There is always relief post-harvest, when the grain is off the field, safe from the perils of drought, floods, pests, disease, hail and frost. Once finally in the bin, you can breathe easy, right? Not quite. Even though safe from outside threats, stored grain still needs protection.
How the grain goes into the bin — wet, dry, hot or cool — will determine how well it stores and what sort of conditioning will help keep it in top shape. Every farmer’s worse nightmare is to discover rot or hot spots in the bin. Even worse is being called out to a bin fire caused by over-heated grain.
Conditioning grain is not as simple as just running fans. Rather, the interaction of inside and outside temperature and moisture should be tracked to ensure that best results are achieved.
When a crop comes off the field hot, the first goal is to cool it as quickly as possible, particularly during the first 24 hours. If the outside air is cooler than the grain — and it’s not rainy or too humid — the fans should be on. Humid air is anything above 70 to 80 per cent relative humidity and will add moisture to the stored product. The target cooling temperature for most grains is around 15 C.
Uniformity is also important as temperatures can fluctuate within the various parts of the bin. For larger bins that can take several days to fill, turning the fans on early will help to get a head start on cooling and drying. Keep in mind that larger bins, when full, have reduced airflow.
Sometimes there is no choice but to harvest grain that is wetter than ideal. In this case, the goal should be to bring down the moisture content. Canola is considered dry at 10 per cent moisture, wheat at 14 per cent, barley (malt) at 13.5 per cent and soybean at 14 per cent. Most other crops range from 14 to 16 per cent. Anything above those levels should be dried down for safe storage.
If only slightly wet, one per cent to two per cent above dry, natural air drying will normally suffice. Circulating cooler outside air through warm grain will have a drying effect. The greater the difference in temperature between the grain and the outside air, the more drying should occur. This will be the case down to about 5 C, at which point moisture will remain relatively constant.
Equilibrium moisture content, or EMC, is a useful indicator of the air’s ability to affect the moisture content of stored grain. When air is being blown into the bin, it is important to recognize whether it will ultimately cause drying or wetting. Based on temperature, relative humidity, and the type of grain that is being stored, the EMC value represents the moisture content that would be achieved if current conditions were to persist.
The graph above shows the relationship between relative humidity and EMC for wheat, canola, and soybean at 15 C. As an example, at 55 per cent relative humidity, the EMC for canola is about 7.5 per cent. If conditions were to remain stable, the canola would eventually reach that level of moisture. If it were wheat, the EMC at 55 per cent relative humidity would be just over 13 per cent.
If air temperatures were to decrease, from 15 C to say 10 C and the relative humidity remains at 55 per cent, the EMC would increase by about half a percent. Likewise, if temperature were to increase to 20 C, at constant relative humidity, the EMC would decrease by about half a percent.
There are certain limitations to EMC. One is that the temperature within the bin is assumed to be similar to the air outside. This is rarely the case as outside air will continuously fluctuate while bin temperatures are slow to change.
When the outside air is cooler than the grain inside the bin, drying is likely to occur. However, if the grain is colder than the air outside, the air will cool when it encounters the grain. The reduction in temperature decreases the water-holding capacity of the air, causing the relative humidity to increase, thus increasing the moisture content within the bin.
Given that temperature and relative humidity are constantly changing, it is important to continuously monitor EMC to ensure that the natural air is achieving the intended results.
Check the Bins
Bringing grain to the ideal temperature and moisture will greatly increase the chances of safe storage. But despite best efforts, bad things can still happen. For this reason, checking bins, whether manually or with automated sensors, or preferably both, remains extremely important and will help detect small problems before they have a chance to escalate. Considering the time, effort, and cost involved in getting the grain into the bin, these steps will help to ensure that it remains in top condition. FF