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In Arkansas, grain sorghum does not typically field-dry to a safe moisture content
(MC) to avoid quality losses (12% wet basis). Therefore, it must typically undergo
artificial drying to achieve the target MC for safe storage. Grain sorghum has its
highest quality at harvest and subsequent post-harvest handling and processing must
be carefully planned and managed to avoid a decrease in quality and market value,
and provide Arkansas producers with a measure of control over the quality and market
value of their grain sorghum.
Grain sorghum is dried by forcing large quantities of unheated or heated air though
the grain bulk. Air temperature and relative humidity (RH) are the two key characteristics
that determine the drying rate and the final MC of the grain. For any specific air
temperature and relative humidity, grain sorghum will only dry until equilibrium is
reached between the moisture in the grain and the moisture in the air. The moisture
content of the grain at this equilibrium point with air at a specific temperature
and relative humidity is called "Equilibrium Moisture Content” (EMC).
Table 1 shows EMC of grain sorghum in equilibrium with air at various temperatures
and relative humidity levels. As an illustrative example, if the grain sorghum is
dried with air at a temperature of 60oF and RH of 70%, then EMC is 14.1%. Therefore if the grain sorghum is placed into
the drying bin at a MC of 18%, and it is dried with air at 60oF and 70% RH, the grain sorghum will dry until it reaches a MC of 14.1% and will remain
at this MC as long as the air stays at a temperature of 60oF and RH of 70%. Note that if the air is heated, not only will the temperature increase,
but the RH will decrease. From Table 1, if the air is heated, EMC will decrease. Thus,
temperature and relative humidity properties of the drying air determine the sorghum
moisture level during drying and storage. The EMC may be determined by measuring air
temperature and relative humidity and using Table 1.
A sling psychrometer is one of the tools used for measuring relative humidity, and
is relatively inexpensive. A sling psychrometer works by measuring the air temperature
with a wet and dry bulb thermometer, and then using a table to determine relative
humidity. EMC of drying air also affects the rate that the grain sorghum is dried.
The difference between the EMC of the air and the actual MC of the grain is proportional
to the drying rate. Therefore, wet grain sorghum at 18% MC from the prior example
exposed to 60 °F, 70% RH air with an EMC of 14.1% will dry more slowly than if the grain sorghum
was exposed to 70 °F, 50% RH air with an EMC of 11.4%. Accordingly, grain sorghum may be dried without
adding any heat to the drying air if the EMC is sufficiently low compared to grain
Careful monitoring of the EMC of the drying air and management of air flow rates and
drying operations is necessary to take advantage of weather periods when EMC is low
will provide the most economical drying. In many cases, particularly at night and
during high humidity weather patterns, the addition of heat is needed to lower the
EMC of the drying air. To determine the value of EMC in your area now, click the link
Determination of the current temperature and relative humidity based on your zip code.
Table 1: EMC of grain sorghum in equilibrium with air at various temperatures and
relative humidity levels
Relative Humidity (%)
To learn more about Equilibrium Moisture Content, see the fact sheet titled: "Grain Drying Tools: Equilibrium Moisture Content Tables and Psychrometric Charts"
To determine the values of EMC for various grains, download the Excel sheet by clicking
the link: "Equilibrium Moisture Content"
One way to cause the grain to reach equilibrium with the air sooner and therefore
dry faster is to increase the flow rate of air over the grain. For most typical in-bin
grain drying systems, drying time is cut in half if the airflow rate is doubled.The airflow rate needed to dry grain sorghum depends on the initial moisture content
of the grain. Airflow rates for drying vary from 0.5 cfm/bushel to more than 50 cfm/bushel
for commercial or batch dryers. Most on-farm airflow rates for drying vary from 0.5
to 6 cfm/bushel dependent on the initial moisture content of the grain and the amount
of heat added to the drying air. Table 2 shows the minimum recommended airflow rate
to dry sorghum at various levels of initial grain moisture content.
Table 2: Recommended Minimum Airflow Rates for Drying Grain Sorghum
Initial Moisture Content
(cfm per bushel)
11% to 13%
13% to 15%
15% to 18%
18% to 20%
20% to 22%
To select and maintain your fan, please see the fact sheet titled "Selection, Performance and Maintenance of Grain Bin Fans"
Sorghum grains contain moisture and dry matter. The actual value of the sorghum is
contained in the dry matter so adding or removing moisture alone should not affect
the price. This is why a base moisture content of 14.0% wet basis is typically used
to calculate price for sorghum at the elevator. Variations of sorghum moisture content
change its weight per bushel as shown in Table 3 and this is used to determine dockage
for overly wet grain.
