Grain Drying Q&A

Grain Drying Q&A

October 12, 2009

In the following Q & A's Extension Educator Tom Dorn shares some of the questions he's been getting about grain drying and the answers he's shared with callers.

Grain Elevator Operator Q: Farmers are bringing in ears of corn for sampling. We're not seeing any "black layer" yet. Will this corn dry down mechanically, and keep until spring with aeration?

A:
I have heard that some corn is not showing black layer this year.  Black layer occurs at the end of the grain filling period when the translocation of nutrients from the ear to the kernel ends. With this summer's cooler temperatures and delayed maturity, some plants may not have received enough heat units to reach this stage before shutting down. Black layer usually occurs when corn is between 30% and 37% moisture content.  If the corn is dry enough to combine, I would not be concerned about being able to mechanically dry it in the high speed dryer or dryer bin with adequate airflow.

Producer Q: Should I turn off my aeration fans when it’s raining or humidity is very high?

A: It depends on the moisture content and temperature of the corn. If you study the Shelf Life Table in the October 17, 2008 CropWatch article, Monitor Grain Condition; Retain the Value of Your Crop, you can see why turning off the aeration fans may not be a good idea under some circumstances.

The shelf life values in the table express the time corn can be stored before it is expected to lose one-half of one percent (0.5%) of its dry matter content. This is the maximum dry matter that can be lost and still maintain the current market grade.

A rule of thumb is when corn (above 17% moisture) is held at a constant temperature, the shelf life is reduced by half for every 2% increase in moisture content. Alternately, when corn (above 17% moisture) is held at a constant moisture content, the shelf life is reduced by half for every 10 F rise in temperature. Corn in a bin that is not aerated to carry away the heat generated by microbial respiration has a shelf life about one-third as long as the values in the table because the heat bulid-up causes higher rates of microbial activity which results in even more heat and it becomes a run-away situation.

If you have corn above 17% moisture in a bin during the cool but not extremely cold months of October and most of November, it is critical to run aeration fans continuously (rain or shine) until the moisture content is below 17% to keep the grain from heating and deteriorating in the bin.

If you can monitor the grain closely for the first sign of heating and the wettest corn in the bin is below 17% moisture, consider intermittent fan operation based on the equilibrium moisture content table. The equilibrium moisture content table shows the driest possible moisture content you can achieve with natural (unheated) air under the stated temperature and humidity conditions. To view an equilibrium moisture content table see How to Reduce On-Farm Drying Energy Costs on the CropWatch Surviving High Input Costs Web site.

Producer Q: If I can't get my corn dry before consistently cold weather sets in, can I hold it in the bin without aeration until more favorable weather conditions return?

A: If ambient air temperature is close to 30°F and predicted to remain cold and the grain temperature is uniform and between 25°F and 35°F throughout the bin, the corn can be held without aeration until better drying conditions return provided it is monitored frequently.

The aeration fan should be run a few hours at least twice a month when the temperature is close to 30°F to ensure the temperature stays uniform in the bin. It is a good idea to climb up and lean into the top access hatch just after starting the fan to monitor for a moldy smell and/or any sign of heat being carried out of the grain. If these conditions are detected, continue to run the aeration fan and make a couple of rounds with the stirring system if so equipped.

Producer Q: Should I try to get 18% moisture grain down to 20°F to prolong the shelf life?

A: I do not recommend getting grain at any moisture content below about 25°F.

If we happen to get one of those January thaws where the air is 50°F and relative humidity is 50%, it would seem to be a perfect opportunity to do more grain drying. If you cooled the grain to 20°F before discontinuing aeration, then start the fan under the seemingly perfect drying conditions described above, you could be headed for a disaster.

When the 50°F air at 50% relative humidity penetrates into the 20°F corn, it quickly loses heat to the frozen corn. Air at 50°F and 50% relative humidity hits its dew point when cooled to 32°F. At some distance into the 20°F grain, the air is cooled to below the dew point temperature of 32°F. At that point, moisture in the air condenses out of the air, creating frost on the surface of the kernels. When the frost layer builds sufficiently, airflow through the grain is blocked. Worse still, the frost dam could hold the grain in place and prevent it from flowing to the center unloading auger.

Producer Q: What should I do if I accidentally cool the corn to 20°F?

A: If the grain gets too cold, watch for a time when the temperature is predicted to stay between 25°F and 35°F with relative humidity below 50% and run the aeration fan. Air at 30°F and 50% relative humidity has a dew point of 15.4°F. Since the grain is warmer than the dew point temperature of the air, no water will condense onto the grain and you can bring the corn back to a safe temperature.

Once the grain temperature is back to about 30°F, you can safely take advantage of that January thaw for some additional drying. Assuming a grain temperature of 30°F, I calculated a few weather conditions that would not result in re-wetting the grain: 35°F and 80% relative humidity; 40°F and 70% relative humidity; 45°F and 55% relative humidity; 50°F and 40% relative humidity; 60°F and 30%.

Producer Q: About how long should it take to dry 20% moisture corn to 15.5% using natural air during October?

A: The High Plains Climate Center data for Lincoln shows the 24-hour mean temperature is 51°F for the final three weeks in October. If we figure 50°F and assume the mean humidity is 50% (dew point of 32°F), and we assume the fan can generate 1.0 cfm/bu airflow, (the minimum recommended airflow for 20% moisture corn), we can estimate the time to dry this grain.
 
Under these climatic conditions and the stated airflow rate, it should take about 34 days to bring the moisture content at the top of this bin of corn to 15.5%.  (Assuming the exhaust air leaving the grain has 85% relative humidity).

Producer Q:
How much faster will the grain dry if I can run my propane burner and add 20°F to the air temperature?

A:
Adding 20°F heat to the assumed 50°F natural air drops the relative humidity of the incoming air to 25%.  If we assume the exhaust air is holding 55% relative humidity,  the drying time is 18.2 days.

Tom Dorn
Extension Educator, Lancaster County