The Value of High Test Weight Corn Grain

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Field Facts written by Agronomy Sciences.

According to corn extension specialists, low test weight grain may be due to any number of causes that reduce kernel fill: lower temperatures, leaf diseases or drought stress during the grain fill period, premature frost damage to late-developing fields and ear rots. Hybrid genetics also has a large influence on test weight. This article discusses test weight and its implications for grain marketing, transportation, storage, quality and feeding.

For Grain Trading, “Bushel” is a Weight Measure – Although “bushel” is technically a volume measure, grain is marketed by weight, not by volume. For this purpose, a “bushel” of corn is defined as 56 lbs. of grain at a moisture content of 15.5%. But because grain differs in kernel hardness or density, the volume occupied by 56 pounds of grain may vary substantially. Grain mass per unit volume, or density, is most important in grain transportation and storage. Denser kernels require less space, which means that fewer truckloads and storage bins are needed to store the same weight of grain. Denser grain also has higher quality characteristics for some end uses.

What is Test Weight? "Test weight" is simply a measure of grain bulk density. An official test weight measurement uses standardized equipment to determine the mass (weight) of a sample quart of grain, and then converts this to a pounds per volumetric bushel basis. Because transporting and storing lower density grain is more costly (on a weight basis), buyers discount grain if test weight is below the minimum standard set by the USDA (Table 1). In addition to density, test weight is affected by how kernels pack in a container, which is influenced by kernel shape and surface friction.

Table 1. Minimum test weight standards
for USDA corn grades*.

USDA Corn Grade	Test Weight (lbs/bu)
1	56.0
2	54.0
3	52.0
4	49.0
5	46.0

* Damaged kernels, % broken corn and foreign
material also affect the USDA grade.

As the table shows, the minimum test weight for No. 2 corn is 54 lbs/bu. Loads testing below this minimum will usually receive a dockage at the elevator.

Transportation and Storage

Higher weight per unit volume of grain has obvious advantages in harvest, handling and storage. More bushels fit in a combine grain tank, on a truck or in a grain bin. These advantages are demonstrated in the following example:

Compare 80 acres of a high test weight hybrid such as Pioneer® brand 35K04 with a low test weight competitor. Assume test weights of 56 lbs/bu and 52 lbs/bu for the 2 hybrids.

The test weight advantage on a percentage basis is 11% (58 - 52 = 6; 6 / 52 = 0.11). Therefore, in a given volume you can hold 11% more of the Pioneer hybrid.
A common semitrailer would hold about 900 bu. for the lower test-weight competitor, and 11% more, or 1000 bu. for the heavier Pioneer corn.
- If the Pioneer hybrid yielded 200 bu/acre this would fill 16 semi loads (200 bu/acre x 80 acres = 16,000 bushels. 16,000 bu / 1000 bu per semi load = 16 semi loads.)
-The same weight of corn for a competitive hybrid with test weight of 52 lbs/bu would require 18 semi loads to haul the grain. (16,000 bu / 900 bu per semi load = 17.8 semi loads.)
The Pioneer hybrid will occupy 11% less space in the combine grain tank as well. For this reason, it is easy to assume that higher test weight hybrids are lower yielding, even when they are higher yielding. (Growers should also calibrate their yield monitors for high test weight grain as well as normal and low test weight grain to ensure accuracy.)
A 40,000 bu grain bin would hold an additional 4,400 bu. of the higher test weight Pioneer hybrid.

Grain Quality and Feed Value

High test weight grain is generally considered to be higher quality because it is preferred by corn wet millers and food processors. In addition, some methods of determining corn breakage have shown less breakage with high test weight grain. Denser grain often has a higher proportion of starch-rich endosperm and a lesser proportion of bran and hull, resulting in a higher metabolizable energy content for livestock feed.

Test Weight and Moisture Relationship

Test weight and grain moisture are inversely related, at least in the range of 20% to 30% moisture where most corn is harvested. As corn dries, the kernel shrinks and becomes more dense, and the kernels pack tighter together due to less surface friction. Studies have shown that corn can gain up to 0.25 pounds of test weight for each point of moisture removed. For example, corn at 28% moisture with a test weight of 54 lbs/bu, may have a test weight of 56 lbs/bu when dried to 20% moisture.