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Understanding Corn Shrink

Shrink is the loss of weight caused by drying and handling the grain and is expressed as a percentage of the original quantity. It is a loss of revenue associated with mechanically drying corn. Corn buyers account for shrink by discounting the price per bushel that they offer (i.e. a charge per bushel) or by calculating the reduction in bushels. When producers are considering whether to sell corn at harvest or store on-farm, all costs, including shrink, must be evaluated.

Moisture Shrink
When corn is dried, the largest component of weight loss is the removal of moisture. This loss of mass is called moisture shrink and is calculated by multiplying a moisture shrink factor by the percentage of water removed.

The moisture shrink factor (% weight loss per % moisture removed) is equal to: 100/(100 – final moisture %)

For example, corn dried to 15% moisture has a shrink factor of 1.176 (100/85). Moisture shrink factors are independent of the initial corn moisture percentage. Moisture shrink factors are dependent on final moisture percentage.

The moisture shrink factor is a constant for each final moisture level achieved. For example, the moisture shrink (weight loss) is 1.176% for each point of moisture removed when the final moisture content of the corn is 15%.

As mentioned earlier, moisture shrink (% weight loss) equals the percentage of moisture removed multiplied by the moisture shrink factor. For example, corn dried from 25 to 15.5% moisture (loss of 9.5 percentage points of water) has a loss of mass equal to 11.23% (9.5 x 1.183). In this example, the percentage reduction from the original weight (11.23%) represents the amount of water removed when drying 25% moisture corn to 15.5%.

Handling Loss
Although the majority of weight lost when drying corn is a consequence of water removal, some dry matter is also lost. Called handling loss (sometimes called invisible shrink), this small reduction in mass results from broken kernels, foreign material, loss of oil or other compounds, and continued respiration of the seed kernels. However, handling losses are usually quite small relative to water loss.

The amount of handling loss depends on several factors, including the physical quality of the corn, the method used to dry the corn, how long the corn is dried, and how the corn is handled during drying. According to Hicks and Cloud (2001), research at Iowa State University estimated on-farm handling losses ranging from 0.22 to 1.71%; and losses from commercial drying systems ranged from 0.64 to 1.33%. Hicks and Cloud found the 3-year on-farm handling loss average was 0.82% compared to 0.88% for the commercial facilities.

Calculating Total Shrink
Grain buyers will account for both moisture shrink and handling loss. This is called total shrink and to calculate buyers will often use either drying tables or a constant shrink factor. Grain drying tables include a water shrink loss plus a loss from handling. Often this handling loss is assumed to be 0.5%. Using the example above, corn dried from 25 to 15% moisture has a moisture weight loss equal to 11.76% (10 x 1.176). Adding in a handling loss of 0.5% gives an assumed total weight loss of 12.26%.

Another method used to calculate shrink is to use a constant shrink factor; often 1.3, 1.4, or 1.5 per percentage point of moisture removed. A constant shrink factor is used to account for both water and handling losses. Total shrink (% total weight loss) is found by multiplying the percentage moisture removed by the constant shrink factor. For example, assuming a constant shrink factor of 1.4, corn dried from 25 to 14.5% moisture (loss of 10.5 percentage points of water) has an assumed loss of mass equal to 14.7% (10.5 x 1.4). In this example, the estimated water loss equals 12.27% (10.5 x 1.169). Therefore the grain buyer is assuming a handling loss of 2.43% (14.7 – 12.27).

The handling loss assumed increases as the constant shrink factor is increased. The handling loss is found by subtracting the water shrink from the total shrink.

Corn shrink occurs whether the grain is stored on-farm or at a commercial facility. Shrink is a reduction of grain weight (mass) and results when corn is dried to a moisture content that is safe for storage. When corn buyers apply a constant shrink factor to estimate shrink (e.g. 1.4), they are accounting for moisture and handling losses. 

Source: iGrow

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