Stored Corn Big Investment: Monitor It

By Jane Fyksen Crops Editor
Thursday, October 30, 2008 7:50 AM CDT

Most corn coming out of the combine is No. 1 grade. To ensure a quality product also comes out of the bin, monitor stored grain frequently.

Grain graders use four criteria to set the market grade of corn: Test weight, broken kernels and foreign matter (BCFM), damaged kernels and odor. Mold growth affects two of the criteria for determining market grade - test weight and odor. Mold growth nearly stops when the moisture content of corn is below 16 percent. As long as grain is above 16 percent, mold organisms will attack the grain, breaking down the starch and using it as the energy source for metabolism. Carbon in the starch is combined with oxygen, creating heat. The heat created in this process results in dry matter loss.

“Think of it as taking some of the corn and setting it on fire to create the same amount of heat. The difference is, instead of burning some corn to ash, a portion of the dry matter is lost from every affected kernel. This reduces test weight,” says Tom Dorn, Extension educator in Lancaster County, Neb.

The rate of microbial respiration increases as the moisture content of the grain increases. It also increases with increasing grain temperature. Table 1 shows the estimated shelf life of corn when held in the laboratory under carefully controlled conditions. “Shelf life is defined as the time the corn can be held under the stated conditions before it loses 0.5 percent of its dry matter (this is about the maximum dry matter loss before corn loses a market grade),” he says.

Since microbial respiration creates heat in the grain, it can result in a run-away process - more heat causes faster respiration rates, which create even greater heat. The shelf life estimates in Table 1 assume a constant temperature is maintained in the grain. If the grain is not being aerated to carry away heat generated in the grain, the shelf life is estimated to be only one-third as long as shown in Table 1.

“This is why we recommend continuous aeration, rain or shine, when corn moisture content is above 18 percent and periodic aeration to maintain a uniform temperature anytime the moisture content is above 15.5 percent,” says Dorn.

A bin of corn is a huge investment and should be monitored at least monthly throughout storage. Check grain temperature with a grain temperature probe several places near the sidewall and near the center of the bin at least monthly. If there is more than a 10-degree difference in temperature, run the aeration fan to push a temperature front through the grain to create uniform temperature in the grain mass. This will stop convection currents from forming in the grain, leading to a wet spot in the top-middle of the bin.

Even when the temperature probe doesn’t indicate a problem, check further, encourages Dorn. Open the access hatch on the roof and start the aeration fan. Lean into the hatch and feel the air on your face. Is the air warmer than expected? Does it have a musty or moldy smell? Can you feel high humidity in the exhaust air as it hits your face? Does condensation form on the bin roof on a cold day?

“These can be signs of the presence of a wet spot somewhere in the grain mass that may have been missed when you probed the grain,” he says.

If you detect a potential problem, open all exhaust hatches and continue to run the aeration fan. If the bin is equipped with a stirring device, make a round or two while aerating to break up pockets of high moisture grain. If the bin does not have stirring devices, he says “it may be wise to remove several hundred bushels of grain from the bin to locate and break up wet spots.”

The amount of time required to push a temperature front through grain depends on the airflow rate. To estimate how many hours it’ll take to push a temperature front through grain, divide 15 by the airflow rate in cubic feet per minute (cfm) per bushel. A temperature front will take 15 hours with one cfm per bushel, 30 hours with 0.5 cfm per bushel and 150 hours with an airflow rate of 0.1 cfm per bushel. Check grain temperature at several locations to determine when the cooling front has been pushed completely through the grain, he notes.

When not running the aeration system, remember to close roof hatches to prevent rain and snow from getting into the bin. Also cover the fan opening whenever it's not running to prevent problems caused by the chimney effect that can draw in moist air at the bottom of the bin and up through the grain. This can result in wet moldy grain on the bottom of the bin caused by condensation of moisture onto the cold grain.

Shrinkage explored

Grain shrinkage is also always a topic of interest this time of year. There are two components of weight losses. Water is by far the major weight-loss component during drying, but a second smaller portion is dry matter or handling loss due to release of oils, mechanical losses from broken kernels and respiration from the seed, says Ohio State University’s Peter Thomison.

While calculating water loss alone is straightforward and doesn’t vary from load to load or chart to chart, the actual amount of handling loss will depend on initial physical quality of the corn and handling during drying. This loss is not measured directly by buyers, but is added to water loss using several different methods of estimation. The most common are: Use of drying tables which include water shrink plus a constant handling loss (usually 0.5 percent of the initial grain weight), or a constant shrink factor in which the assumed handling loss varies depending on the initial grain moisture content.

“In order to accurately compare custom drying quotes or grain sale alternatives, corn growers must determine the shrinkage costs associated with each and choose the alternative which returns the greatest dry bushels or highest net sale,” says Thomison.

For examples of how to do such comparisons (and a more detailed description of shrinkage calculation differences), see the National Corn Handbook Publication NCH-61, "Calculating Grain Weight Shrinkage in Corn Due to Mechanical Drying," online at http://www.extension.iastate.edu/Publications/NCH61.pdf

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