Ken Hellevang, Ph.D., PE, Extension Engineer, Professor
NDSU Extension Service, Ag & Bio systems Engineering Department
Progression to maturity is very slow in October, even if there is not a killing frost. The average number of growing degree-days (GDDs) during the first two weeks of October is about 100. In 2008 there were about 70 to 80 GDDs during the first two weeks of October. If the corn has just reached full dent on October 1, a small reduction in test weight and yield might be expected for the corn this year.
The following table summarizes information from various sources on corn development.
Stage |
Days to Maturity |
GDDs Until Mature |
Yield Loss (%) |
Test Weight (lbs/bu.) |
Grain Moisture Content (%) |
Dough |
30-40 |
600-700 |
50-60 |
35-40 |
60-80 |
Early Dent |
20-25 |
450-550 |
25-40 |
45-50 |
50-55 |
Full Dent |
12-17 |
250-400 |
10-20 |
50-53 |
35-45 |
Half Milk Line |
10-15 |
150-300 |
5-10 |
54-55 |
35-40 |
Mature |
0 |
0 |
0 |
56 |
25-35 |
*GDD needed varies with corn RM and hybrid
The amount of drying in the field depends on parameters such as corn maturity, hybrid, and moisture content, air temperature, and relative humidity, solar radiation, and wind speed. The moisture content to which corn will dry is determined by the corn's equilibrium moisture content(EMC), which is based on air temperature and relative humidity. The rate of drying has been estimated based on the corn growing degree-days. Another predictor of the drying rate is potential evapotranspiration( PET), which is based on the parameters similar to those that affect drying. Values for PET calculated by the North Dakota Agricultural Weather Network for 2008 are shown in the following table. There appears to be a good correlation between PET and field drying experience. The table shows these EMC, GDD, and PET values, as well as the estimated percentage points of field drying for each month.
"Estimated" Corn Field Drying
Month |
EMC (%) |
GDD |
PET (in) |
Est. Drying (%pt) |
Month |
Week |
Sep |
15 |
250-350 |
4.0-5.0 |
18 |
4.5 |
Oct |
16 |
100-125 |
2.8-3.5 |
11-12 |
2.5 |
Nov |
19 |
20-30 |
0.8-1.2 |
4-5 |
1 |
Dec |
20 |
0 |
0.5-0.8 |
2 |
0.5 |
Jan |
21 |
0 |
0.5-0.8 |
2 |
0.5 |
Feb |
21 |
0 |
0.5-0.9 |
3 |
0.8 |
Mar |
19 |
0 |
1.3-1.6 |
5 |
1 |
Apr |
16 |
50-90 |
3.2-4.5 |
16 |
4 |
May |
14 |
200-300 |
6.5-8.5 |
30 |
7 |
PET=Potential Evapotranspiration, NDAWN, Weather, Total PET, Estimate: 1-inch = 4percent drying
EMC-equilibrium moisture content, GDD-growing degree-days, percentage pt. - percentage point of moisture reduction, for example from 20 percent to 15percent are 5percentage pts.
Standing corn in the field may dry about 0.3 to 0.4 percentage points per day during October and 0.15 to 0.2 per day or less during November. Corn at 35 percent moisture content on October 1 might be expected to dry to about 24 percent by November 1 and about 20 percent by December 1. Therefore, corn moisture content at harvest will likely be in the mid-20 percent range again this year.
Immature corn dries more slowly in the field than mature corn. Frosted high-moisture corn can mold on the stalk.
Field drying is normally more economical until mid to late October, and mechanical high-temperature drying is normally more economical after that point.
Field drying is extremely slow during winter months. Corn will only dry to about 20 percent to 21 percent moisture content based on the equilibrium moisture content for average monthly air temperature and relative humidity. Corn in the field over winter in 2008-2009 dried from 25 percent to 30 percent moisture in November to 17 percent to 20 percent when harvested in February and early March. Corn losses were generally small if the corn stalk was strong in November. Accumulated snow and cover from the corn resulted in wet fields in the spring.
Post harvest Management of High-Moisture Corn
Corn at moisture contents exceeding approximately 23 percent should not be stored in a grain bin because the kernels may freeze together and may deform and bind together. The corn may not flow from the bin for unloading. Corn above this moisture content should be placed so it can be unloaded with a front-end loader or something else that can mechanically dislodge the corn.
It is critical to provide aeration to keep the corn cool. Wet corn will deteriorate rapidly unless kept cool. Corn will deteriorate even with airflow, but without airflow through the corn, storage temperatures will increase, resulting in rapid deterioration. Condensation and icing occurs on bin vents at temperatures near or below freezing, so it is best to leave bin covers open to serve as a safety opening when operating fans near or below freezing temperature. There were numerous reports last year of bin vents freezing over and the fan pushing the roof up and damaging the bin roof.
