Test Weight

Description

Test weights are measured on grain loads by weighing a known volume of grain, which is used to compare grain densities. If lower test weights than the accepted standard are recorded than more grain volume is needed for storage or transportation. If high test weights are recorded than less grain volume is needed. Low test weights tend to result from poor grain fill and environmental conditions in the field before harvest.

The Test weight concept was developed many years ago by the grain trade as a means of accounting for the varying densities of grain caused by weather and/or production practices. When grain density is lower than the accepted standard (low test weight), more volume is needed to store and transport a given weight of grain, thus increasing storage and transport costs. Different grades of each grain have different standard test weights. No. 2 yellow corn has a standard of 56 pounds per bushel while No. 3 yellow corn has a lower weight. Test weight is determined on each load of grain sold by weighing a known volume of the grain. If the weight is lower than the acceptable range, the sale is "docked" on a percentage basis. The seller of grain with test weight greater than the acceptable range is usually not rewarded for a superior product. Varieties of a crop often vary in their inherent test weight. Two common causes of low test weights are:

1. Grain is prevented from filling completely and/or maturing and drying naturally in the field due to a killing frost, hail, or insect damage. When this happens the starch molecules inside the grain are prevented from the natural process of shedding absorbed water molecules that allows the grain to shrink to a normal size. Artificial drying with heat removes this excess water but the starch molecules do not shrink and grain size doesn't change appreciably, so test weight (density) remains low.
2. Grain matures and dries naturally in the field but is sometimes reweted by rainfall, dew, or fog causing the grain to initiate the germination process (precocious germination) before harvesting. During germination, oil, starch, and protein are digested to provide energy and molecules to produce a new seedling. This process leaves small voids inside the grain. Although the grain may again dry in the field, the seed size does not change and the small voids inside the seed result in a decreased test weight. Maximum test weight is achieved when grain is harvested on the first dry-down and also at higher moisture. For example, the ideal harvest moisture for soybeans and corn is 16-19% moisture and 20-25% moisture respectively.

Read More: http://ohioline.osu.edu/agf-fact/0503.html