David P. Shelton, Extension Agricultural Engineer

David D. Jones, Associate Professor, Biological Systems Engineering

Once the last grain has been augered into the bin and the hatches closed, there is often a tendency to forget about what is needed to maintain the grain at a high level of quality. However, without proper management, that grain can rapidly deteriorate, becoming a worthless mass.

Grain spoilage is usually the cumulative result of several different handling and management operations and decisions. Thus, the better the overall management program, the better the chance for maintaining grain quality. Four factors which greatly affect grain storability are (1) grain moisture content; (2) grain temperature; (3) initial condition of the grain; and (4) insects and molds. These factors are all interrelated.

Moisture Content

If grain moisture content is too high, even the best aeration equipment and monitoring management will not keep the grain from spoiling - it only delays the inevitable. Recommended moisture contents are given in Table 1. These recommendations assume the grain is high quality and aerated to control temperatures and moisture migration. Reduce the recommended moisture contents by 1 percentage point when storing low quality grain. This includes immature grain, severely cracked and damaged grain, and grain subject to previous insect or mold activity. Also reduce the recommended moisture contents by at least 1 percentage point for grain in temporary or emergency storage.

Table 1. Maximum recommended moisture contents for properly managed, high quality, aerated grain.
Storage Period Corn and Sorghum Soybeans
Fed by April 18% 13%
Marketed by June 15.5% 13%
Up to one year 14% 12%
Over one year 13% 11%

Going into storage at the proper moisture content does not guarantee grain will remain at that moisture. Grain may be rewet as a result of bin roof or sidewall leaks. Moisture can also enter through downspouts from a bucket elevator or through hatches that have been left open.

Moisture condensation can also cause localized increases in grain moisture content. Condensation, particularly on bin roofs and sidewalls, is common when warm grain (50o F or above) is cooled during cold weather (30o F or less), or when hot grain from a dryer is cooled in a storage bin. Minimize these condensation problems by gradually cooling stored grain in stages during the fall, and by cooling hot grain from the dryer in a separate cooling bin before moving it to storage. Condensation can also be minimized by providing adequate exhaust vents in the cooling bin.

Grain Temperature

Whether holding wet grain for a short period of time or storing dry grain for longer periods, it is important that grain temperatures be controlled by moving air through the grain mass. Because both wet grain and molds respire and give off heat, aeration is needed to keep the grain cool and to slow mold growth. Properly aerated grain can generally be safely held about four times longer than non-aerated grain.

Aeration is needed, even if grain is dry and cool when placed in storage, to keep the grain mass at the desired temperature and to keep temperatures equalized. Differences in grain temperatures create convection currents which can move and concentrate moisture in the top center of the bin. Problems caused by this moisture movement, or moisture migration, often become obvious in the spring when outside air temperatures begin to warm. The first indication of trouble is usually damp or tacky feeling kernels at the grain surface, followed by the formation of a crust. Moisture migration is more of a problem in a peaked storage because the moisture is concentrated in a smaller volume of grain.

Moisture also moves by vapor diffusion from warmer to cooler areas in the bin. If grain is not properly cooled for winter storage, there is a tendency for moisture to move to the cool grain along the bin sidewall, causing spoilage. Moisture movement problems can be prevented or minimized by keeping grain mass temperatures equalized and within 10 to 15o F of the average outside air temperature.

Aeration can also be used effectively to control insect activity by keeping grain temperatures at or below 60o F. Aeration is especially critical early in the fall to cool grain from warmer harvest or summer storage temperatures. Waiting until late in the fall to cool grain invites insect activity. Cooling the grain in the fall and keeping grain temperatures below 60o F as long as possible into the summer will help control insects and increase the chances of getting through the summer without having to fumigate the grain.

When grain from a dryer is cooled in a storage bin, it is critical that cooling be completed within 4 to 6 hours. Do not turn the fan off until all the grain has been cooled, regardless of weather conditions. Holding warm grain in a bin for even a few days is a needless risk to take.

Initial Grain Condition

Grain quality will not improve during storage. At best, initial quality can only be maintained. To help assure that only high quality grain goes into storage, the following is recommended:

Clean around the bin site. Remove any old grain, grass, weeds, and other debris.

