After earlier concerns about a potential late harvest and higher-than-normal moisture contents, the harvest of corn (maize) and soybeans in 2020 in Iowa and across the midwestern US was completed rather quickly and resulted in better grain quality than expected. In Central Iowa, many farmers and commercial grain handlers never turned on their grain dryers because corn came off the fields well below the market moisture content of 15%. Similarly, soybeans were generally lower than the market moisture content of 13%. Thus, crops should store well into the new marketing year.
The reason for this change in harvest conditions was warm, dry weather accompanied by steady winds that caused substantial drying in the field. Field drying is similar to natural (or ambient) air drying in silos (or flat storage structures) except that corn is on the ear and soybeans are in the pods. Knowing the air-grain moisture relationship for specific conditions will give an indicator as to the moisture content grain will equilibrate toward over time when exposed to certain air temperature and relative humidity conditions whether in a field or in a silo. For example, ear corn will equilibrate to 14.3% moisture content when air temperature is 25°C (77°F) and relative humidity is 70%. When relative humidity reduces to 50%, ear corn will equilibrate to 11.6%. Under the same conditions, soybeans will equilibrate to 13.3% and 8.6%, respectively.
In the past, these air-grain moisture relationships had to be looked up in charts, handbooks and extension publications. More recently, one can find them on the internet with the right combination of search terms. Neither is as convenient as having them in an app on your smart phone. Now such an app adding intelligence for stored grain management at your fingertips is finally available.
Overview of the Grain Aeration and Storage App
The app is currently only available for the Android platform and may be found in the Google Play Store with the search term “Grain Aeration & Storage.” It will display with the following symbol: The programmer name is Maximiliano Batelli. The most current version as of this writing is 1.4 updated on Oct. 15.
This app is intended to serve as an engineering tool for grain storage and aeration processes. It was jointly developed by the co-authors of this article representing the Grain Postharvest Group located at the Experimental Research Station of INTA Balcarce, Argentina, and the Post-Harvest Engineering and Feed Technology Group of the Department of Agricultural and Biosystems Engineering and the Iowa Grain Quality Initiative at Iowa State University.
The base settings of the app allow the user to choose among four interface languages (English, Spanish, Portuguese, French), metric or imperial units, and whether to use the smartphone’s GPS function to find a weather station within a certain distance of the current location. When the GPS function is turned off, the user has the flexibility to enter any geographic location in the world and use the app tools for grain stored based on that location’s weather forecast. The app has the option to select among five common weather forecast platforms to give the user flexibility in case one is preferred over another, or perhaps one is unavailable at a particular geographic location in the world.
The app allows the user to choose from among 26 different grain types, including cereals (barley, oats, wheat), coarse grains (corn/maize, popcorn, sorghum), oilseeds (soybeans, sunflower, rapeseed, safflower), expelled soybeans, edible beans, rice, and shelled peanuts. For each grain type, the user can view the model equation and grain-specific parameters as well as the scientific reference source.
The app consists of three tools that are described in detail in the following sections. Additionally, a related podcast is available at https://www.extension.iastate.edu/grain/grain-aeration-storage-app-released
Air-grain moisture relationships
This tool calculates the Equilibrium Moisture Content (EMC), Equilibrium Relative Humidity (ERH), and recommended Safe Storage Moisture Content (SSMC) for different grain types and air conditions. It indicates whether certain air conditions (ambient, heated, refrigerated/chilled) are suitable to achieve a stored grain manager’s goal for lowering, maintaining, or increasing moisture content.
The above example for ear corn and soybeans exposed to the harvest weather conditions this fall in Central Iowa were determined with this tool (Screenshot 2, page 60). In a more typical harvest year, when air temperature averages 15°C (60°F) and relative humidity 50% or 70%, ear corn would equilibrate in the field to 12.4% or 15.1%, respectively, and soybeans to 9% or 13.7%, respectively. The effect of a lower temperature at the same relative humidity indicates a 0.8 points higher moisture content for ear corn, but only about half that (i.e., 0.3 to 0.4 points) higher for soybeans. This example points to the difference in the equilibrium moisture relationship among grains, especially with respect to those with higher starch content compared to those with higher oil content. The effect of weather influences harvest conditions in a similar manner as aerated storage conditions.
