Wheat Test Weight – What is It?

Robert Kratochvil, Extension Agronomist, University of Maryland; rkratoch@umd.edu

Most reports I have heard this year about wheat test weight indicate this measure of crop quality is running lower than usual. Why is this? In order to answer this question, let’s look at what test weight is and what characteristics comprise it.

Test weight is the quantity (measured in mass or weight) of wheat that can be contained in a standard volume. The standard volume used in the U.S. is the bushel (32 quarts). The required weight of wheat in a bushel for No. 1 and No. 2 U.S. Soft Red Winter Wheat are 60 and 58 lb/bu, respectively. What are the components of test weight?

The first is kernel density, i.e. the weight of each individual kernel. Each kernel is composed of the bran (seed coat), the germ, and the endosperm (the largest component). Kernel density is influenced by both genetics (varieties of sound wheat can differ by as much as 3-4 lb/bu) and environment (weather). The endosperm is primarily comprised of carbohydrates (starch) with protein woven among the starch granules. Starch weighs more than protein. The tighter the starch molecules are woven within a kernel, the better the test weight. The cooler than usual and wet weather that caused corn planting problems this year has not been conducive for maximum grain fill; consequently, starch development and packing in the kernel has not been tight leaving more air space and resulting in lower test weight. There has also been more disease pressure for the crop this year resulting in lower starch production.

The second component is kernel shape. This characteristic also is influenced by both genetics (varieties do have subtle shape differences) and the weather. Kernels that are more ovate and smooth (no humps, bumps, and wrinkles) will pack better than kernels that are more angular and less smooth. Weather also causes differences in kernel shape. Test weight is greatest when the crop first reaches suitable moisture content for harvest (approximately 14-15% moisture content). Rain events even when moisture content is as high as 18-20% interrupt the dry down process causing the kernels to swell slightly. Even though the kernels will dry down to suitable harvest moisture following the rain event, they do not re-shrink to the size they had attained prior to that event and test weight is reduced. The more rain events that interrupt harvest, the more swelling events resulting in a continuing decease in test weight.

What can be done to minimize elevator penalties caused by low test weight? When selecting wheat varieties, not only choose those with the best yield potential, but also select those with good disease resistance, and high test weight. This practice will give you the best opportunity to produce a crop with better than average at-harvest test weight. Another practice a number of farmers are using is to begin harvest of the crop when moisture content reaches approximately 20% and using a grain dryer to dry the crop to acceptable moisture content. This will help capture the crop’s test weight at its maximum. This practice is particularly beneficial if the weather forecast for an upcoming harvest week will be rainy but you have couple of good harvest days before the rain is predicted to occur.

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