Wet Weather Creates Challenges for Harvest

September 25, 2016
ICM News

The 2016 growing season was wet with two distinct temperature patterns—hot during early pollination and cooler in August. Most of Iowa had 125% up to 200% of normal rainfall up to Labor Day, and even more after Labor Day. The outlook going forward into October is continued above average temperatures and above normal rainfall. These conditions produced high yield potential at the start of September, but deteriorating quality since Labor Day. While yield is determined over the full season, crop quality is often established by conditions at the very end of the growing season. The recent massive rains in the northern third of Iowa will complicate the need to move quickly on deteriorating grain.

Field Mold

Corn is susceptible to field mold, which we are seeing emerge rapidly. Some yield will be lost, but more importantly, field dry-down and storage properties will be reduced. Test weight of 54-55 lb/bu may be high this year. If wet and humid weather persists, regardless of temperature, expect wetter than normal corn at harvest. Fungi are invading stalks, which makes stalks weak in many cases. Stalk rots are progressing into the cobs, and eventually into the kernels. Scouting and early harvest will be needed to prevent field loss from both downed stalks and mold activity. We had a similar situation last year, but the weather turned to very low humidities at the end of September through November, which removed the mold threat.

The effect on soybean quality will be slower field drydown. While soybeans typically dry down to 12-13% or less with warm, dry weather. Equilibrium moisture contents are staying in the 14-16% range all day, with the current high humidities.


The amount of grain needing temporary storage, typically in piles, will be larger this year. Given the carryover 2015 corn, I expect about 1 billion bushel of the expected 2.7 billion bushel Iowa corn crop will require temporary storage. At the rate corn is used, this will take three to four months to use up. Carryover is projected to increase again over the 2016 marketing year, which means a significant share of this year’s harvest will have to remain in condition more the 12 months. There will probably be a rush to commercial storage in the second half of harvest, because both corn and soybean crops are expected to be large. Grain handlers will need to choose, based on local conditions, which crop year grain to hold in long term storage through 2016-2017. In some cases it may be best to hold 2015 grain another year.

Some specific impacts from wet field conditions and tips for grain handling and storage:

  • Fungus development. Continued wet weather and warm humid conditions are retarding drydown and are encouraging fungus development on ears (see Figures 1 and 2 below). Some fungi produce mycotoxins harmful to animal and human health. The wide range of fungi on samples this year creates the unusual potential for several mycotoxins. Some fungi, such as diplodia ear rot, do not produce mycotoxins while others such as fusarium or aspergillus can be toxigenic. Expect feed mills and ethanol plants to screen early harvest samples for toxins. Field scouting can help identify problems before harvest.

  • Moldy grain. Moldy grain will have shorter storage life than 'clean' grain held at the same moisture and temperature. The data in Table 1 represents the long term averages. For grain carrying field mold, expect perhaps 60% to 70% of normal shelf life. 

  • Storage life. The storage life of grain starts immediately after harvest and is shortened significantly when stored at warm temperatures. One day of holding warm, wet corn before drying can use as much as 50% of the storage life, therefore increasing the likelihood of storage problems next spring and summer. Holding wet corn can also result in blue eye mold development, which lowers the Grade but does not produce mycotoxins. Soybeans are sometimes overlooked in the need to cool grain immediately; yet soybeans are more often harvested in warm weather compared to corn.

  • Harvest early. Harvest early and expect to dry the corn. Avoid holding wet grain before drying. Dry rapidly.

  • Watch dewpoint temperatures. Cooling the corn out of the field and after drying will be a challenge; watch the dewpoint temperatures. You will never cool the grain below the dewpoint. Ongoing temperature monitoring is key to successful storage. Recent developments in carbon dioxide monitoring have created another way to monitor spoilage; deteriorating organic matter always produces CO2 emissions.

  • Drying methods. Slower drying methods will require careful monitoring. There will be potential for additional spoilage before the grain dries. This would apply to both natural air and low temperature systems. You can increase drying speed in these systems by reducing the depth, but the tradeoff is having less grain in the dryer. 

  • Bin dryer temperatures. Operate gas-fired bin dryers at temperatures above 120 F, but not over 160 F.

  • Management of pile grain. Temporary piles should be at commercial facilities, not on farms. Small piles spoil more rapidly than large ones. Management of pile grain is labor intensive, and often requires very short notice decisions about moving the grain. Larger piles are also easier to aerate.

  • Do not mix old and new crop grain in the same bin. The old crop has used much of its storage time and the new crop is still equilibrating moisture. Rotate stocks if you can. Be sure to separate the crop years and draw the center cores out of all bins, corn and soybeans alike.

  • Testing for toxins. Commercial grain handlers and processors should expect to receive grain with higher levels of damage and foreign material. We do not know yet if this will create mycotoxin issues. One way to track the situation in a given area is to collect a 10-lb daily composite (some grain from each of the deliveries in a day) for toxin testing. The USDA-GIPSA grain inspection agencies have the capability to test for the mycotoxins of most concern - aflatoxin, vomitoxin, zearalenone, and fumonisin. Toxin test kits are now simplified to the point where elevators and processors can do their own, but the USDA results provide a good benchmark. Results after 7-10 days should indicate whether there is a problem in an area. The composites also provide an opportunity to check the accuracy and completeness of on-site grading.

  • Grain handling safety. High volumes and wet corn, are the common ingredients for grain handling accidents, both entanglements and engulfments. Nothing is worth taking shortcuts on safety.  


In summary, given expected weather patterns, there will be large volumes of wetter and potentially moldy corn this year. Use test weight and visual mold presence as a measures of future storability. Identify in advance bins that will be kept for the long term, including possibly retained 2015 corn. Separate crop years in storage, and scout fields for grain molds before harvesting. Low test weight and moldy corn are likely to go together. Dry quickly with as little wet corn holding time as possible. Once in storage, keep track of temperatures regularly and consider adding CO2 monitoring to your stored grain control process.

We will have updates on crop quality as the harvest progresses.

Table 1.  Allowable storage time for corn and soybeans.


Figure 1.  Cob rot progressing into kernels.


Figure 2.  Ears harvested in mid September

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Charles Hurburgh Professor, Agricultural and Biosystems Engineering

Dr. Charles R. Hurburgh, Charlie to most everyone, is a native Iowan from Rockwell City (Iowa, USA). He continues to operate the family farm, and is a professor of Agricultural and Biosystems Engineering at Iowa State University. He has a bachelor's degree, master's degree, and doctoral degree fr...