Wet weather raises corn quality concerns

Encyclopedia Article

19 Oct 2007 -

Rain has caused significant delays and potential damage to the Iowa corn crop. As of Oct. 14, 63 percent of the nearly 14 million acres of corn in the state remained mature yet unharvested. Little has been harvested since then.

Several Iowa State University (ISU) Extension crop and grain quality experts offer advice on how to handle the extreme moisture affecting the unharvested corn crop in Iowa. 

Current Conditions

The majority of corn is standing, yet some is lying on moisture-saturated soils, and, therefore, is vulnerable to all sorts of factors that could reduce yield, quality and profit. As delays continue  producers can expect to experience plant and grain quality issues. Extended rain periods expose crops to saturated soil conditions, ponding and continual wetting of plant tissue. There are two areas of concern for the corn crop remaining in the field: stalk quality and standability, and grain quality.

Stalk Quality and Standability

Eighteen percent of Iowa’s unharvested corn is either moderately or heavily lodged, according to the USDA National Agriculture Statistics Service report released earlier this week. There are few more discouraging harvest problems at this time of year than to have fields or areas of fields that are lodged. Lodged corn not only slows down harvest dramatically, but it also increases the possibility of harvest losses. Flooded corn will not likely stand for much longer, which will predispose the ears to an increased chance of mold infections.

Ears on some hybrids stay upright and/or have open husks. Upright ears and open husks can serve as funnels and receptacles for water. Given enough moisture and warm temperatures, corn on the ear will germinate reducing grain quality.  

With the persistent damp weather, many have noticed a change in the color of plant tissue described as blackish soot. A sample brought into the ISU Plant and Insect Diagnostic Clinic contained this black growth. This growth was primarily due to a fungus called Alternaria, although there were likely other fungi present (including Colletotrichum). Alternaria is a common decay fungus, which decomposes dead organic matter. Although Colletotrichum is often a pathogen earlier in the growing season, it is also a common saprophyte, an organism that feeds on decaying dead organic material late in the season. This discoloration of plant tissue will not affect grain yield or quality yet it will likely increase stalk degradation. 

Field Grain Quality

High moisture conditions favor growth of many ear and stalk rot fungi, increasing  the risk of ear rots and mycotoxin contamination

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 as soon as possible. The longer the corn remains in the field, the greater the chance of toxin production. The toxins of most concern to increase in the field at this time are the fuarium-based toxins, vomitoxin and fumonisin. Adjust harvest equipment to minimize damage to kernels since mold and mycotoxin levels tend to be at greater levels in damaged kernels. Dry (<15 percent moisture) and cool (<45 degrees F) grain as quickly as possible to reduce further mold growth and toxin production.

Elevator operators report cases of mold damage levels 5 percent and higher in the current corn crop, with an unusual mix of mold types, and sprouting from prolonged moist conditions.  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. Corn that had black layered, even if high moisture, was past experiencing economic injury from frost.  However, corn that was green will likely have lower test weights, which is another source of reduced storage life.

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 frost damaged areas. Do not mix damaged grain with good grain in storage either. The damaged or low test weight grain must 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). 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. 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.

Grain Storage Quality

Precleaning grain prior to storage is generally a good practice but can be critical for badly damaged product. Pre-germinated corn can typically be removed using most screening operations.  Kernels with extensive mold damage can frequently be removed with simple air separations but this may not be sufficient to reduce mycotoxin levels to safe levels. Separation equipment is available for removal of additional infected kernels but the additional operations required are rarely economically viable for commodity corn. In these situations, level bins after filling, provide adequate aeration to reduce moisture migration, and monitor temperatures for “hot spots” to help control growth of fungal storage pathogens.  

Corn that has not dried early in the harvest period often stops at 17 to 18 percent.  It will be very important to cool this corn immediately and dry it as rapidly as possible.  Natural air drying systems will have difficulty if the humidity levels remain high. Check stored corn frequently and consider delivering to market if problems start to appear. 

Grain Quality for Livestock Feed

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.

The wide variety of molds on these samples has created a range of mycotoxin possibilities – aflatoxin, vomitoxin, fumonisin primarily. Most recently fumonisin has been found in a number of high-rainfall areas. The 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 to 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 (<50 lb/bu).  See ISU Extension publications PM 1800, Aflatoxins in Corn, and PM 1698, Corn Ear Rots, Storage Molds, Mycotoxins, and Animal Health for guidelines on sampling and sample handling. The black light test responds to only aspergillus that produces aflatoxin and will not detect fusarium. 

Food Grade Corn

Current conditions are a serious concern for edible products such as popcorn or white corn intended for human consumption. These are typically higher value crops with limited product volume so additional separation steps to remove contaminated kernels are a fairly common practice. 

Separation of Contaminated Grain

Fungal toxins are frequently concentrated in a few highly contaminated kernels.  Individual kernel levels of aflatoxin have been reported up to about 8,000 ppb. Removal of the infected seeds to reduce mycotoxin levels prior to milling is a fairly common practice. Limited research has also been conducted on corn separation, but industrial applications are less common due to typical lower mycotoxin levels and the commodity nature of the product. A portion of infected kernels can frequently be removed using more advanced cleaning devices to remove low density and/or discolored kernels. Both methods have been fairly effective at reducing mycotoxin levels by removing kernels with different physical or optical properties.   

Several ISU Extension specialists contributed to this report. They include:

Roger Elmore, extension corn specialist

Lori Abendroth, extension agronomy specialist

Christine Engelbrecht, plant pathologist, ISU Plant and Insect Diagnostic Clinic

Alison Robertson, extension field crops pathologist

Alan Gaul, seed conditioning specialist, ISU Seed Science Center

Charles Hurburgh, professor in charge, Iowa Grain Quality Initiative 

ISU Extension publications PM1800, Aflatoxins in Corn, and PM 1698, Corn Ear Rots, Storage Molds, Mycotoxins, and Animal Health can be ordered through any ISU Extension county office, online at https://store.extension.iastate.edu/ or by calling the ISU Extension Distribution Center at (515) 294-5247. 

Contacts :

Roger Elmore, Agronomy, (515) 294-6655, relmore@iastate.edu

Charles Hurburgh, Iowa Grain Quality Initiative, (515) 294-8629, tatry@iastate.edu

Alison Robertson, Plant Pathology, (515) 294-6708, alisonr@iastate.edu

Iowa State University Agronomy Extension Corn Production