"Approximate" Allowable Storage Time (Days) for Cereal Grains
|
Temperature (°F) |
MC (%) |
30° |
40° |
50° |
60° |
70° |
80° |
14 |
* |
* |
* |
* |
200 |
140 |
15 |
* |
* |
* |
240 |
125 |
701 |
16 |
* |
* |
230 |
120 |
70 |
40 |
17 |
* |
280 |
130 |
75 |
45 |
20 |
18 |
* |
200 |
90 |
50 |
30 |
15 |
19 |
* |
140 |
70 |
35 |
20 |
10 |
20 |
* |
90 |
50 |
25 |
14 |
7 |
22 |
190 |
60 |
30 |
15 |
8 |
3 |
24 |
130 |
40 |
15 |
10 |
6 |
2 |
26 |
90 |
35 |
12 |
8 |
5 |
2 |
28 |
70 |
30 |
10 |
7 |
4 |
2 |
30 |
60 |
25 |
5 |
5 |
3 |
1 |
This 0.5 percent maximum dry matter loss composite is calculated based on USDA research at Iowa State University; Transactions of ASAE 3330337, 1972; and "Unheated Air Drying," Manitoba Agriculture Agdex 732-1, rev. 1986.
* Approximate allowable storage time exceeds 300 days.
High-Moisture Corn Storage be Ensiling
Shelled corn should be at 25 to 30 percent moisture for anaerobic (without oxygen) high-moisture storage in silos or silo bags. Any tears in the plastic bag must be promptly repaired to minimize storage losses. Whole shelled corn can be stored in oxygen-limiting silos, but a medium grind is needed for proper packing in horizontal or conventional upright silos. Wet grain exerts more pressure on the silo than corn silage, so conventional concrete stave silos may require additional hoops, or the silo must not be completely filled. Corn at moisture contents below 25 percent will not ensile, so it will need to be dried for storage. If oxygen is not adequately removed as the corn ensiles, heating and severe deterioration will occur.
Natural Air and Low Temperature Corn Drying
To minimize corn spoilage during drying, corn above 21 percent moisture should not be dried using natural air and low temperature drying. An airflow rate of 1.0 to 1.25 cfm/bu is recommended to reduce drying time. Because the drying capacity is extremely poor at temperatures below 35 to 40 degrees, natural air system drying is not effective during the fall months. Instead, cool the corn to 20 to 25 degrees for winter storage, and start drying in early April. Adding heat does not permit drying wetter corn and only slightly increases drying speed. The primary effect of adding heat is to reduce the corn moisture content. Natural air drying in the spring is the most energy- and cost-effective method of drying. Corn depth should be limited to about 20 to 22 feet to obtain the desired airflow rate for drying. Turn fans off during extended rain, fog, or snow to minimize the amount of moisture moved into the bin by the fan.
High-Temperature Drying
Using the maximum drying temperature that will not damage the corn increases the dryer capacity and can reduce energy consumption. The amount of energy required to remove a pound of water is about 20 percent less using a drying air temperature of 200 degrees than at 150 degrees. Be aware that high drying temperatures may result in a lower final test weight and increased breakage susceptibility. In addition, as the drying time increases with high-moisture corn, it becomes more susceptible to browning. During 2008, dryer temperatures needed to be reduced below 200 degrees to minimize the corn kernel damage.
Housekeeping during drying was critical during 2008 because of condensation that occurred on the dryer, creating a wet surface for debris to accumulate. The debris sometimes reduced airflow through the dryer, which in turn reduced drying capacity and created a fire hazard.
In-Storage Cooling
Use in-storage cooling instead of in-dryer cooling to boost capacity of high-temperature dryers. Cooling corn slowly in a bin rather than in the high-temperature dryer will also reduce the potential for stress cracks in the kernels.
In-storage cooling requires a positive-pressure, airflow rate of about 0.20 cfm/bu or 12 cfm/bu-hr of fill rate. Cooling should be started immediately when corn is placed in the bin from the dryer. Dryer capacity is increased 20 to 40 percent , and about one percentage point of moisture is removed during corn cooling. Condensation problems can be reduced by cooling the corn in the dryer to about 90 degrees before placing it in storage.
