Farmers filled bins on their farms to the brim following Iowa’s huge corn and soybean harvests in 2025. Facing continued low prices in 2026, grain will be held in storage longer than usual.
“With drier than normal weather in Iowa during harvest last fall, grain came out of fields drier than usual,” noted Charlie Hurburgh, a retired Iowa State University Extension and Outreach agricultural engineer. “That doesn’t mean you can put it in a bin and forget about it. Always keep an eye on stored grain, checking it often. If it shows signs of starting to deteriorate, you must act quickly. Take the necessary steps to avoid quality problems and possible spoilage.”
In some cases, both corn and soybeans came out of fields overly dry last fall.
“Farmers had the opportunity to cool the grain after harvest by running fans on grain in bins,” Hurburgh said. “The challenge is to keep grain cool, especially with the variable weather and temperatures we’ve had this winter.”
Iowa had wild swings in temperature and humidity, as well as days of dense fog after harvest.
“Running bin fans on foggy days doesn’t dry the grain,” Hurburgh warned. “It puts moisture back into stored grain. We had that situation on foggy days last fall.”
Be careful with big bags
With high yields and big crops, farmers filled many large white plastic bags with corn, and some with soybeans, that sat in fields this winter. To manage the limited storage capacity as they ran out of bin space, more farmers used temporary storage such as poly bags. Bags of grain require careful monitoring for moisture, pests and damage due to weather.
The key to storing corn in bags temporarily during winter is to keep the air out of the bags once they are full, Hurburgh said. There’s no way to change the temperature of grain in bags other than to slow the path of warming and cooling from the outside. You can’t aerate or dry grain in bags like you can in bins.
Also, check bags to see if holes have been created. A hungry raccoon could eat a hole in the plastic.
Grain in bags should go in as dry grain.
“I’ve seen more than one attempt to put 17%- or 18%-moisture corn, or even higher, in storage bags,” Hurburgh said. “That doesn’t turn out well. Grain will spoil at those moistures, especially without the capability to aerate the grain.”
Moisture content guidelines
Corn grain that’s at 15% moisture or less — and in bags that are sealed well with no holes or patched holes — should stay in good shape at least until spring.
What is the ideal moisture content for storing grain in bags? Hurburgh said 15% or below for corn.
“As you start to go toward 16% or 17% grain moisture, spoilage occurs more rapidly,” he said. “With potential for spoilage, I don’t recommend having corn wetter than 15% in storage bags.”
For corn stored in bins aerated with fans, he advised drying the corn to a safe moisture content when it enters the bin at harvest. A 15% moisture content is safe for corn storage through winter. Still, successful corn storage hinges on proper cooling of the stored grain after it’s dried.
“If you don’t cool the stored grain by aerating with a bin fan, you might have some problems, especially if you put corn in a bin when it was 70 or 80 degrees [F] at harvest,” Hurburgh noted. “Dry grain can spoil and go out of condition if you don’t cool it with aeration.”
What about soybeans?
More soybeans than normal are stored on farms this year as farmers wait for better market prices.
Hurburgh isn’t a big fan of storing soybeans in big bags.
“I hope you don’t have beans in bags,” he said. “Priced more per bushel than corn, soybeans are too valuable to risk storing them in bags.”
With beans coming out of fields quite dry last fall, farmers were asking: What’s the recommended moisture content for storing soybeans in big bags? Some beans were harvested as low as 8% moisture.
“Those beans are more likely to have spoilage issues, even if they went into the bin or the bag extra dry,” Hurburgh said. “You can have moisture migration issues in stored grain if you don’t periodically change the air and make sure the grain has uniform temperatures. Stored grain temperature needs to be reasonably uniform — in the 30- to 39-degree [F] range. Dry grain will spoil if you don’t have air moving through it. Proper aeration is needed.”
A matter of monitoring
It’s not just corn that’s at risk of spoilage; soybeans can have storage problems too. In a big bin, beans will spoil if they don’t have enough air moving through during storage. It’s very important to monitor corn and beans during storage.
How about putting moisture into soybeans that are too dry in a storage bin? Some people try doing this, and there are control systems that claim to operate the fans in times of humid weather, such as on foggy days.
“Trying to put moisture into soybeans with fans in bins is risky,” Hurburgh said, “especially if you are wetting beans in a bin from the bottom up. Beans rewet very slowly. It takes a lot of energy, and the beans swell. Trying to rewet stored grain is a risky process I don’t recommend.”
Summing up
Stored corn and soybeans need to be checked regularly so a problem can be detected before it becomes a disaster. In spring and summer, it’s a good idea to check grain weekly, Hurburgh said.
Turn on the fans, and smell the air that moves through the grain. You can start the fans while someone else waits at the roof access door to smell the first flush of air.
Musty or sour smells indicate mold growth. To get an earlier indication of mold growth, you can use a handheld carbon dioxide monitor to check for rising CO2 levels coming from the grain.
For more information on managing stored grain, visit the Iowa Grain Quality Initiative website at extension.iastate.edu/grain.