Another feature of this tool is the recommended safe storage moisture content (SSMC), defined as the moisture content at which grain should be stored to prevent mold development. The trigger for mold development is relative humidity of air within the stored grain mass higher than about 65% to 68%. For soybeans, the recommended SSMC at 15°C (60°F) is 12.8% compared to 12.4% at 25°C (77°F). For shelled corn, the recommended SSMC values are 13.8% and 13% at 15°C (60°F) and 25°C (77°F), respectively. These are similar differences in percentage points of corn versus soybeans over this temperature range as observed for the EMC values. Oil content has a substantial effect on SSMC. For instance, the SSMC of sunflower seeds with 44% oil content at 15°C (60°F) is 9.8%, but for 53% oil content it is 7.8%. This shows that for the safe storage at these temperatures, sunflower seeds and soybeans need to be stored at a lower moisture content than corn, and the SSMC is lower the higher the oil content and stored grain temperature.
Cooling aeration prediction
This tool estimates the temperature and moisture content grain will cool (or warm) and shrink (or increase) to for different ambient air conditions. It estimates the temperature and moisture content at which grain will equilibrate when aerating with certain air conditions while accounting for the evaporative cooling effect based on the wet bulb temperature within the grain mass.
The app interface allows the user to enter the geographic location (or allow the smartphone GPS to determine the current location) and to choose the grain type. In this example, we allowed the GPS to confirm our location in Ames, Iowa, US, and assumed low moisture content shelled corn at 12.5% from this harvest placed in a storage silo that is aerated with a typical airflow rate of 0.11 m3/min/t (0.1 CFM/bu). Our target moisture content is 15%, indicating that we would rather see the corn increase in moisture during storage than shrink it further. Based on a current grain temperature of 15°C (60°F), what would the grain cool (or warm) to and what grain moisture content would the corn equilibrate toward if we turned on the aeration fan and had a consistent air temperature of 7.2°C (45°F) and 80% relative humidity?
The tool predicts corn would cool within 150 hours of constant aeration to 10°C (50°F) and cause the corn to equilibrate toward 17.4% moisture content. That level of potential rewetting results in a red caution flag by the app warning the user that this is not a recommended practice. The yellow caution flags indicate that air conditions are poor for achieving the target moisture content of 15% primarily because of the high air relative humidity of 80%, which will expose the bottom layers of grain to conditions above 65% to 68% relative humidity that could trigger mold development. Both moisture contents are above the recommended SSMC of 14.3% as corn cools from 15°C (60°F) to 10°C (50°F) along the wet bulb temperature.
It should be noted that during the evaporative cooling process the heat energy contained within the grain causes moisture loss of about 0.25 percentage points for every 5.5°C (10°F) of cooling. Once the grain has cooled, additional moisture loss due to air equilibrium conditions takes much longer than a temperature change. In the above case where the air equilibrium conditions would be driving moisture into the corn while it is being cooled, 150 hours would not suffice to increase the entire grain mass to 17.4% or even 15%. Practically speaking, grain moisture content may increase only slightly (0.2 to 0.3 percentage points) over this period, which would prevent shrink loss and “pay for” the electricity to run the aeration fans. Nevertheless, the concern regarding the potential to trigger mold development in the bottom grain layers remains.
We can also apply the tool to evaluate aeration of US corn exported at 14% to 14.5% moisture content under tropical conditions in a location such as Veracruz, Mexico. However, we need to first understand the typical local weather conditions. Figure 1 summarizes the monthly temperature and relative humidity values averaged over five years, and Figure 2 represents the corresponding wet bulb temperatures and EMC values for corn and soybeans determined using this app tool.
