In previous articles he's written in Iowa State University's Integrated Crop Management Newsletter this fall, Charlie Hurburgh has discussed the aflatoxin issue from several angles; scouting, testing, use of the corn and handling the corn. The newsletter is available to the public online at ISU Extension's website. Hurburgh is a well-known grain quality expert at ISU and a professor of agriculture and biosystems engineering.
One key point is that once grain is dry and cold, or even just cold, the Aspergillus flavus fungus is rarely able to grow and produce more toxin, says Hurburgh. However, at least two problematic situations are arising -- bin dryers operated at medium temperatures (below 120 degrees F) and high variability of moisture within fields.
The optimum temperature for aflatoxin production is 75 to 95 degrees F with grain moisture content greater than 18%, he explains. A bin dryer operating with input air below 120 degrees F will "store" the grain during drying at these temperatures. If the bin is full, drying times of four to six days are not uncommon. In this case, grain already containing the Aspergillus fungus can experience increased aflatoxin levels.
To avoid problems, boost the grain dryer temperature beyond 120 degrees
The correction you need to make is to increase the grain drying air temperature beyond 120 degrees F. Hurburgh says some bin drying systems with rapid stirring systems can go as high as 160 degrees F; others with less grain circulation may be limited to around 140 degrees F. Half batches of grain will also help; shallower grain depth will increase airflow and cause less grain be held at higher moistures. "It would be better in this case to dry two half batches instead of one full batch," he advises. "The outside air temperatures have fallen enough that the corn in the field is now less likely to increase in toxin. Holding that corn in the field may be preferable to having it warm in a dryer."
High temperature batch dryers and continuous flow dryers are not susceptible to this problem, but wet corn should be held in a high airflow wet holding bin (to maintain cold temperatures) or else hold the corn in the field.
As an example, Hurburgh says today's conditions of about 80 degrees F and 30% relative humidity will hold aerated wet grain at about 45 to 50 degrees F because of evaporative cooling of dry air. This is below the growth conditions for Aspergillus flavus mold, although in time other more temperature resistant fungi will grow at those temperatures. Low temperature-natural air drying will also work under these conditions because the wet grain will not be warm enough to sustain the fungus.
Dryers won't get the job done in one pass, more than one cooling cycle is needed
In some cases, very high ranges of grain moisture content for corn are being experienced within the same cornfield. For example 15% to 30% moisture corn is coming out of the same field. What should you do? Dryers will not equalize this moisture in one pass; there will be some wet corn remaining even after the average for the corn reaches 15% moisture. "Low temperature drying is the only control method for this situation," says Hurburgh. "You need to use extra cooling cycles to bring the grain temperature immediately below 50 degrees F."
Corn will segregate somewhat by moisture content if the grain is drop-filled into a bin, he adds. This means that both the high moisture corn and the fines will collect in the center of the bin as it is being filled. "It will be very important to remove the center core right away," he emphasizes. "In large bins (those over 50 feet diameter) two removals of grain from the core of the grain mass would be advisable."
Hurburgh says you should pay attention to the yield monitor's moisture reading when you are driving along in the combine cab. Watch the estimate it gives for moisture and use that information to make your to estimate regarding which fields are likely to be a storage problem from grain moisture variations. "Manage your harvest and drying and aeration of corn properly," he emphasizes. "Remember, crop insurance will not cover quality issues after harvest.
"Also, take the time to cool your stored corn in the bin down to proper grain temperature," adds Hurburgh. "And remember, once grain is dry and cold, aflatoxin is rarely able to grow."