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| Wednesday, January 03, 2007 |
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Frequently Asked Questions - Grain Storage Considerations
How does weather affect my stored grain? To get an answer to your own question, click here
How does weather affect my stored grain? In the winter, cold air outside the bin cools the stored grain along the walls causing moisture (water vapour) to migrate downwards and inwards into the grain mass. When it reaches the warmer centre area it migrates upward and as it reaches the cooler top surface, condenses and moisture accumulates. This is a critical area where spoilage and other problems can occur (Fig. 1. A). 
During summer months the reverse occurs whereby warm air outside of the bin warms the grain at the edge causing moisture to migrate upwards and turn in at the top of the grain mass. From there, moisture moves downward through the middle of the grain mass, which is cooler, and condensation and moisture accumulation occurs at the bottom (Fig. 1. B). For this same situation, some researchers suggest that moisture may begin to accumulate anywhere from 0.3 to 1.0 metre below the surface of the grain. High moisture levels found near the bottom of the bin result when snow contaminates the grain, melts, and flows down through the grain accumulating in that region. In either case, it is important to monitor the grain for moisture and temperature changes.
What is the difference between aeration and “near-ambient” drying?
Aeration systems provide a means to lower stored grains to the lowest possible temperature and to equalize the temperature throughout the grain mass. Typically, aeration systems have airflow rates that range from one to two Litres of air per second per cubic metre [one to two [L/s]/m³]. With one Litre of air per second per cubic metre it should take between 150 and 200 hours of fan operation to change the temperature of the grain in a bin, 2[L/s]/m³ requires about half the time. This is why it is necessary to run the fans for a period long enough to equalize the temperature throughout the grain mass. The purpose of aeration is to reduce the temperature of damp or wet grain extending the time it can safely be stored until better drying conditions become available. “Near-ambient” drying (figure 2) works on the principal that dry air is helping to reduce moisture from the stored grain. In this system, air temperature should be above 10° C and relative humidity (RH) below 70%. This kind of system requires greater airflow rates (typically 10-20 [L/s]/m3) than those found in aeration systems. The purpose of “near-ambient” drying is to reduce the moisture content of the grain to a safe allowable level before spoilage occurs.In a ‘near-ambient’ drying system the grain below the drying zone will have a moisture content which is in equilibrium with incoming air conditions. Table 1 shows the equilibrium moisture content for cereals and canola. Table 1. Equilibrium Moisture Content
Relative storage time for cereals and oilseeds depends on the combination of grain temperature and moisture content. Oilseed requirements are different from cereal requirements as shown in the charts below.
What kinds of perforated flooring materials are available? Producers can choose between perforated ducts or perforated flooring. The perforations must be sized to prevent the seed from passing through. The material should be at least 10% perforated. The amount of perforated floor or duct area should be sized so that the air velocity through the total perforated area is less than 0.15 m/sec. Floors and ducts may be fully perforated or partially perforated. Fully perforated flooring allows the greatest flow of air and the best air distribution and provides the best means to facilitate drying or temperature alteration. Partially perforated systems may restrict airflow and reduce or affect the air distribution. What is the best way to fill the bin with grain?
With increased levels of fill there will be increased requirements of time to aerate or dry stored grain. Grain that is peaked will also require more time to have drying fronts or cooling fronts pass completely through the bulk mass. Fig. 5 shows relative drying time for three different scenarios. To help facilitate airflow producers should try to reduce the amount of fine material and broken grain that might contaminate the stored grain. Since this material tends to accumulate in the center part of the grain mass it is helpful to have ‘fines’ spreaders available to distribute the fine material over a wider area. Passing the grain through a simple screen machine can also reduce the contamination levels. Producers who experience problems after the bin is filled may alleviate the problem by auguring some grain out of the central area of the grain mass. Where can I find more information about selecting the proper fan? In order to facilitate the race to condition grain before spoilage occurs, it is important to have proper equipment. Selection of a suitable fan to accomplish “near-ambient” drying is imperative. To help you select the proper fan size there is a program called, ‘Grain 89’ that can help. ‘Grain 89’ is part of the Canadian Grain Storage CD-ROM that is downloadable from the following website, CANSTORE VERSION 1.0, http://res2.agr.ca/winnipeg/storage/pages/cnstr_e.htm. Or call Tannis Mayert at 204-984-6494 to order the CD-ROM.