Remove all traces of old grain from the bin and harvesting and handling equipment.

Properly adjust the combine to minimize grain damage.

Clean the grain as it is put into the bin, preferably using a rotating grain cleaner.

Cool the grain to the prevailing outside air temperature as soon as it is put into the bin.

(Refer to the article "Initial Condition Determines Quality of Stored Grain" for additional details on assuring that only high quality grain is put into storage bins.)

Insect and Mold Control

Insects are generally not a problem in grain stored for less than 10 months or a year. However, if grain is to be stored for longer than this, or if a bin has had an insect problem in the past, special precautions should be taken. These include:

Spray the inside of the bin with protective insecticides 2 to 3 weeks before new grain is added.

Treat the grain with an approved insecticide as the bin is filled.

Top-dress the grain with an approved insecticide after the bin has been filled and the grain surface has been leveled.

(Refer to the article "Initial Condition Determines Quality of Stored Grain" for additional details on insecticide application recommendations.)

Left untreated, an insect infestation will eventually lead to other storage problems. Insects give off moisture which can cause grain moisture contents to increase enough to create a mold problem. Mold activity will in turn raise temperatures and result in an increased rate of insect reproduction. Greater numbers of insects create more moisture, and the cycle is repeated at an ever increasing rate.

Aeration System Management

The primary objectives of aeration are to keep the grain at a seasonally cool temperature and to maintain uniform grain temperatures - preferably no more than a 10o F difference in temperature from one part of the bin to another. These objectives can be achieved by keeping grain temperatures within 10 to 15o F of the average ambient air temperature. Thus, seasonal temperature changes require changes in aeration fan operation.

There are a number of fan operation schedules that can be used to maintain the quality of stored grain. Following the management procedure outlined below will help assure that basic aeration requirements are met. Adapt it as necessary to meet individual needs and conditions.


Move at least one (preferably two) cooling zone(s) through the grain to remove field or dryer heat and help equalize moisture contents.

Thereafter, move one cooling zone per month through the grain until it is cooled to between 35o and 40o F and to equalize grain mass temperatures.

Check the grain temperature and condition every two weeks and as needed to monitor cooling zone progress.

The initial cooling is important, so do not skimp on fan operation. Turn the fans on as soon as grain covers the perforated floor or aeration ducts, and operate continuously until the grain has been cooled to the prevailing outside temperature. Since cooling is the primary concern, especially if the grain has come from a dryer, do not turn the fans off during rainy or humid weather. Failing to properly cool the grain can cause more problems than the small amount of rewetting that occurs from running the fan on a humid day.


Check the grain temperature and condition at least once a month.

Aerate as needed to maintain grain temperatures between 35 and 40o F.

During the winter, the aeration system needs to be operated only on a maintenance schedule to control localized temperature increases. In fact, it may not be necessary to run the fan at all during the winter if the grain remains dry and in good condition, and if grain mass temperatures are stable. One aeration strategy is to operate the fan for a few hours as part of a bi-weekly or monthly grain checking program. This allows the operator to check the exhaust air for off-odors, an indication that the grain requires immediate attention.

Avoid operating the fan on warm days. When air temperatures are warmer than grain temperatures, fan operation can result in moisture condensing and possibly freezing on the cold grain. This condensation problem can be prevented by operating the fan only when air temperatures are the same as or cooler than grain temperatures.

Freezing grain is not recommended because of the increased likelihood of condensation problems if the grain is not properly warmed in the spring. However, freezing the grain becomes a secondary concern if the grain begins to heat or go out of condition. If a problem occurs, operate the aeration fan continuously, regardless of weather conditions, until the problem is corrected.


If the grain is frozen, thaw by moving a warming zone completely through the grain as soon as outside air temperatures remain above freezing.

If the grain is not frozen and will be fed or sold by June, aerate only as needed to control "hot spots" and heating problems.

If the grain will be held into or through the summer, move one warming zone per month completely through the grain until the grain mass is uniformly warmed to about 60o F.

Check the grain temperature and condition at least every two weeks and as needed to monitor warming zone progress.

