Integrating temperature and pest management for successful grain storage
October 01, 1997
by Teresa Acklin
Maintaining stored grain quality requires an integrated approach incorporating various techniques to prevent quality deterioration.
A proper storage management program integrates a range of tools and practices to assure maximum stored-grain quality. These include sanitation, application of residual insecticides to structural surfaces and grain, temperature control through aeration, moisture control through drying, frequent grain and pest sampling, grain cleaning and low-impact handling, biological agents and fumigants.
The emphasis is on using a combination of these tools and practices to prevent quality deterioration, rather than using a single “big gun” approach to treat a quality problem once it occurs. Relying on a single tool to take care of a problem is an approach of the past that is doomed to fail in the future. Single solutions, especially if they are chemical in nature, are under intense public and regulatory scrutiny and will continue to be a limited option. Prevention is the only acceptable way to maintain grain quality.
Elevator managers are in a unique position to apply integrated stored-grain management programs successfully if they take advantage of the ability to control critical system parameters such as grain temperature and moisture content, storage time, market destination and pest movement into their facilities. Because grain is generally binned with some initial population of molds and/or insects, the control of environmental conditions throughout the storage period is critical to prevent quality deterioration.
To maintain quality grain in storage, the storage environment must prevent the growth of microorganisms and insects. The most important management factors include temperature, moisture, length of storage and the condition of the grain when placed in storage.
One of the primary management practices to maintain quality is aeration. Aeration involves moving relatively low volumes of air through grain to control and maintain uniform grain temperatures throughout the pile and to reduce the risk of spoilage and damage from molding and insect infestation. Non-uniform temperatures in the grain bulk generate air currents that can lead to moisture migration.
Aeration generally utilizes ambient air. However, an alternative gaining popularity is chilled aeration, which allows for cooling independent of the ambient conditions at any time of the year. Grain chilling is especially beneficial during summer storage because grain temperatures can be maintained at or below 13° to 16°C.
For aeration to be successful, the grain has to be level and at moisture contents safe for storage because aeration airflow is inadequate to dry the grain. Table 1 on page 24 summarizes the recommended maximum moisture contents for aerated grain storage in temperate climates such as the U.S. Midwest.
Recommended aeration airflow rates for upright storage are 0.05 to 0.10 cubic meters of air per minute per tonne. For flat storage, the rates are 0.025 to 0.05 cubic meters per minute per tonne.
Most storage problems result from improperly cooling the grain in the storage bin. The most common mistake is to stop running the aeration fan before the cooling front has moved through the entire grain pile. This can lead to condensation and crusted layers of spoiled grain in the bulk.
Four separate storage management periods can be distinguished: fall cool down, winter holding, spring warming and summer holding. In the fall, it may take up to two aeration cycles to cool the grain to below 7°C; at an airflow of 0.10 cubic meters per minute per tonne, it would require 150 hours per cycle regardless of grain depth. For winter storage in the Northern Hemisphere, grain should be cooled below 2°C before the end of December.
The fan operation time depends solely on the airflow rate in the storage bin. An aeration fan is usually sized for about 0.1 cubic meters per minute per tonne, while an in-bin drying fan is usually sized for 1 cubic meter per minute per tonne.
It is very important to recognize the difference in order to operate the fans long enough to move the cooling front completely through the bulk, and yet not so long as to waste electricity. Table 2 above summarizes the approximate times for the last cooling cycle before the winter holding period for a range of common airflow rates.Pest Management
Correct drying, aerating and managing of stored grain minimizes the risks of quality deterioration caused by pests. Several species of insects may infest stored grain, including internal grain feeders such as rice weevil, lesser grain borer and grain moth; and external grain feeders (or bran bugs) such as Indian meal moth, saw-toothed grain beetle, red and confused flour beetles and flat grain beetles. Grain stored at excess moisture levels is likely to be infested by mold (or fungi) feeders such as the foreign grain beetle and hairy fungus beetle.
Sanitation in and around stored grain facilities is the most effective and economical management practice to prevent insect infestations in stored grain. Prior to storing grain, all surfaces that may come in contact with the newly harvested grain should be cleaned.
In addition, storage bins with false floors and aeration ducts may need to be fumigated or treated with diatomaceous earth powder. The grain and dust that accumulate in these areas are an excellent source of insect infestations.
If fumigation is selected as the optimal procedure, seek a licensed applicator to do the job. Fumigants are highly toxic to humans and must be applied with proper protective equipment.
After the storage is cleaned, an approved residual insecticide should be applied on both the outside and inside bin walls and floors. Table 3 lists the approved residual insecticides (protectants) for bin and grain treatment. Pesticide applications without adequate cleaning generally are a waste of time and money.