Which insect control measures should I consider? Sanitation is the key to minimizing potential insect problems. Producers should clean storage bins completely and consider applying registered insecticides, if necessary, prior to filling the bin. Residues of old grains and organic matter should be discarded in a manner than prevents further infestation. Cleaning the surrounding area where bins are positioned is also important since long grassy areas and areas with spilled grains often house insects that later infest the grains in the bin. Weatherproofing grain bins will help reduce the chances for deterioration in conditions required for safe storage. All openings should be repaired and, when not required, aeration ducts should be well sealed. If grain is not dry, aeration should begin as soon as possible. When weather conditions are such that drying is no longer likely to occur, producers should then focus on trying to lower the grain temperature to 0° C as soon as possible. For the first few weeks of storage, it is important to monitor your stored grain often - at least once every two weeks. You can do a simple check by probing the grain and feeling for hot spots or crusted areas, or you can rely on a more elaborate electronic measuring system. Changes in temperature indicate that further action is required, if temperature remains constant over time, the grain has stabilized and monitoring times can be reduced. However, if your grain is stored at higher than recommended moisture content you need to check the grain from time to time to ensure it is keeping properly. When weather conditions change you will likely need to resume aeration or take other remedial action. What kinds of organisms should I look for? Common insects found in stored grains are rusty grain beetle and the red flour beetle, (Fig.6). However, there are many more insects that you might find. A list of beetles can be found on the website maintained by the Cereal Research Centre, AAFC. Mites can infest stored grains and they do damage similar to grain beetles (Fig. 6). High moisture contents and warm temperatures facilitate development of these insects. Also, various moulds can grow in stored grains that have high moisture content or warm temperatures. These moulds contribute to deterioration of grains over extended time and they also attract other insects that feed on mould. These moulds will cause grain quality degradation by creating foul odours and decomposing the grain. The by-products formed by some storage moulds are called mycotoxins and these are potentially dangerous to livestock or humans consuming products made from the grain. Temperature requirements to keep moulds from developing are usually cooler than what is required to halt insect development. To control mould development producers need to drop the grain temperature down to -8°C.
Fig. 6. Organisms that can be found on stored grain. What is the current tolerance for insects in stored grain? Canadian law currently adheres to a ‘zero-tolerance’ ideal. This means that even one insect found in a sample of stored grain is grounds for refusal. Grains that are infested must be controlled before delivery will be accepted. For Saskatchewan, if fumigation is the chosen method, it must be performed by persons holding a valid fumigation license. See the following web page for more control options http://res2.agr.ca/winnipeg/storage/pages/smmry_e.htm
What are Spoutlines and how can they be prevented or eliminated? Spoutlines are areas within stored grains where the density of the stored material is higher due to increased levels of small weed seeds and broken kernels. These particles tend to accumulate in the center of the stored grain forming a core that is difficult for forced air to penetrate. Consequently, the producer’s attempt to condition the grain is thwarted due to impeded air flow rates resulting increased chances for insect infestation and mould development. The best way to check for spoutlines is to sense changes in the force required as you push a probe through from the edge of the grain into the center. If you detect such changes you should remedy the condition by removing the grain and cleaning it. Alternatively, with the use of a centre draw unloading auger, you can draw the center material out and eliminate the spoutline. Agriculture and Agri-Food Canada, Cereal Research Centre, res2.agr.ca/winnipeg/cgs_e.htm Acknowledgements Dale Risula, P. Ag.,
Integrated Cropping Management Systems Specialist, SAF | ||||||
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