It may seem counterproductive to warm grain in the spring after cooling it down in the fall. If fact, there is little reason to warm the grain if it is to be marketed or fed by summer. One exception is that frozen grain should always be thawed before being handled in warm weather. Operate aeration fans continuously when thawing frozen grain to prevent freezing of condensed moisture on the grain.

Since average outside air temperatures change at the rate of 2.5 to 3o F per week, move one warming zone per month through the grain to maintain uniform grain temperatures and to warm the grain to 60o F in preparation for summer storage. This temperature is cool enough to slow insect activity, yet warm enough to minimize condensation if the aeration fans need to be operated to control localized heating in the bin. Fans should be operated continuously for each successive warming zone.


Check the grain at least once every two weeks to monitor temperature, moisture, and insect activity.

Consider operating the fan one cool night per week through June to help maintain grain temperatures at 60o F.

Cover fan openings during June, July, and early August.

Grain checking is very important during the summer because grain is being held at higher temperatures and aeration conditions are less favorable than during the rest of the year. Grain temperatures need to be checked and recorded on a regular basis. Insect activity is also at a peak during the summer, and frequent checking is required if infestations are to be controlled before they develop into major problems.

Not all of the grain going into the summer at 60o F will remain at that temperature. The grain along the bin sidewall and the grain surface will be gradually warmed over the course of the summer. Operating the aeration fan on cool nights helps to bring these temperatures back down. However, aeration is normally beneficial for only part of the summer because of high temperatures during July and August. Do not operate aeration fans during these months unless a problem develops.

Although aeration fans are not normally operated during this period, there are still some temperature control measures that can be effective. One is to ventilate the roof space using gravity vents or roof exhaust fans to prevent high summertime temperature build up. Roof vents should be sized so that 1 square foot of vent space is available for every 1000 cfm of air flow. Openings at the eaves between the bin sidewall and roof can be considered as vent space.

Perhaps more important than moving air through the roof space is to keep the warm air from moving down through the grain. The best way to prevent this is to cover the aeration fan openings when the fans are not in operation. If the fans are not covered, the cooler air in the grain will move out of the bin through the fan and draw warmer air down into the grain. This reverse chimney effect can gradually warm all of the grain in the bin to 70 to 80o F. These temperatures increase the risk of mold problems and provide a favorable environment for insect activity.

Monitoring Grain Condition

Following the above aeration schedule will help maintain grain quality. However, grain condition needs to be monitored to verify that the desired temperature control is being achieved. Further, a regular checking schedule is essential if mold and insect activity are to be detected and controlled in a timely fashion. The method and frequency of checking will vary with time of year, initial condition of the grain, and aeration procedure. Generally grain should be inspected at least once a month during the winter and every two weeks over the spring, summer, and fall.

Grain checking is extremely important during the summer because grain is being held at higher temperatures and aeration conditions are less favorable than during the rest of the year. Grain temperatures need to be checked and recorded on a regular basis. Without temperature records, it is difficult to tell whether elevated grain temperatures are caused by normally occurring outside temperatures or by heating due to mold activity. The grain needs to be probed to locate any moisture pockets where molds will develop rapidly as temperatures warm. Insect activity is also at a peak during the summer, and frequent checking is required if infestations are to be controlled before they develop into major problems.

Failure to monitor grain condition throughout the entire storage period is a frequent mistake. A small area which starts to heat or otherwise "go out of condition" can quickly get out of control and spread within the bin. Think of the grain as being cash in the bin, and consider how frequently it would get checked if that were the case.

Some areas and conditions to check when monitoring grain quality include:

Grain surface for condensation, crusting, wet areas, molds, and insects.

Bin roof for condensation and leaks.

Grain mass for non-uniform temperatures, high moisture pockets or layers, molds, and insects.

Exhaust air for any off-odors.

If problems are detected, they need to be evaluated and corrected as soon as possible. This may include cooling with aeration, further drying, or fumigation for insect control.


It is critical to carefully manage stored grain to prevent grain deterioration and possible serious economic loss. This management should include:

A well-designed and properly-operated storage system with adequate aeration capacity.

Storing only clean grain at the proper moisture content and temperature.

Checking the grain condition regularly and correcting problems before they get out of hand.

September 1998