Table 3: Sorghum Weights corresponding to One Bushel at Market Standard Moisture Content
Moisture Content (%)
The above conversion contains the weight of sorghum at various moisture levels. Here
is an example to illustrate how these values were calculated.
How many pounds of 18.0% moisture sorghum is equivalent to a standard bushel?
The percentage of dry matter in a standard bushel (14.0% MC) = 100.0-14.0 = 86.0%.
The percentage of dry matter in one bushel of sorghum at 18.0% MC = 100.0-18.0 = 82.0%.
The shrinkage weight corresponding to one bushel at 18.0% MC = (86.0 ÷ 82.0) × 100
The shrinkage weight corresponding to one bushel at 18.0% MC = 104.87 × (55.00 lb)
÷ 100 = 57.68 pounds equivalent to a standard bushel.
Natural air-drying and low temperature drying typically occur in bins with one or
more fans attached to the bin. The fans may also be attached to a heater for low temperature
drying. Natural air-drying does not utilize a heater. Natural air or slightly heated
air (<10°F) is forced up through the grain with fans for a sufficient time to reduce
grain moisture content to safe storage levels. This is typically done in bins with
a perforated floor to ensure even airflow, but can also be done using air ducts laid
on the bin floor prior to adding grain. Grain sorghum offers higher resistance to
airflow than corn or soybeans. Thus, bin-drying methods used for other grains can
be adapted to grain sorghum if some adjustments are made to compensate for the high
resistance to airflow. The simplest way is to reduce grain sorghum depth in the bin
to half that is used for corn or soybean. In addition, the installation of centrifugal
fans may help to deliver higher airflow rates under higher static pressures.
Bin capacity, as measured in bushels of grain, increases by increasing the bin diameter
and/or the grain depth as shown in Table 4. A grain bin with 28 ft diameter and 16
ft height can hold up to 7,882 bushels of grain sorghum. Increasing the grain depth
increases the static pressure that the fan must overcome to provide the required airflow
rate. See Figure 6 for a graphical representation of the relationship between fan
size, airflow rate, and static pressure requirement.
The following is an illustrative example of how to use the information presented to
this point for estimating the fan requirements for drying grain sorghum. A 30-foot
diameter bin is filled to a depth of 8 feet with grain sorghum at a moisture content
of 18% wet basis that must be dried. Table 2 recommends a minimum airflow rate of
3 cfm/bu for 18% MC grain sorghum. Table 5 shows that the capacity of the bin at this
depth is 4,524 bushels. Therefore, the total airflow rate required from the fans in
cfm is 3 cfm/bu * 4,524 bu = 13,572 cfm. Table 4 shows that the static pressure requirement
for 8 ft. depth at 3 cfm/bu = 5.11 in water. Figure 5 shows that 1-15 hp fan will
provide about 15,000 cfm to the grain sorghum, which exceeds the 3 cfm/bu (13,572
cfm) requirement. Therefore, 1-15 hp fan will provide the proper airflow required
to dry grain sorghum with an initial moisture content of 18% wb in the 30 ft. bin
filled to a depth of 8 ft. It should be mentioned that for a 30-ft or 32-ft bin, grain
sorghum depths greater than 20 feet would generally reduce airflow rates to less than
1 cfm /bu even with the use of 3-15 fans. Accordingly, Table 6 could be used to simplify
the selection of grain depth as a function of grain sorghum MC and fan size.
If high moisture grain sorghum is to be dried and stored in the same bin, extra care
is advised. Stirring devices, re-circulators, or automatic unloading augers can be
used to increase the drying rate. For high moisture content grain sorghum, it is recommended
that after drying grain sorghum to 17%, use unheated air to dry it to about 15%. During
this period, run the fan continuously to maintain uniform moisture content. Once the
grain sorghum has dried to 15%, run the fans only during low humidity hours to finish
drying to around 13%. This management scheme will minimize the amount of grain sorghum
overdrying in the bottom of the bin. Never use air heated above recommended temperatures
as this can cause severe overdrying resulting not only in wasted energy that is uncompensated
at the elevator buying point, but also damage quality of the grain sorghum.
