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10/20/2008 - 10/26/2008

Corn Quality Issues in 2008 - Moisture and Test Weight

By Charles Hurburgh, Department of Agricultural and Biosystems Engineering; Roger Elmore, Department of Agronomy

Weather this fall has benefited the corn crop by allowing late planted corn to mature. Much of the state recently experienced its first killing frost. The USDA October yield estimate of 172 bushels per acre in Iowa surprised many. If this is the final yield, 2008 will produce the third highest yield recorded, behind the 173 bushels per acre of 2005 and 181 bushels per acre in 2004.

Although high grain yields are expected, reports of quality issues are surfacing. A cool, long, growing season will often result in high yields with high grain moistures and low test weights. The lower test weight is the result of more starch and lower protein on a relative basis, a condition that also reduces field dry down rates and increases drying costs. 

Corn that has not dried early in the harvest period often stops at 17 to 18 percent. This may well be the case this year, as the number of favorable drying hours is much less after Oct. 20.  Moisture contents on that date were hovering around 20 percent, with some in upper teens and many up to 23 or 24 percent moisture.  At this time, with the cool wet near-term forecast, there will not be much additional field drying.

 Expect drying to cost about 5 cents per point of moisture removed. Eight points removed, down to 15 percent moisture, would cost about 40 cents per bushel plus the weight shrink. For this reason, there will be an incentive to hold corn at higher moistures, awaiting better drying conditions in the spring, blending opportunities, or higher moisture feeding.

Test weight
Test weight is expressed as pounds per volumetric bushel. Corn test weights can range from 45 to over 60 pounds per bushel.  The market standard is 54 pounds per bushel, the grade limit for No. 2 Yellow corn. Kernel size, shape, and density all affect test weight.

Higher test weights mean better filled kernels with a higher percentage of hard endosperm. Low test weights frequently imply that the crop did not mature entirely or that it was subjected to some stress conditions. In 2008, dry corn test weights of 52-54 pounds per bushel (compared with the more typical 55-57 pounds per bushel) are indicative of incomplete maturity.

Food processors are especially sensitive to lower test weight. Product yield and quality is reduced. Ethanol processors may not be greatly affected by the somewhat lower test weight this year; lower protein and higher starch yields more ethanol, but does reduce DDGS quality. Pay attention to the moisture limits of ethanol plants; most stop taking corn at either 17 or 18 percent.

Some researchers report that early-season hybrids grown in their area of adaptation have lower test weights than later-season hybrids grown in their area of adaptation. Early planting dates, higher N rates or less N loss, lower plant populations, and years with better growing conditions all contribute to higher test weights. Loss of N from extreme rainfall may be a contributing factor to lower test weight this year.

Corn storability issues
Test weight is a good indicator of corn storability. Corn that is below 54 pounds per bushel after drying should not be stored into warm weather and should be dried to less than 15 percent moisture before storage of any duration. Lighter corn also will break more in handling.  

Corn normally gains about 0.25 lb/Bu per percent of moisture removed, more with low temperature drying and less if corn temperatures exceed 150F. We are also learning that corn that has reached 150F or more in drying is more difficult to ferment in ethanol plants. A discount for test weight in wet corn can be simply an extra discount for moisture, depending on the actual values for moisture and test weight.



Charles Hurburgh is a professor of Agricultural and Biosystems. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production.

Corn Quality Issues in 2008 – Field Molds

By Charles Hurburgh, Department of Agricultural and Biosystems Engineering; Alison Robertson, Department of Plant Pathology; Roger Elmore, Department of Agronomy


Fall weather has allowed late planted corn to mature, increasing grain yields expectations - the USDA October yield estimate is 172 bushels per acre in Iowa. Although high grain yields are expected, reports of quality issues are surfacing.

