Corn marketed at the standard moisture content of 15.5% and 56 lbs per bushel typically contains 47.3 lbs of dry matter and 8.7 lbs of water. At harvest, a farmer has to decide whether to sell (or even store) his corn at ‘as is’ moisture content or mechanically dry before taking it to the buyer. For proper storage, grain moisture content has to be reduced to acceptable levels. Bruce McKenzie from Purdue University suggests 15.5% for storing up to six months, 14% up to 12 months and 13% for more than one year. Total weight loss while drying grain to desirable moisture levels is called ‘shrink’ and grain buyers usually take into consideration how much actual grain they will have after it is dried down.
Moisture Shrink, Shrink Factor, and Total Shrink
Moisture shrink can be calculated by the following formula:
For example: Corn with 25% initial moisture and 15.5% after drying has a moisture shrink of 11.24% Moisture Shrink Factor is determined as
From the example above, water shrink is calculated as 1.18% for each point of moisture removed when the final moisture is 15.5%. The table below shows the moisture shrink factor when drying corn grain at different moistures.
(% Shrink per point)
The total shrink accounts for both moisture shrink and handling loss. When shelled corn is dried from 25% to 15.5% total weight reduction is 11.24% of the original weight. When handling loss is added the total weight reduction would be greater than 11.24%. To estimate the total shrink or total weight loss grain buyers generally use a constant shrink factor which can range from 1.2% to 1.5%. The handling loss alone can range from 0.22 to 1.71% and from the research conducted at Iowa State University, less than 1% handling loss is considered ‘reasonable’. This handling loss can vary depending on corn quality, drying method, and the handling throughout drying. When the constant shrink factor increases, total handling loss also increases.