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Test Weight in Wheat (07/20/17)

During my travels to western North Dakota last week to attend field days, growers grappling with the decision of whether or not to hay their wheat crop were expressing concern about the test weight of their crop if taken to harvest.

Test Weight in Wheat

During my travels to western North Dakota last week to attend field days, growers grappling with the decision of whether or not to hay their wheat crop were expressing concern about the test weight of their crop if taken to harvest. I did not anticipate that question, assuming the main criterion for not taking their crop to harvest would be its yield. Their rationale was that with the current price of spring wheat, harvesting the crop for grain would be more profitable than haying it. However, that would only be the case if it had sufficient test weight so that it would not be downgraded to feed (currently below 52 lb/bu is the value level for feed grade, as posted by one elevator).

Test weight is the measure of the weight of grain that can fit in a standard volume and in the case of wheat is expressed as pounds per bushel. Test weight in wheat is a measure of its quality. High-test weight wheat usually has relatively more extractable flour and less bran and is therefore more valuable to the end-user. The minimum test weight for No. 1 grade is 58 pounds per bushel and discounts usually are applied when values fall below that value. To reach this standard, kernels must be dense and well filled so that they pack well. Kernel density is effected by the ratio of starch to protein and how tightly the starch granules are woven into the kernel. Factors that impeded starch deposition during grain filling, such as leaf diseases, scab, lodging, drought and high temperatures can reduce kernel density. How well kernels pack is determined largely by their shape. Smooth, rounded kernel pack more tightly than wrinkled ones. Shriveled kernels that are not completely filled will have a low test weight.

With that background, the critical question is how will current droughty conditions impact test weight. One grower at the field days mentioned to me that based on his experience with a similar drought, that his crop had very good test weight, even though the yield was low. This was because there were lower stem numbers (few tillers survived the drought) and smaller spikes than normal, so the remaining kernels could fill well. Another field day participant, from South Dakota, mentioned that he had heard of wheat harvested from a drought stressed area this year that had a test weight of greater than 60 lb/bu. I do not have enough experience with end of season drought to be able to look at a crop and estimate its test weight prior to maturity. Basic biology would suggest that the following factors ought to be considered when trying to determine if low test weight could be a problem with your maturing crop.

  1. Consider the stage of kernel development and the status of the plant. Plants with limited green tissue will not have the capacity to produce the carbohydrates needed for optimum kernel development. The flag leaf is the primary source of photosynthate for the developing kernels, so the health of the flag leaf is of particular concern. Stressed plants that are in the early phases of grain filling are at greater risk of poor kernel development than those that are near maturity. Also, note that plants with taller and thicker stems have the capacity to mobilize more carbohydrates to the developing kernels than short, spindly ones. In many areas of the state, early stress this season resulted in not only fewer surviving tillers, but also stems that were shorter than normal.
  2. Temperatures that exceed 85 degrees during the day and 60 degrees at night during grain filling hasten kernel development at a faster pace than the rate at which the kernel fills. Under these conditions kernel will reach physiological maturity before all of the space available for carbohydrate deposition are filled. As these kernels dry, they shrivel as the seed coat collapses over the voids in the unfilled kernel.

 

Joel Ransom

Extension Agronomist for Cereal Crops

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