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Controlling Heat, Humidity Reduces Insects and Mold

Uncontrolled pests and mold can quickly devalue or ruin a harvest

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18 special report

When storing grain, taking care to reduce temperature and relative humidity helps ensure that an entire harvest isn’t ruined; or in the case of large scale grain operations, that the work of multiple growers is protected. Moisture and temperature can work either together or independently to cause trouble. Each insect species has its preference for temperature and moisture content combinations. Some flourish in warm temperatures and high moisture while others like lower moisture content.

“Moisture is the most critical element to monitor because insects thrive in warm, moist environments,” says Dr. Carol Jones, a professor at Oklahoma State University’s Stillwater campus. “It also allows mold to cultivate into big clumps that stick to the sides of a bin or become crusted on top of grain.”

Whether it is moisture from condensation, leaking or insect infestations, problems can occur at different layers within the bin. That’s because most new grain is usually added to the top of old inventory, creating pockets with different moisture and temperature levels. “Managers often opt to place grain with higher moisture content into a bin with drier grain. Unless the grain is stirred, spread or mixed with the other grain, however, it is highly likely that pockets will occur,” says Jones.

Moisture can cause grain to clump to the point it can’t pass through an auger system. That poses a safety issue because many people are tempted to enter a bin to break up the clumps, resulting in dangerous entrapment, says Jones. The best practice is to put good grain in the bin at the onset, and that’s done by ensuring it is dry enough to store. Most of the time, that is the grower’s responsibility, and facilites need to check incoming grain to ensure material is dry before accepting the shipment. Once inside the bin, it’s important to use aeration to control temperature and moisture.

“Proper aeration actually works like crop insurance to protect a harvest,” says Jones. “A temperature swing of as little as 15 degrees can allow condensation to occur which helps form a breeding ground for insects or works as an incubator for mold. Installing temperature cables in a bin to automatic controllers can pay for itself in a single season by activating blowers to cool grain without requiring trained employees to constantly monitor the material, she adds.

Equivalent moisture content

Each type of grain has its own equivalent moisture content (EMC), an internal rulebook. Depending upon the grain, there is a certain level of moisture content considered stable for storage. Without automatic controllers, employees need to look up the current air temperature and relative humidity and compare it to an EMC chart to determine proper moisture content before deciding to begin aeration. Running fans when conditions are not optimal is costly from an energy standpoint and may encourage insect and mold development instead of reducing their damage, says Jones.

When humidity is higher, like when it’s raining, special care must be taken to ensure the process doesn’t put more moisture into grain, she adds. If moisture level is already low, however, aeration could work to dry out the grain too much. Once mold or moisture is present, the only other option is to move grain from one bin to another or into trucks for storage or sale. That usually results in shrinkage, however, and is costly and labor intensive to complete.

Once a problem develops, it creates a huge dilemma. Mixing mold and insects with good grain works to reduce overall quality of the load.

“Then it’s hard to sell because buyers deduct points for poor quality,” says Jones. “Unless we’re constantly monitoring the grain in storage, problems won’t be discovered until it’s moved or sold. By then, it’s too late to remedy the situation.”

Insects the biggest problem

Insects are usually the biggest problem with storage of summer-harvested crops like wheat, says Mark Casada, a research agricultural engineer with the U.S. Department of Agriculture, Agricultural Research Service, Center for Grain and Animal Health Research in Manhattan, KS.

“Insects respond more to temperature. When grain is too warm, they breed quickly. They can attack grain even when dry, but there is less chance of seeing weevils, such as the rice weevil, in drier wheat,” he explains. “The lesser grain borer, which is the most prevalent internal infesting species impacting dry wheat in central and southern plains, is less affected by grain moisture,” says Casada. “Low moisture stops some insects, but it doesn’t eliminate the problem. Growers and storage centers must make sure temperatures remain cool.

“When evaluating grain condition, the natural tendency is to look at average moisture content; however, people need to be more concerned with the highest moisture content,” he adds.

For example, if there is one or two loads of higher moisture grain adds to a bin that otherwise had low moisture content, it creates an area wetter than the rest of the grain. It will require intervention and the highest moisture levels must always be the focus when managing that bin.

“Look what happens to warm grain sitting in a bin in late summer and into fall,” says Casada. “As the temperatures begin to cool, edges of the stored grain will cool off, but those cooler temperatures won’t penetrate to the core.” So what happens to unaerated, warm grain as temperatures fall in winter?

“The middle part remains warm. But as the edges cool, the warm air rises and the cold air falls, creating convection currents that could cause condensation,” he adds. “If grain is not aerated, warm moist air will move to the cool top edge where it can cause moisture accumulation.”

A key temperature for grain storage is to keep it below 60 F, he adds. At that temperature, insect populations will not increase. Even when grain is aerated to match external temperature, the sun is always working to heat the sides and top of a bin so grain in that area gets warmer right away. But, at 50 degrees, insects stop feeding and they will eventually die if they stay that cool.

