October 30, 2003

 

Warm Harvest makes Cooling Stored Grain more Important

The warm weather was excellent for grain harvest, but it increases the potential for grain storage problems according to Ken Hellevang, engineering specialist with the North Dakota State University Extension Service.

Optimum grain temperatures for insect activity and grain spoilage are 70 F to 90 F. "Harvested grain temperatures may be in this range," Hellevang says. "Grain is an excellent insulator, so it will remain warm unless cooled by aeration."

Insect reproduction and feeding activity are reduced at temperatures of 50 F to 70 F. Insects are dormant at temperatures below about 50 F, and insects may be killed if the grain temperature is below freezing for a few weeks, Hellevang says. The allowable storage time for grain, based on mold growth, is approximately doubled for each 10 degrees that the grain is cooled. The allowable storage time of 20 percent moisture corn is only about 25 days at 60 F, about 50 days at 50 F, 100 days at 40 F, and is more than 200 days at 30 F.

Average monthly temperatures permit cooling grain to about 70 F in August, 60 F in September, 50 F in October and about 25-30 F in November. Average nighttime temperatures permit cooling grain to below 60 F in August, 50 F in September, 40 F in October and below 30 F in November. "These are average temperatures, so cooler air is frequently available," Hellevang says. "Grain should be cooled to 20 F to 25 F for winter storage."

Large differences between grain and outdoor temperatures during aeration increases the potential for moisture condensation on the bin roof if air is "pushed-up" through the grain, Hellevang notes. This condensation may form ice that blocks the exhaust vents when outdoor temperatures are below freezing. With the vents blocked, the fan may develop enough pressure to damage the bin roof.

Hellevang recommends leaving the fill hole cover unlatched, so it can operate as a pressure relief valve if the vents become blocked. The vents can also become blocked with ice if the fan is pulling the air down through the grain. Ice may form on vents if outside air is damp when fans are operating with outside air temperature near or below freezing.

Moisture will accumulate in the top central portion of a grain mass due to moisture migration if the grain is not cooled as outdoor temperatures cool. Cool air along the bin wall settles to the bottom of the bin and pushes air in the center of the grain mass toward the top center of the bin. Moisture from this air is deposited on the cool grain near the top surface, because cool air cannot hold as much moisture as warm air, Hellevang explains.

Moisture migration can increase the moisture in the top central portion of the grain mass by 10 percentage points, which leads to grain spoilage. Because about a 20-degree temperature differential in the grain mass will cause moisture migration, aeration should start before the average outdoor temperature is 20 degrees cooler than the grain temperature. For example, cooling wheat may require aerating 3 to 4 times; just after harvest and additional times during the fall.

"The amount of time required to cool the grain depends on the airflow rate," Hellevang notes. The time to cool 56- to 60-pound grain can be estimated by dividing 15 by the airflow rate. For example, about 75 hours is needed with an airflow rate of 0.2 cubic feet per minute per bushel (cfm/bu).

"Check the temperature at several locations to determine when the aeration cycle is complete. Shut off aeration fans during periods of fog or rainy weather to minimize rewetting," Hellevang says. However, rewetting in not normally a concern when aerating with high humidity air. Typically moisture is lost during cooling. Cover fans and ducts after the grain has been cooled for winter storage to prevent snow from blowing into the grain bin.

"Check the condition of the stored grain every two weeks until the grain has been cooled to winter storage temperature," Hellevang says. "Check the grain at least monthly during the winter. Check the temperature of the grain in several locations and collect grain samples to check the moisture content."

Because the accuracy of moisture measurements are dependant on the grain temperature, it is best to let the sample warm to room temperature in a sealed plastic bag or other container before checking the moisture content. Record the data for future reference in managing the stored grain.

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Source: Ken Hellevang, (701) 231-7243, kjh-eng@ndsuext.nodak.edu
Editor: Tom Jirik, (701) 231-9629, tjirik@ndsuext.nodak.edu

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