Grain pile
Circular outdoor pile with tarp partially removed in preparation for unloading of grain in summer 2016 after more than nine months of successful storage.
Photo courtesy of  Dirk Maier. 
Grain stored in covered outdoor piles can be successfully maintained for medium to longer-term “temporary” storage if variables such as site preparation, layout design, aeration use, and initial grain moisture content have been properly addressed before grain was piled outdoors during the 2016 harvest.

What cannot be controlled but challenges the ability of a manager to maintain grain quality are the prevailing weather conditions during the storage period, the initial quality of the crop, and to some extent the length of storage. Minimizing storage losses in covered outdoor piles requires attention to storage management and frequent monitoring. Maintaining quality is always a greater challenge than in permanent structures such as bins, silos, tanks and buildings. Outdoor piles that are not covered should always be considered short-term “emergency” storage only and should be moved to permanent structures as soon after harvest as possible. This is even more critical if it rained on uncovered piles during any length of the storage period.

Starting with the wheat and sorghum harvest across the Great Plains and continuing with the corn and soybean harvest across the Midwest, this year’s record U.S. fall harvest has forced grain handlers to place more grain in outdoor piles than perhaps ever.

Based on estimates by the Iowa Grain Quality Initiative, as much as 530 million to 670 million bushels of mostly corn may have been stored in outdoor piles in Iowa. This is based on the difference between harvests of 3.05 billion and 3.15 billion bushels of corn and soybeans in 2015 and 2016, respectively, and only 2.90 billion bushels of permanent storage capacity on farms and at grain elevators in Iowa.

Much of the corn piled outdoors during the 2015 harvest was picked up this past spring and summer. Some of that corn was damaged and may be a challenge blending off into a 2016 crop that to some extent is also quality-challenged. Storing corn in outdoor piles has become common practice and is calculated as part of an elevator’s total storage capacity. It is not unusual to see some corn remain in covered outdoor piles for more than one season, especially while commodity prices are low and damage loss is less costly as a result. A number of covered outdoor piles containing 2015 corn are being carried over into the current storage season.

Having prepared well

In order to successfully manage covered outdoor piles, hopefully managers started by placing only grain that was dried below the safe storage limit of 15% for corn (and ideally closer to 14% if corn is to be carried into the spring or summer months), was cooled before leaving the dryer to within 5-10°F (2-5°C) of ambient temperature, was run over a screen cleaner to remove broken corn, fines and foreign material, and was covered quickly before any rainfall.

Placing corn at sufficiently low moisture content into covered outdoor piles was especially important this past fall harvest because across the U.S. Midwest a number of regions were affected by late-season ear corn diseases resulting in mold-damaged kernels before drying and storage. For mold-damaged corn, one should err on the conservative side of these maximum values.

Broken corn and fines will concentrate under the fill spouts of loading or transfer conveyors restricting airflow. The lightweight foreign material such as cob pieces will flow toward the outer edges of a pile whether circular or rectangular. Any placement of higher moisture grain at any location in the pile or exposure to rainfall may result in the grain to self-heat due to respiration and mold spoilage, causing higher damage levels. Every one inch (2.5 centimeters) of rainfall penetrates grain by one foot (30 centimeters) and increases grain moisture content by approximately nine percentage points. For example, during the fall 2015 harvest Iowa experienced much rainfall. Uncovered outdoor piles were penetrated up to six feet (1.80 meters) in depth, increasing moisture content from 14% to 23% in that thick of a layer with surface moisture contents as high as 30%.


A hand-held CO2 sensor placed at the outlet  of an aeration fan in down draft aeration system of an outdoor pile is used for the early detection of onset grain spoilage.
Photo by Dirk Maier.
All properly designed, built and covered outdoor piles intended for longer-term storage of grain include aeration systems. These are critical to maintain grain temperatures and to hold down the tarp on the pile.

If a pile is not aerated, whether covered or not, grain temperatures in the core will change little because grain is a good insulator. In the case of wheat placed in a pile during the Kansas summer harvest or corn placed in a pile during the Iowa fall harvest, both will remain at the average temperature when piled. Quality will deteriorate in wheat (remaining at 80-110°F; 26-43°C) due to insect attack and in corn (remaining at 50-80°F; 10-26°C) due to mold spoilage.

The negative pressure from an aeration system creates the suction required to hold the tarp on the pile. Depending on wind velocity, the amount of suction may be increased by operating more fans. Typically, aeration fans are controlled automatically based on wind anemometers installed above the pile. The minimum number of fans operated to hold down the tarp will maintain grain temperature in the pile at average ambient temperature. Additional fans are started when wind velocity increases above a certain level. For peace of mind, most covered outdoor pile fans are connected to backup generators that turn on automatically during electrical outages, for example, caused by storms.

Immediate cooling with aeration is critical for maximizing allowable storage time. Low grain temperatures minimize mold growth, limit moisture movement and condensation, and control insect development. Ideally, this past fall all aeration fans installed around a pile were operated to maximize airflow until grain temperatures were uniform and equal to the average outdoor temperature of 40°F (5°C) or below. At a typical design airflow rate of 0.10 cubic feet per minute per bushel (about 0.10 m3/MT), this should have taken at least 150 hours (cooling hours = 15/airflow rate). More likely it will have taken nearly double that amount because of non-uniform airflow distribution due to the concentration of broken grain and fines below the peak or ridge of the pile.

Given that a minimum number of aeration fans have to be operated around the clock to keep the tarp on the pile, grain temperatures will change as a function of ambient temperature but off-set by the approximate time it takes to move an aeration front through the pile (e.g., 150 to 300 hours or more). In the Midwest and Great Plains, grain temperatures in a pile will continue to decrease during the winter to well below freezing, then increase above freezing in late winter, then above 50°F (10°C) during early spring, and to higher temperatures depending on how long grain will be stored in the outdoor pile into late spring and summer.

The usual practice of keeping cooled grain cool in permanent storage structures is not available. Therefore, carrying grain in outdoor piles into the warmer period of the year is especially risky because biological activity from molds and insects increase exponentially with temperature.

Monitoring Quality

Grain Pile
Grain is unloaded into an outdoor pile at a grain facility in Minnesota, U.S.
Photo by Adobestock. 
Grain quality deterioration in covered outdoor piles is a function of the change in grain temperature from continuous aeration and from any infiltration of external moisture through the tarp due to rain or snow. Condensation on the underside of the tarp and surface of the grain is generally not a problem because continuous aeration keeps air moving on the underside of the tarp and keeps grain surface temperatures near the average ambient temperature.

The best ways to monitor grain quality in outdoor piles is with CO2 monitors, and with temperature and “moisture” cables.

Probing grain is labor intensive, only samples the top of the pile and also raises safety concerns because workers would have to walk onto the pile to extract samples. Tarps have slick surfaces to shed rain and snow, which makes these inclined surfaces slippery and difficult to walk on even when hooked to a fall protection system. Workers have been blown off tarps that got loose and started to flap during high winds. Steel cables with temperature and relative humidity sensors can be installed before a pile is covered, albeit with much effort. A number of commercial systems are available. Traditionally, cables only included temperature sensors, but more recently cables with both temperature and relative humidity sensors have become available commercially. Given the changing temperatures and associated variation in relative humidity of the air moving through the grain pile due to continuous aeration, installing cables to monitor both grain temperature and moisture content is the more comprehensive approach for successful quality monitoring of grain stored in outdoor piles.

CO2 sensors are a complement or even an alternative to cables for monitoring the quality of stored grain in outdoor piles. Given that aeration fans draw air out of the grain pile at ground level, a manager can hold a CO2 sensor at the outlet of each fan and determine whether CO2 content in the exhaust air is stable (e.g., 400 to 600 ppm, normal values) or is increasing from week to week. Increases in CO2 readings often are picked up in a certain section or slice of an outdoor pile. This will allow a manager to detect the onset of limited spoilage earlier than temperature and “moisture” cables.

How to utilize CO2 sensors and interpret observed CO2 readings has been described in a previous article, which is accessible on-line in the July 2016 issue of World Grain (

Grain Reclaim

Outdoor piles should be picked up as soon as permanent storage space becomes available. The costs and benefits of partial reclaim should be weighed against penalties associated with quality deterioration that accrue when waiting until space becomes available for the entire pile. Once it has been decided that grain will be reclaimed from an outdoor pile, the tarp has to be at least partially removed together with the inlet aeration ducts placed on the surface of the grain.

Although generally not possible, if a section or slice of an outdoor pile previously was identified to have higher CO2 readings, it would be preferable to start unloading the pile at or near that location. Grain from that area likely will have higher damage levels. Rectangular piles positioned with the long axis pointing from east to west may contain grain with greater quality deterioration along one face of the pile. Grain from these areas should be segregated from the rest of the piled grain as it is brought into permanent storage or shipping bins.

As grain is reclaimed, grain with higher levels of damage typically from the grain surface, locations where water penetrated through holes or seams in the tarp, or from locations where grain with high levels of broken corn and fines accumulated, should be segregated for additional conditioning such as cleaning, drying and blending.

Grain with objectionable odor also should be segregated from the rest of the grain. It could be treated with ozone to remove the objectionable odor, a practice that has been utilized successfully by a number of grain companies.

Grain samples should be taken from all loads and analyzed for moisture content and grade factors (especially damage). Some grain, especially along the base and edge of a pile, may be sufficiently spoiled that it should be left on the ground after the rest of the pile has been removed. It often contains non-grain materials such as gravel (if not paved) from the bottom surface of the pile and needs to discarded.

Overall weight and quality loss should be assessed, and costs for storing grain in outdoor piles for the season should be determined. Consideration should be given to improvements for the next season such as paving the surface, improving the aeration system and tarping a pile that will be regularly used, or adding more permanent storage structures to avoid weight and quality losses from temporary storage in outdoor piles.

Dirk Maier and Sam Cook are post-harvest engineers with the Iowa Grain Quality Initiative at Iowa State University. They may be reached at [email protected] and [email protected].