Much ink has been devoted to the nationwide dilemma of pre-harvest damage to mature soybean seed that occurred because harvest of the mature crop was delayed by lengthy periods of rainy weather. However, once these weathered seed are harvested and stored in on-farm facilities, the period of concern is not over just because these seed have been removed from the adverse field environment.
Weathered/damaged soybeans that are harvested will carry with them to the storage facility the agents–i.e. pathogenic organisms–that caused the damage in the field. Thus, if the storage facility is not equipped to maintain conditions that will prevent these field fungi plus storage fungi from proliferating during storage, then the damage that was occurring in the field before harvest will only get worse in storage. Remember, newly harvested soybeans, even those that have weathering damage, are at their highest quality when they come from the field to the drying/storage facilities. So here are a few pointers to ensure that the situation does not deteriorate further during storage after harvest. Click here for the reference resource from the Univ. of Arkansas that was used to compile the following points.
• When in storage, soybeans are dried to get rid of the excess moisture in the stored seed so that there is a lessened chance of activity by pathogens that are on the seed. Fungal activity in stored soybeans is enhanced by high moisture levels and high humidity. Soybeans are typically dried to a seed moisture content of 12 to 13% to prevent fungal activity.
• The quality of the drying air that is used in the process determines the moisture content in the stored seed.
• A specific volume of air at a certain temperature can hold a specific amount of moisture. Thus, the drying capabilities of air entering the storage bin can be increased by adding energy (heat) to that air or by passing larger volumes of ambient air over the stored seed. See Table 15-1 in the above-linked article for minimum air flow rates (cubic ft./min./bu.) for drying soybeans with a known moisture content.
• The equilibrium moisture content (EMC) of soybean seed in a storage bin is the moisture content that seed in the bin will maintain at a given air temperature (AT) and relative humidity (RH). See Table 15-2 in the above-linked article for soybean EMC’s at selected AT’s and RH’s, and note the AT and RH necessary to maintain EMC’s in the 12-13% range.
• The preferable moisture content for soybean seed at harvest is about 14%. A producer with storing and drying capability can harvest at higher moisture contents to avoid potential weathering damage to seed that are still in the field when unfavorable weather patterns occur.
• To avoid soybean seedcoat cracking while in storage, keep the RH of drying air above 40%. This places a heavy emphasis on careful monitoring of drying air temperature since hotter air will lower the RH of that air.
• The proper temperature of drying air is determined by the final intended use of the stored seed; e.g., seed that will be used for oil and food production can be dried at temperatures no higher than 130°F, while seed that will be used for planting should be dried at air temperatures no greater than 100°F.
• Batch and continuous-flow driers should not be used for drying soybean seed because the heat input may be too high. Thus, bin drying systems–natural-air and low-temperature drying–are likely the best drying options for soybeans.
• Natural-air drying (unheated air) can be used under favorable weather conditions that will allow a net moisture transfer from the stored soybeans to the passing air. A general rule is that natural air can be used to achieve and maintain EMC in a normal year if AT is above 60°F and RH is below 75%. Thus, natural-air drying is sensitive to prevailing weather conditions.
• Low-temperature drying may be needed in a cool, wet fall. With this technique, the drying air is heated to 10° to 20°F above ambient conditions. This drying technique requires the same bin components as natural-air drying.
• Regardless of drying technique used, RH of the drying air should be >40%. Since RH of the drying air is dependent on the temperature of that air, drying air temperature should be closely monitored during the drying process.
• It is generally accepted that stored soybeans should be kept at a temperature no greater than 40°F to prevent damaging fungal activity.
• Grain bin fans are used to circulate air through grain in the storage bin, and these fans determine the rate of air circulation. Click here for a Univ. of Arkansas publication that provides details for the selection, performance, and maintenance of grain bin fans.
There is little if any remedy for the damage to soybean seed that will occur when adverse weather conditions prevent timely harvest of mature seed. However, following proper storing and drying techniques will prevent further deterioration of seed after they are harvested and placed in storage facilities.
Composed by Larry G. Heatherly, Dec. 2018, email@example.com