The cool wet fall conditions also favor the development of fusarium fungi; the white or pink ear rots that are often found in ear corn stored too wet.  Field moistures in the low 20s over a long period are favorable for these fungi, which in turn can produce several toxins harmful to people and livestock – vomitoxin, zearalenone, and fumonisin.  Grain with field mold should be tested for mycotoxins before feeding.

Scouting and harvesting fields with disease problems
High moisture conditions favor growth of many ear and stalk rot fungi. Fields should be scouted as soon as possible to determine the extent of disease problems. To minimize losses due to ear rot and increased mycotoxin levels, it is recommended that producers harvest problem fields (greater than 10-15 percent incidence of ear rot) as soon as possible.

The longer the corn remains in the field, the greater the chance of toxin production. The toxins most likely to increase in the field at this time are those associated with fusarium ear rots - namely vomitoxin, zearalenone and fumonisin.  

Giberrella ear rot

Gibberella Ear Rot - a consistently important mycotoxigenic fungus in the northern corn belt, producing vomitoxin, zearalenone, and other toxins.


Diplodia ear rot

Diplodia Ear Rot - characterized by the appearance of raised black bumps on the moldy husk or kernels.


Diplodia ear rot is more prevalent this season than in previous years. Although mycotoxins are not usually associated with Diplodia ear rot, grain quality will decrease substantially if the corn is allowed to remain in the field, thus early harvest is also recommended in these fields. 

Adjust harvest equipment to minimize damage to kernels since mold and mycotoxin levels tend to be at greater levels in damaged kernels. Dry (to less than 15 percent moisture) and cool (to less than 45 degrees F) grain as quickly as possible to reduce further mold growth and toxin production.

Elevator operators, especially in Eastern Iowa report cases of visible mold damage levels 5 percent and higher. In normal years, overall mold damage levels are generally less than 2 percent in freshly harvested corn. High damage levels in harvested grain create challenges for grain grading, particularly in the harvest rush.  Damaged corn sharply reduces the future storage life of the grain.

Storage issues
Storage and harvest management will be particularly important. Field damaged grain, regardless of reason, should not be mixed with good grain. Producers should harvest around water holes, downed grain and immature areas. Do not mix damaged grain with good grain in storage. Regardless of condition, all grain should be aerated immediately to reduce temperature and equalize moisture. 

Field damaged grain will not store beyond the winter months. Maintain 1 to 2 percentage points lower moisture than normal grain (for example, 13 percent corn instead of more typical 15 percent).  Do not try to hold field damaged corn at higher moistures to avoid drying expense. If you suspect mycotoxin problems, check with crop insurance providers to see if adjustments may be needed, and how to represent the areas to be adjusted. Your veterinarian or the local USDA grain inspection service provider (see below) can assist with obtaining mycotoxin testing. Crop adjustments for quality problems, including mycotoxins, must be done on standing corn at or before harvest.

Accurate grading of field-damaged grain is always difficult in the rush of harvest. Expect end users, such as ethanol plants, to increase their level of grading because mold and weather damage reduce processing yields/byproduct quality.

An Official USDA grade is the standard against which buyer analysis should be compared. It is important that company graders be trained to match USDA graders. Alternatively, samples can be submitted to USDA grading agencies but this process is slower and more costly. In the event of a dispute, use an Official grader. The variety of damage types will be very challenging to evaluate.  You can locate the official agency in your area from the USDA list of official inspection and/or weighing services.

Livestock Feed Issues
Livestock (swine, cattle, horses, poultry) are susceptible to certain mycotoxins.  Therefore any grain that is fed to livestock should be tested for mycotoxins.  Dairy producers should be particularly sensitive to mycotoxins. (See tables below.)

The wide variety of molds on these samples has created a range of mycotoxin possibilities – primarily vomitoxin, zearalenone and fumonisin. Prolonged cloudy, humid weather encourages production of the fusarium-based toxins. 