Dryeration
Dryeration will increase the dryer capacity about 50 to 75 percent, reduce energy used by about 25 percent, and remove about 2 to 2.5 points of moisture. (0.25 percent for each 10 degrees the corn is cooled.) With dryeration, hot corn from the dryer is placed in a dryeration bin with a perforated floor, allowed to steep four to six hours without airflow, cooled, and then moved to a storage bin. There will be a tremendous amount of condensation during the steeping and cooling process, so the corn must be moved to a different bin for storage, or spoilage will occur along the bin wall and on the top grain surface.
Dryer Energy Efficiency
Dryers can capture heat from when cooling the dry corn. A portion of the air from the final drying can reduce the energy used to dry the corn by about 20percent or more depending on outdoor temperature. Newer dryers typically have incorporated features to make them more energy efficient than older models.
Estimating High Temperature Drying Cost
Propane costs for high-temperature drying corn can be estimated using the following formula.:cost/bu. - pt. = 0.022 x propane price/gal. For example, the drying cost is $0.022/ bu.-pt. if the cost of propane is $1.00, 0.022 x $1.00. It will cost about $26.00 for propane to remove 10 percentage points of moisture from 120 bushels of corn using $1.00 propane.
Estimating Drying Fuel Requirement
The estimated quantity of propane needed to dry is 0.02 gallons per bushel per point of moisture removed. For example, 24 gallons of propane is needed to dry 120 bushels of corn from 25 percent to 15 percent . (0.02 x 120 bu. x 10 pts.) This is based on 0.72 pounds of water being removed per point of moisture per bushel, 2,500 Btu of heat required to remove a pound of water in a high-temperature dryer and a propane heat content of 91,500 Btu/gallon.
Moisture Shrink
Moisture shrink is the reduction in weight as the grain is dried one percentage point. The moisture shrink factor equals 100 ? (100 - final moisture content). The shrink factor drying corn to 15.5 percent is 1.1834. The shrink drying corn from 20.5 to 15.5 would be 5 x 1.1834 = 5.92 percent.
Obtaining an Accurate Moisture Content Measurement
Moisture meters will not provide accurate readings on corn coming from a high-temperature dryer. The error will vary depending on the drying temperature and the amount of moisture removed , but the meter reading may be about 2 percent lower than true moisture. To find the true moisture, check the moisture of a sample, place the sample in a closed container for about 12 hours, and then check the moisture content again to determine the amount of error. Moisture meter errors increase as corn moisture contents increase, so readings above 25 percent should only be considered estimates.
In addition, moisture meters are affected by grain temperature. If the meter does not automatically measure the grain temperature and adjust the value, then it must be done manually. Even if the meter does it automatically, it is recommended to cool a sample in a sealed container to room temperature before measuring the moisture content. Then compare the moisture content of the room temperature sample to the initial sample to verify that the adjustment is done accurately.
Moisture meters are not normally accurate when grain temperatures are below about 40 degrees. Place the corn sample in a plastic bag or other sealed container, warm it to room temperature, and then measure the moisture content.
Corn Test Weight
Normally, corn test weight increases about 0.25 pound for each point of moisture removal during high-temperature drying. However, the increase in test weight is affected by the amount of mechanical damage during harvest and the gentleness of the drying. Due to the high mechanical damage involved with harvesting 25 to 30 percent moisture corn and high drying temperatures used in 2008, there was frequently no increase in test weight during drying. There will be little or no increase in test weight on immature or frost-damaged corn.
Corn Storage
More fines are produced when corn is wet, because more aggressive shelling is required. Aggressive shelling causes more kernel cracking and breaking. There is also more potential for stress cracks in kernels during drying, which leads to more breakage potential during handling. In addition, immature corn contains more small and shriveled kernels. Fines cause storage problems because they spoil faster than whole kernels, they have high airflow resistance, and they accumulate in high concentrations under the fill hole unless a spreader or distributor is used. Preferably, the corn should be screen-cleaned before binning to remove fine material, cob pieces, and broken kernels.
Because of their short storage lives, corn with damage to the seed coat and immature corn are not recommended for long-term storage.. Mature corn may be stored long-term under the right conditions. It should be cooled to about 20 to 25 degrees for winter storage and dried at a percentage point lower in moisture content. More frequent checking of the storage is also recommended.
Safety
Ice accumulation on fans leads to imbalance and vibration. Monitor fans periodically and and remove any existing ice accumulation. Bridging of corn in a bin transfers more of the load to the bin wall that may lead to bin failure. Follow recommended storage management to minimize the potential for crusting or bridging, and watch for the grain flow when unloading. Never enter a bin while unloading grain. Also, do not enter a bin to break up grain bridging. Two people lost their lives from 2008to 2009 due to grain entrapment, and several more narrowly escaped.
For more information, look up NDSU corn drying on the Internet.
Back to Top
