to push a temperature front through a bin of grain divide 15 by the airflow rate in cubic feet per minute per bushel (cfm/bu).

If you don't have a temperature probe, turn on the aeration fan and lean into the access hatch or climb into the bin. Does the air hitting your face feel warmer than expected, or do you detect a musty odor, or does condensation form on the inside surface of the bin roof on a cold day, if you detect any of these symptoms, continue to run the fan long enough to push a temperature front through the bin. If the bin is equipped with a stirring system run two or three rounds to break up hot spots and to equalize the moisture throughout in the grain mass.

If the warning signs are present and the bin is not equipped with a stirring system, pull some grain out of the bin and monitor the condition of the grain coming out of the auger. If you detect heating, run the aeration fans to cool the grain and to dry the grain if air properties allow. Level the grain surface if the remaining grain will be left in place.

Tom Dorn
UNL Extension Educator in Lancaster County