Insects worse at grain elevators

Integrated pest management techniques prevent insect infestations. One way is to get rid of the core funnel of grain, then cool it off and always be sure it is dry before storing it.

Another method involves cleaning storage bins by using heat, which can kill any residual insect populations before the grain is put in the bin. Sweeping and cleaning the bin with brooms is helpful, but using a shop vacuum is even better to help suck grain out of cracks and crevices which trap grain that could become food for insects.

“Many entomologists recommend treating empty bins with a residual insecticide like Tempo before filling,” says Casada.

Insect problems are generally worse at grain elevators than on farms, he explains. On farms, grain tends

to sit for a few weeks or months before it moves to grain elevators. But, at the elevators, grain starts with any insects accumulated from farm storage, and then sits stagnant for additional periods, which increases breeding opportunities for insects.

Aeration doesn’t dry grain

“You can’t use aeration to dry grain. You need at least 10 times the aeration airflow to dry grain,” Casada explains. “Aeration fans don’t have capacity to move enough air to dry grain in a reasonable time. But, if needed, a much larger drier fan could be used to cool grain quickly.”

Even with the standard aeration cooling time of 150 hours at 0.1 cubic feet per minute per bushel of airflow, most of the grain in a bin won’t even start to dry. Casada says timing of the harvest offers options. For example, wheat deposited into a bin in early summer, such as July 4, will be hard to cool in late July and August. In those situations, stored grain managers are often limited to partial cooling during only the coolest portion of each night depending on local weather conditions.

With crops harvested in fall, however, aeration fans often provide immediate effective cooling and may be able to run 24 hours a day to cool grain in many regions of the United States.

“If you harvest wheat in summer, you’ll never get it down to 60 F in the central and southern plains, even with an automatic controller turning fans on at night,” he explains. “However, over a few weeks, you might get the grain down to 70 degrees with a controller. It’s not ideal, but insects won’t reproduce as fast.

“Come September, when temperatures at night drop to 50 F or 60 F to allow further cooling, the insect population won’t have become nearly as big a problem as it would have if you had not aerated at all during the summer,” he adds.

Casada notes that even in the hotter summer months, there are generally enough nighttime hours to aerate grain so the temperature gets below 75 F and, maybe, even 70 F — enough to slow breeding until autumn temperatures reach a more desirable 50 to 60 degrees. To control insects that have gotten out of hand, Casada recommends using closed-loop fumigation during which a small fan pushes air mixed with phosphine through grain from the bottom and ensures it is applied uniformly. Another method involves pushing aluminum phosphide tablets into the top of the grain and allowing diffusion to move phosphine gas into the grain.

“The gas is about the same density as air when coming off the tablets,” he says. “So, it doesn’t fall into the bin, but just sits where it is. We recommend using a small fan to circulate the fumigant to get uniform distribution.”

Grain quality impacts international sales

Knowledge of proper grain storage is essential when working with international shipments of grain, says Carlos Campabadal, an outreach and extension specialist with the International Grains Program Institute, which is part of the Department of Grain Science and Industry at Kansas State University. He assists international grain buyers to understand the best methods to store grain imported from America.

“Grain that is not dried to the right moisture content can make it difficult for shipment to other countries,” he adds. “It all depends upon the type of grain being stored and the location.”

For example, grain stored in southern states, which is exposed to conditions more similar to semi-tropical and tropical weather, must be maintained at a lower moisture level. Campabadal envies growers in northern states because they have an easier job of controlling moisture content than those in southern states, which are hot most of the time during long-term storage.

Moist climate and high moisture content can produce dangerous mycotoxins that can render an entire bin unsafe to use or sell. In North America, growers and grain operators use aeration to cool grain, but in Europe and, in some cases, Latin America, special grain chillers are employed to combat the problem. The equipment tends to be expensive, which contributes to why it isn’t employed at a wider level in North America.

To combat the problem, Campabadal suggests four options:

  1. Dry the grain to the safe moisture content. For example, in much of America, moisture content of corn cannot exceed 15%
  2. Clean the grain. This can be difficult for processors and elevators as it requires separating broken pieces and fine particles that attract insects and allows mold growth
  3. Aerate the grain to push ambient air through it to cool the material. But, this only works when air is cooler than the internal temperature of the grain
  4. Move the grain. A likely last resort is to move grain from one bin to another or from top of the bin to the bottom. This can mix a hotspot into the rest of the grain. It also creates more broken pieces and devalues the crop.

“It should be relatively easy to aerate every night when humidity is at 70% or below,” says Campabadal. “If humidity is above 70%, mold grows faster. We definitely don’t recommend aerating when it’s raining.”

Grain storage is a necessity, but with special attention to heat and relative humidity, the material can be protected from mold and insects to retain value. ❚

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