Information Management to Maximize Your Yields

Understanding Corn Test Weight

From agAnytime, Monsanto

Hundreds of years ago grain was sold by the bushel (32 quarts). This was because it was easier to see a volumetric measurement than it was to weigh grain accurately and consistently. Then in 1916, the United States Grain Standards Act was passed. It included the concept of measuring grain test weight to account for differences in grain density.

The USDA established the standard test weight of a bushel of corn as 56 lb/bu based on 15.5% moisture content.

Any stress that prematurely stops or reduces grain fill and/or interferes with photosynthesis has the potential to lower yield potential as well as test weight. Grain yield is determined by the number of kernels per acre along with the weight per kernel. In comparison, test weight measures the weight of corn in pounds that will fit into a bushel. Yield is a direct measure of kernel weight and kernel number.

However, test weight is not a direct factor of grain yield. Test weight is only partially related to kernel weight because there is also the volume component associated with the measurement. Factors that affect test weight but not corn yield are those that influence how kernels fit or pack together. These may include slipperiness of the seed coat as well as kernel shape or size. Due to its volume component, test weight will influence how many bushels a grower can fit into a bin, wagon or truck, but not yield per acre.

Many think yield will always be lower if test weight is lower and vice versa. If this were true, corn products with high test weight grain would regularly outyield corn products with average test weight grain. Yield can be high and test weight can be low. For example, if the time for grain fill is shortened the seed size may be smaller but the density of the individual seeds remains unchanged.

Conversely, yield can be low and test weight can be high. Popcorn is a good example of this as it is more dense than field corn and has a relatively high standard test weight of 65 lb/bu compared to 56 lb/bu for field corn. Yet, average popcorn yields are half as much as field corn. In some instances, corn test weights may be lower than expected due to changes in kernel weight from stresses during kernel fill.

Lower kernel weights can also result in lower yield and therefore, the test weight can be low as well. It is important to remember that changes in yield and test weight are not always proportional or correlated.