Veterinarians can submit samples to the Iowa State Veterinary Diagnostic Lab.  Alternatively, Official USDA grading agencies can do quick tests for these toxins.  Toxins concentrate in the distillers grains three to four times the levels in the corn. Ethanol plants are doing quick screening tests on inbound grain, especially on low test weight corn (less than 50 lb/bu). 

Please see ISU Extension publications  Aflatoxins in Corn - PM1800, and Corn Ear Rots, Storage Molds, Mycotoxins, and Animal Health - PM 1698, for guidelines on sampling and sample handling. The black light test will not respond to fusarium.


FDA fumonisin guidelines table



Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Charles Hurburgh is a professor of Agricultural and Biosystems. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production.

Corn Quality Issues in 2008 – Storage Management

By Charles Hurburgh, Department of Agricultural and Biosystems Engineering; Roger Elmore, Department of Agronomy

Grains have a shelf life just like any food product. Shelf life is primarily determined by moisture content and temperature. It is gradually used through the time before use, and each operation or storage regime consumes a portion of the life.

Check combine settings between fields because fines and cracked kernels spoil much faster than whole, sound kernels. Grain that starts to heat or get moldy has essentially used its storage life. The goal of grain storage management is to reduce the rate at which the life is lost. Every action taken after harvest affects the ultimate length of time grain can be stored and the quality at the time of use.  Always get grain cool quickly and minimize variations both from the dryer and from the field.

Holding wet grain, especially without aeration, shortens shelf life considerably.  Fungi grow very fast in corn above 20 percent moisture. Overnight storage of wet corn in a wagon or truck can have a marked effect on future storability.  Always get wet corn into an aerated storage immediately. Likewise the practice of holding medium moisture corn (16-20 percent) for future blending or feeding opportunities will cause problems for corn stored (even after drying) into the following summer. 

Aeration Practice
Phase 1: Fall Cool Down
   • Lower grain temperatures stepwise 
         • October 40-45 F
         • November 35-40 F
         • December 28-35 F
Phase 2: Winter Maintenance
   • Maintain temperatures with intermittent aeration
         • January, February 28-35 F
Phase 3: Spring Holding
   • Keep cold grain cold
         • Seal fans
         • Ventilate headspace intermittently

This year there will be more wet corn held because of high field moistures and expensive drying.  Wet corn should be checked weekly, and monitored for temperature increases. Wet corn should have 0.2 cfm/bu of aeration, double the normal rates for dry corn. Problems will start to show up in February and March as temperatures rise.

Options when large amounts of wet corn exceed drying capability
 1) Dry to 17-18 percent moisture and cool in the storage bin. Corn will end up at about 16 percent moisture.

2) Dry to 20 percent moisture, cool in bin, hold wet corn for spring.

The less you dry, the more risk you are accepting. But spreading out the drying into spring may be the only choice. Risk will require more constant attention.

Be selective about what corn is placed in storage versus moved at harvest.   Deliberately decide which corn and bins are going to be kept into the summer. This should be your best (highest test weight) corn, harvested below 20 percent moisture with careful combine settings to minimize trash and placed in storages with good aeration rates/airflow distributions.

Low test weight corn should not be put in temporary storages or outdoor piles. It is also not wise to mix corn of different crop years in the same storage bin; the mix is generally much less stable than each year’s crop stored separately. The 2008 corn will be more susceptible to mold and heating in storage than average corn at the same moisture, which means that holding wetter corn should only be done in cases where there is drying or other options to halt spoilage if it starts.

Remove the center core and use a grain distributor if possible. Check your grain at least every two weeks, with some way to take grain temperatures.  If a slow rise is noted, aerate. If a hot spot starts, make that the next corn to be moved out; one storage problem always leads to another.

Understand your buyers' needs, and match storage and drying practice to intended marketing time. For example, corn sold for July or August 2009 delivery should be dried more fully right away.

Maximum storage time chart


Charles Hurburgh is a professor of Agricultural and Biosystems. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production.

This article was published originally on 10/27/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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