A key conclusion is that the narrow range of relative humidity values of 75% to 81% throughout the year is above the safe storage moisture content in equilibrium with 65% to 68% relative humidity in the stored grain mass. Therefore, a high potential for activating mold spores exists if this imported corn were aerated because it would be warmed to an average of 26.6°C (80°F) and increase to 14.8% equilibrium moisture content. This amount of warming and moisture increase due to aeration would be costly and would substantially decrease the storability of the imported corn. The recommended best practice is that unless this corn is utilized within three weeks of receipt, it should be stored at 13% to 13.5% moisture content and as cool as possible to prevent onset of spoilage and insect development, which are slowed substantially below 20°C (68°F) and an equilibrium relative humidity of 65% to 68% within the stored grain mass.
Aeration weather forecast
This tool will forecast the suitability and hours of aeration based on a 6- to 10-day weather forecast for a specific geographic location. It provides an estimate of the number of suitable fan run hours based on historic average and minimum temperatures for the location and a percentage amount of fan run time the user can enter. The user can also allow the app to enter the average and minimum temperatures based on the geographic location and weather. The output also includes a set point recommendation for a thermostat that could turn the aeration fans on and off automatically for the forecast period. The fans could also be turned on and off manually, or via an IoT (Internet of Things) switch activated from the user’s smartphone.
When we used this app tool for Des Moines weather conditions during the recent harvest period and entered for mid-October the five-year monthly average of 15°C (60°F) and 7.2°C (45°F) for the average minimum temperature, the forecast for a 50% fan run time predicted 129 hours of excellent (green) aeration weather over the following six days. This was based on a thermostat setting of 15°C (60°F), which the program sets at that percentage of fan run time based on the monthly average temperature entered. For the typical airflow rate of 0.11 m3/min/t (0.1 CFM/bushel) for a fan (or fans) attached to a storage silo (or flat storage structure), it would take a minimum of 150 hours to move a cooling front through the grain mass provided it has been cored and un-peaked. Thus, an extra day beyond the six in this forecast would be required to complete the aeration cooling goal. The user can update the forecast each day and adjust operational strategy according to the newest six-day forecast.
The app allows the user to click on the days that do not yield 24 hours of fan run time. It shows certain hours during the middle of the day when ambient temperatures are predicted to be high, and so the user could shut off the fan to avoid blowing warm air back into the bottom grain layers. A few hours later, the user could tell their smartphone, “Alexa, turn on the fan for Silo 1.”
If a user wanted to take advantage of a lower thermostat setting, a 30% fan run time percentage would utilize the average between the minimum and average temperatures entered, and a 1% fan runtime would utilize the minimum temperature as the set point. These adjustments in settings can be utilized by stored grain managers for a second and third aeration cooling cycle, which is typically needed to prepare grain for winter storage. During the first cooling cycle during the fall harvest period, the 50% fan runtime setting will reduce grain temperature aggressively. After the initial cool down is achieved, the user can reduce the fan runtime to 25% to 30% to initiate the second aeration cycle. This usually occurs in the late fall/early winter during which the average aeration air conditions are substantially lower and achieved less aggressively. An eventual third cycle would be initiated with 15% to 20% fan runtime to restrict fan operating hours during of the cold winter weather.
Conclusion
Key to preserving quality of stored grain is understanding the local weather conditions and its relationship to the temperature and moisture content of the corn, soybeans or other grains to be stored and maintained. Specifically, ambient temperature and relative humidity determine the cooling potential of ambient air, estimated based on the wet bulb temperature, and whether ambient air will increase or decrease moisture content of the grain, estimated based on the equilibrium moisture content. For the first time, the newly available Grain Aeration and Storage App combines these engineering equations with local weather forecasting and puts this intelligence at the user’s fingertips for any grain type and global location — and best of all, it is free of charge.
To view Figure 1 and 2 referenced in this story please visit the November digital issue.
