There are two topics you are told not to discuss, politics and religion. But there are two topics most corn growers will discuss some time throughout the year, yield and test weight. Both provide for lively debates and provide many different avenues for arguing way or the other. But in every good “discussion” there needs to be a small bit of truth or fact to really stand firm. Facts on yield are easy, pounds of grain per acre = yield, while the facts on test weight seem to be a little fuzzier at times. The following article will hopefully shed a few truths in the next test weight “discussion” at the coffee shop.
Simply what is test weight?
Test weight is simply a volumetric measurement. An official bushel measures 1.2445 cubic feet or 32 quarts. Test weight is simply the weight of that volume. Typically test weight is measured in a one quart container and then multiplied by 32 to obtain the actual test weight of the bushel. Historically a bushel is a measurement of volume, not weight, but because of the limitations of selling on volume, it was decided that each bushel of corn weighs 56 pounds. Therefore today corn is sold in 56 pound “blocks” called a bushel.
Why does test weight differ between years, fields and/or hybrids?
There are many factors that influence test weight: grain moisture, physical characteristics of the kernel, plant stresses and hybrid differences to name a few. Depending on the year and growing conditions some or all can impact test weight at harvest.
Grain moisture: Grain moisture is probably the largest factor that influences test weight. As the grain moisture decreases, grain test weight increases. Why? There are two reasons: The first is as corn dries it also shrinks, thus allowing more kernels to fit into the quart measurement container. Second, as the corn dries the corn naturally becomes more slick (seen in how “wet” corn stacks higher in wagons than “dry” corn) which allows for better packing.
Determining exactly how much test weight increase there will be when dried is variable. Individual hybrids react differently to drying, the overall condition of the grain as lower quality will gain less than higher quality. Also, the speed and heat that is used can affect the gain of test weight. Grain that is dried at temperatures higher than 180 degrees will have less of an increase due to “cooking” of the grain. But for a general rule of thumb, refer to the graph to the right.
Physical characteristic: The physical characteristics of individual kernels play a huge role in determining test weight. Essentially how well do the kernels pack together in the measuring bucket. The kernels size, density, shape and “slickness” of the outer kernel impact the pack ability and ultimately the test weight.
Plant Stresses: Plant stresses are a major factor in determining the final test weight. Plant stress is defined as anything that keeps or slows the movement of water and nutrients to the kernel during the grain fill period or causes the kernel to be damaged (ear rots & molds). The stresses can include leaf or stalk disease, low fall temperatures, insect damage, poor fertility or poor nutrient uptake, drought, hail and premature frost.
Hybrid differences: Each genetic family carries different test weight characteristics and can greatly influence the final test weight.
What is test weight an indication of?
Grain quality: Test weight is a general indicator of grain quality. The higher the test weight for a given hybrid genetics normally means higher grain quality. For example hybrid A’s normal test weight is 55 pounds, if it carries a 57 pound test weight it generally means it is of higher quality than if it was 54 pounds. Test weights do decrease as grain deteriorates.
Grain storability: Test weight tends to be a good indicator of corn storability. Corn that is below 54 pounds per acre should not be stored into warm weather and dried below 15% moisture. Also softer corn can be more easily damaged during handling.
Is there any correlation to test weight and yield?
Often high test weight is associated with high yields and low test weight with low yields. “Your corn would have yielded more if the test weight was higher,” is a familiar tune that very seldom rings true. According to Mike Rankin, Crop & Soils Agent for University of Wisconsin Extension, “In fact, there is a poor relationship between test weight and yield. The same test weight can exists across a wide range of yield environments and genetics. Similarly, there can be a wide range of test weight values across the same high or low yielding environments.”
Are growers selling test weight or pounds per acre?
There is a difference as to what is being measured. Test weight measures the weight of a “volume” bushel of corn which is equivalent to 32 quarts. But growers are paid on the “weight” bushel that is based in units of pounds. The “weight” bushel of corn is exactly 56 pounds. Therefore in the following example a grower hauled in 85,000 pounds of 54# test weight corn and another grower hauled in 85,000 pounds of 59# test weight, each grower got paid for 1,517.9 bushel.
“Cost” of low test weight corn….
The following example demonstrates how test weight figures into what a grower is paid at an elevator for differing test weight levels.
While the economic factors of test weight are negligible, there is another factor to test weight not discussed to this point and that is the volume piece of the puzzle. The heavier the test weight the more pounds that can be transported and stored in the same size container as compared to lighter test weight corn. The volume line in the above graph demonstrates that point.
Summary:
Corn yield is simply the number of pounds or corn harvested in a certain area, while test weight is the measure of how many pounds of corn can fit into a certain volume. Test weight is only partially related to kernel weight, other factors such as grain moisture, hybrid genetics, the physical characteristics of the kernel and the level of plant stress during grain fill.
References:
Bern, Carl & Brumm, Thomas. “Grain Test Weight Deception.” Iowa State University Extension. PMR 1005. October 2009.
Hurburgh, Charles & Elmore, Roger. “Corn Quality Issues in 2008 – Moisture and Test Weight.” Integrated Crop Management News. Iowa State University Extension. October 2008.
Nafziger, Emerson. “Test Weight & Yield: A Connection?” University of Illinois Extension. Crop Development Bulletin. October 3, 2003.
Rankin, Mike. “Understanding Corn Test Weight.” Extenstion Team Grains. University of Wisconsin. October 2009.