As the grain is binned, preventive measures include applying a protectant if the grain will be in storage for more than a year. Grain protectants kill insects as they crawl about or feed on the treated grain. However, grain protectants should not be applied to high moisture grain or above 32°C because they can lose their effectiveness.
After binning, some grain protectants can be applied as a surface treatment (top-dress) to protect the grain from surface feeders such as Indian meal moth and invading beetles. Legal tolerances can be exceeded if a product is applied both as a grain protectant and top-dress, so the label must be read and followed.
Storages should not be overfilled. Furthermore, insecticide treatments, aeration and fumigation cannot be done effectively when the grain surface is not leveled.
Above 13° to 16°C, the grain should be inspected at least every two weeks for insect activity. Plastic grain probe traps are excellent sampling devices that can help determine insect activity below the grain surface.
Increasing the frequency of inspections may be necessary if numerous insects are trapped. Keeping record of where and how often a trap captures insects is part of a well-conducted integrated stored grain management program.
Insects collected in the traps should be identified before a treatment routine is selected. Infested grain is not automatically “weevily.” Depending upon the type of insects found, treatment may not be necessary, but other management practices may be indicated.
If grain is found to be weevil infested but needs to remain in storage, fumigation may be the only solution. However, one needs to keep in mind that although a fumigation treatment may dramatically reduce the insect population, there is no residual effect. The grain is susceptible to re-infestation as soon as the gas is vented unless additional preventive measures are taken.
To prevent stored grain insects, effective measures can be as simple as maintaining grain temperatures below 16° or above 38°C. Grain temperatures above 38°C have proven effective against insect development in dry wheat (below 10% to 11% moisture) binned and stored during the summer time in the U.S. south-central states.
Research conducted by Oklahoma State University has reduced fumigation needs to less than one a year using this strategy. For other crops, such as maize, or wheat above 10% to 11% moisture, grain temperatures can be reduced below 16°C any time of the year using chilled aeration to prevent insect development.Mold and Mycotoxin Management
Grain spoilage is the result of microorganisms using the nutrients within the grain for their own growth and development. During this process, they produce heat and increase the temperature of the surrounding grain, which may result in hot spots.
Heat damage significantly reduces grain quality. If environmental conditions in the grain are right, the major storage mold species Aspergillus, Fusarium and Pencillium may produce mycotoxins such as aflatoxin, fumonisin, DON and zearalenone. These may cause serious illness and even death when consumed by livestock or humans.
The presence of mold does not mean mycotoxins will be present, but rather that the potential for their development exists given the right combination of temperature, moisture content and storage time. Even more frustrating is the fact that the absence of mold does not guarantee a mycotoxin-free commodity. This is because the growth of the mold may not be extensive enough to cause visible damage, but nevertheless it can still produce toxins.
Generally, broken, ground and dead grain is more vulnerable to fungal attack than whole grain; stored grain dried at high temperatures is more vulnerable to molding than is grain dried at low temperatures; and grain stored for long periods of time is more vulnerable than freshly harvested grain. Although molds are diverse in their requirements, all mold growth can be prevented by low moisture, low temperature and low oxygen environmental conditions.
This article is based on a paper written by Linda J. Mason, entomology; Dirk E. Maier, agricultural engineering; and Charles Woloshuk, botany and plant pathology, all of Purdue University, West Lafayette, Indiana, U.S.Table 1 Maximum moisture content for aerated grain storage in temperate climates
Table 2 Aeration times to cool grain for winter storage
|Grain||Up to||Six to||More than|
|six months||12 months||12 months|
Table 3 Residual insecticides for bin and grain treatment
| (cubic meters per minute per tonne)|
|Airflow rate||Cooling time, hours|
|Bacillus thuringiensis||Maize||Best if used as a top-dress treatment to|
|Wheat||prevent or control lepidopterous pests|
|Sorghum||such as Indian meal moth larva.|
|Pirimiphos-methyl||Maize||Can be used as either a grain|
|Sorghum||protectant or top-dress treatment, not|
|both. Is effective on all stored grain|
|Malathion||Maize||Can be used for bin wall treatments,|
|Wheat||top-dress or a grain protectant. Read|
|Sorghum||and follow label directions so that legal|
|tolerances are not exceeded. Do not|
|use for Indian meal moth.|
|Sorghum||Can be used for bin wall treatments,|
|top-dress or a grain protectant, only for|
|the crops listed.|
|Wheat||Diatomaceous earth (D.E.) kills insects|
|Sorghum||by scratching the body surface and|
|Soybeans||causing dehydration. Grain buyers may|
|be reluctant to purchase grain treated|
|with D.E. because of apossible lower|
|grade, reduced flowability, reduced test|
|weight and increased wear on grain|
|moving equipment. Its use as an empty|
|bin treatment, especially beneath the|
|slotted floor, shows promise.|