Table 4: Number of Grain Sorghum Bushels in Grain Bins
Grain depth (ft)
Bin Diameter (ft)
Figure 5: Graphical representation of the relationship between fan size, airflow rate,
and static pressure requirement. Single and multiples fans static pressure and airflow
Table 6: Maximum Safe Drying Depth for Grain Sorghum with Typical Bin and Fan Combinations.
Moisture content of grain placed in the bin
Safe Depth of grain – ft
High temperature batch or continuous flow dryers are usually used to dry large capacities
of grain sorghum. These units typically have very high airflow rates, and they do
not require supplemental heat for daytime drying when harvesting grain sorghum at
18-20% moisture range. If heat is used, the drying air temperature can be limited
by cycling the burner on and off or by changing the gas burner orifice size using
an automatic modulating valve.
Example of on-farm grain drying and storage facility
Pass or continuous flow dryers are often utilized to speed up the drying process.
These high flow driers pass very large amounts of high temperature air over the grain
sorghum. Three to six moisture percentage points may be removed from the grain in
a single pass. The grain exiting these types of dryers must be cooled to near ambient
temperature prior to storage or transport. Continuous flow drying helps to rapidly
prepare grain for shipment to market if needed. Quickly drying the grain down to values
of 16% or less will lessen potential spread of toxins for affected grain.
Grain sorghum growers who use on-farm grain sorghum drying and storage list numerous
advantages of these technologies. On-farm drying and storage costs may be less than
the drying price reduction at a commercial dryer, particularly if some other crop
may justify the capital cost of their on-farm equipment. For comparison to reductions
at a commercial dryer, on-farm energy costs for electricity and liquid propane range
from 3 to 4 cents per bushel per point of moisture removed. In addition, on-farm drying
may make it economically feasible to harvest grain sorghum early, at moisture content
higher than the dockage level at a commercial dryer. This provides the grower with
more flexibility for managing their business. Additionally, early harvest and proper
drying can produce better quality grain sorghum compared to grain sorghum that is
re-wetted in the field several times while field drying to a dockage level.
Following are some tips that may help grain sorghum producers achieve better grain
quality while minimizing the drying cost:
Never add more heat than necessary to adjust the humidity of the drying air below
55%. The maximum heat needed, even in rain or 100% humidity will be about 15-17oF above the outdoor temperature. Adding too much heat can over-dry the bottom layers
of grain to as low as 8% and that reduce the market weight. Recommendations for heated
air temperature rise are to: add 15 degrees F if the outside air humidity ranges between
85 to 100%; add 10 degrees F for 75 to 85% humidity and 5 degrees F for 55 to 75%
humidity. If the humidity is 55% or less, use natural air for drying.
During long-term storage, grain sorghum quality is affected by several factors including
moisture content, temperature, and insect infestation, which if not properly controlled
can shorten the safe storage period. Grain sorghum shares these factors with all other
grains. Sorghum cleanliness, however, is another factor that might affect air movement
within the grain bin during storage and reduce the risk of spoilage. The following
are some tips that may help sorghum producers to achieve better grain quality during
It is essential to maintain the on-farm grain sorghum drying cost at a minimum in
order to maximize the return on investment. To estimate drying costs for producers
interested in drying their grain sorghum need to first determine the total pounds
of water they will remove from one bushel of grain. The number of BTUs to extract
1 pound of water will vary from 1,100 to 1,400 BTU/pound depending on how easily moisture
is given up by the kernel. A good estimate is to use an average of 1,200 BTU/pound
of water removed to calculate the energy needed to get rid of 1 pound of moisture.
Table 7 summarizes the BTU/unit of fuel as well as the burning efficiencies. Burning
efficiencies estimate the conversion of raw gas use to energy. Therefore, if one gallon
of LP gas is consumed by the heater with a burning efficiency of 80%, then the amount
of heat added to the air is 0.8*92,000 = 73,600 BTUs.
Table 7: Heating value of fuel as well as their corresponding efficiencies
Total grain sorghum drying costs may be estimated using the following equation(s):
Fan motor cost:
Fan motor cost ($/h) = fan HP x 0.7475 (kW/HP) x electricity cost ($/kWh)
Fuel cost ($/bu) = [BTU/lb water x (lb water removed/bu) x fuel cost ($/unit of fuel) x 100] /
[(BTU/unit of fuel x burning efficiency %]
Grain sorghum drying cost ($/bu) = fan cost ($/h) x drying time (h/bu) + fuel cost
Working with either moving and/or stationary grain requires special precautions and
training. Therefore, it is necessary to keep in mind the following tips while working
with grain sorghum as well as other grain: