Delayed harvest of corn and soybeans in 2018 across the midwestern U.S. are raising concerns about grain quality and how well this crop will store long term. With commercial grain facilities holding a large carryover from the big 2017 crop, some 2017 carry-over corn and much of the 2018 corn crop will end up in outdoor piles again. Managing grain in outdoor piles was the subject of a previous grain operations article (see World Grain February 2017, pages 78-83). This article focuses on managing stored corn and soybeans long term in permanent structures such as bins, silos, tanks and horizontal sheds where managers have more tools available, including coring, controlled aeration, temperature and moisture monitoring, and partial unloading.
Ideally, storage structures were cleaned and checked for insects in aeration ducts, under perforated floors, and in handling equipment before filling, and residual grain inside and debris around the facility were removed. For long-term storage into next summer and possibly beyond, an approved residual insecticide should have been sprayed on the inside walls and floor of the structure. External wall surfaces up to about 6 ft (1.80 m) above the foundation, the foundation including cracks and crevices, and the aeration fan inlets and conveying equipment outlets should be sprayed well in advance of the warm weather storage period to prevent insects from getting to the grain. Grain protectants are not needed on cold, dry grain carried into the warmer periods of the year except perhaps on the grain mass surface.
Low commodity prices and large crops will result in long-term storage of the 2018 crop well into or through 2019. Ideally outdoor piles are picked up before weather turns warm and humid in April or May. Corn carried in outdoor piles longer must be closer to 13% moisture content to maintain its shelf life and minimize damage due to mold spoilage and insect damage. Damage levels of corn not managed properly in outdoor piles can easily exceed 30% to 40%. Typically, soybeans are not piled on the ground.
Moisture content concerns
Delayed harvest has resulted in higher moisture content than normal in corn and soybean fields. In the case of soybeans, moisture content as high as 16% has been reported, which would be equivalent to storing corn at 19%, and thus may require heated air drying if harvested late in the season. The maximum recommended drying air temperature for commercial soybeans is 130°F (55°C).
Even at that temperature, 20% to 90% of skins may crack, especially if the drying air relative humidity is 30% or less. Moldy and sprouted soybean kernels are an additional concern, which means moisture content should be reduced by 1 to 2 percentage points below the recommended safe storage level of 13%.
In the case of corn, some mold-damaged ears and the occurrence of mycotoxins have been reported. Mold-damaged corn should be dried in high temperature dryers because the heated air will surface-sterilize the kernels. This will reduce the chance for mold development during storage as long as moisture content is reduced by 1 to 2 percentage points below the recommended safe storage level of 15%.
Coring the grain mass
Ideally, corn and soybeans should be screen-cleaned to remove some of the broken kernels and foreign material as storage structures are filled. As grain flows into the storage structure, segregation of smaller particles from whole kernels occurs based on size and density.
Fines accumulate under the fill spout and form a densely packed core that substantially reduces airflow. Cooling of the core and peaked region of the grain mass can take 2 to 4 times longer than the rest of the grain mass. This increases electricity cost by the same factor and causes additional shrink loss.
Unloading grain from under the fill spout (i.e., coring the grain mass) until 1/3 to 1/2 of the center grain surface is inverted will remove the densely packed core of fines and result in uniform airflow distribution through, and thus cooling of, the grain mass.
Aeration after drying
Dried corn and soybeans should be aerated immediately after filling permanent storage structures. In upright bins, aeration systems should be designed to generate at least 1/7 to 1/10 cubic feet of air per minute per bushel (CFM/bushel). This should result in cooling fronts moving through the grain mass in a minimum of 105 to 150 hours.
For grain depths exceeding 50 feet (15 meters) such as tall concrete silos, airflow rates of 1/20 to 1/40 CFM/bushel are often selected to reduce excessive static pressure and horsepower requirements. These rates should result in cooling fronts moving through the grain mass in a minimum of 300 to 600 hours. In flat storage sheds, airflow rates lower than 1/10 CFM/bushel may be used to limit in-floor and above-floor aeration duct sizes.
By late October, running fans on corn and soybeans in the midwestern U.S. should cool them to below 50°F (10°C), by late November temperatures should drop below 40°F (5°C), and by late December corn and soybean temperatures should be cooled to around the freezing point (28-36°F; -2-2°C). During January and February, it should not be necessary to run aeration fans unless temperature rises are observed due to self-heating grain.
Excessive aeration causes additional shrink loss below the recommended safe storage moisture content for corn and soybeans. At today’s low prices and thin margins, losses from excess energy cost and unnecessary shrink can be substantial. Shrinking corn by 1 percentage point will cost about 4 cents per bushel in weight loss and about 0.5 cents per bushel in energy cost. For soybeans, the cost would be about 10 cents per bushel in weight loss and 0.5 cents per bushel in energy cost.
If corn and soybeans remain in good condition and the grain mass has been cored, aeration fans should be sealed before spring weather arrives. Operating roof exhausters independent of the aeration fans at sundown long enough to remove warm temperature and high humidity air from the headspace will prevent condensation under the roof and on the grain surface. Most surface crusting occurs during that period of the year.
Managing headspace conditions was the subject of a previous article (World Grain, April 2017, pages 68-73). Cold, dry grain will stay cold well into the summer as long as aeration fans remain sealed.
Monitoring temperature, moisture content and CO2
The key to successfully managing stored grain long term is effective and consistent temperature, moisture content and carbon dioxide (CO2) monitoring. Thermocouple-based temperature cables have been available for decades and should be used widely. If 25% or more of the thermocouples no longer give reliable readings, it is time to replace them as they cannot be repaired. Newer generation thermistor-based temperature cables are digital and can incorporate relative humidity sensors that allow for moisture content determination.
At a minimum, the cable in the center of the grain mass should include the moisture content measurement option as self-heating most often occurs there, especially if the grain mass was left peaked and fines remain concentrated below the fill point. An increase in stored grain temperature of 3°F (1.5°C) degrees in two weeks, if fans have not been operated during that period, indicates self-heating within close proximity of the temperature sensor.
Spoiling grain gives off carbon dioxide that can be detected using inexpensive handheld CO2 monitors either in the headspace of the storage structure or in the exhaust air of a downdraft aeration fan. Some commercially available digital temperature and moisture sensing systems include CO2 sensors in the headspace. An increase in CO2 over the baseline of 400 to 600 ppm is a more sensitive and much earlier indicator of the onset of spoilage than temperature. Monitoring CO2 in stored grain was the subject of a previous article (World Grain, July 2016, pages 64-69). A delay of three to five weeks between detecting higher CO2 readings and higher temperature readings is typical. This gives the stored grain manager sufficient time to consider options such as aeration to move a cooling front through the grain mass and determining whether temperature sensors pick up heat from spoiled grain before cooling off.
Another option is unloading some grain and determining whether more coring of the grain mass reveals any breaking up of surface crusting or problems with grain flowability.
Allowable storage time
The table on page 90 shows the allowable storage time (AST) of corn and soybeans in months as a function of temperature and moisture content. For successful long-term storage, AST needs to be understood and respected, especially when initial grain quality is lower than normal.
Shelf-life is progressively used from the time grain is harvested through its end use. If wet corn is held before drying, a major portion of the storage time can be consumed, leaving less for the summer months. For example, corn stored at 22% moisture and 60°F (15°C) for four days has used up a quarter of its AST. If it is dried to 14% and cooled to 40°F (5°C), the remaining AST is 112 months (3/4 x 150 months).
In a year with high carryover and a large harvest, long-term storage is likely and thus allowable storage time needs to be preserved; it cannot be recovered.