Grain test weight, often used as a grain quality indicator, is a volumetric measurement based on an official bushel being 1.244 cubic feet. For U.S. No. 1 yellow corn, the official minimum test weight is 56 pounds per bushel.

If test weights drop below this standard, the grain price is discounted, says University of Nebraska Extension educator Todd Whitney. Usually, test weights are determined by weighing grain samples filled in a standard dry one-quart measure.

Lower test weights are more common when crops have endured stress at some point during the grain-filling period or when frost ends the growing season prior to physiological maturity. Any disease, insect, or environmental condition that reduces the movement of nutrients to the kernel during grain fill (once it is filled) will likely lower grain test weights.

Test weights can be misunderstood and attributing strong correlation between test weight and grain yields may be incorrect. High-test weights are not automatically associated with high yields, and lower test weights do not always mean lower yields. Many factors influence test weight.

Factors Influencing Test Weight

Grain wetting between maturity and harvest can lower test weights. Research indicates that the number of times grain experiences wetting and drying cycles impacts test weights more than the total amount of precipitation. Prior to wetting at maturity, normal mature kernels are smooth, well-shaped, and fit well into a volume.

If a wetting event penetrates the outer husks on mature corn ears, the kernels may swell. Upon drying, the kernels may not shrink back to their original volume, shape, and smoothness. This results in more space between kernels; thus, the kernels will not pack into a bushel volume as well as before the grain-wetting event. Even though the total dry weight harvested from the field will be the same regardless of the moisture events, it may require extra trucking trips to haul the same field production when test weights drop.

Higher test weight grains have more nutrient density, since the grain has a greater proportion of starch-rich endosperm and less bran and hull. As a result, livestock producers may prefer buying higher test weight grains due to higher energy values. Lower test weight grains can also be good energy sources, but their value per bushel will likely be lower than higher test weight grains.

Hauling High Test-Weight Grain

During harvest, farmers may appreciate higher test weight grain, since theoretically more pounds can be hauled using the same grain trucks (full capacity). This translates into fewer trips hauling grain to the elevator or storage facility to move the same total grain weight. 

Farmers, though, must be cautious when moving high test-weight grain on highways so they don’t exceed truck hauling weight limits. Higher test weight grains take up less storage “volume” for equivalent “bushel weights” than lower test weight grains. Therefore, combine operators may easily overfill trucks when filling them to the usual volumes. Further, harvesters may need to recalibrate their truck “full” lines (based on volume) if grain test weights are higher.

Department of Transportation fines for overweight grain trucks on the highway may exceed the value of the extra grain being hauled. So, especially when grain prices are lower and test weights are higher, it makes sense to check truck load weights prior to the loaded vehicles travelling on the highway. Also, truck drivers may need to communicate to combine operators if trucks are being overloaded.