NDSU Extension Service, Published August 23 2012
Grain yield not related to test weightFARGO - Don't confuse grain yield with test weight, cautions North Dakota State University Extension Service agricultural engineer Ken Hellevang.
Grain yield is expressed as bushels per acre, and the number of bushels is determined by dividing the pounds of grain by a standard bushel weight. For example, a bushel of wheat has been determined to be 60 pounds per bushel. Therefore, grain yield is the pounds of grain harvested per acre divided by the standard weight.
So, does the test weight affect the yield? The answer is "no" unless the number of pounds of grain being harvested from a field changes.
"Because a bushel is defined in two ways, confusion may occur unless it is specified as volumetric bushel or weight bushel," Hellevang says. "The pounds of grain in a bin will be affected by test weight, but the pounds of grain harvested per acre are not affected by test weight."
Test weight is a density measurement that is used as an indication of grain quality but is not a factor in determining grain yield in bushels per acre. It is how many pounds fit into a specific volume. It is expressed as pounds per bushel, with a bushel being a volume of 1.244 cubic feet.
The procedure for determining test weight is accurately weighing a small container that has been filled using a specific method and then converting that to pounds per bushel.
The size and shape of kernels, moisture content and the composition of the kernels primarily affect the weight of grain fitting into the volume or test weight. Large kernels or kernels that do not fit tightly together generally will have a lower test weight than smaller kernels or kernels with less void spaces between them.
Wheat test weight will decrease after each time it is rewet due to the kernels swelling when wet and not returning to their original size as they dry. The pounds of wheat being harvested have not changed, but the test weight is reduced because the kernels are larger, resulting in a lower density.
"The lower test weight might affect market price but does not affect the pounds of wheat harvested," Hellevang says.
The grain moisture content will affect the test weight because a kernel with high moisture content will be larger than a kernel with lower moisture content and because of the difference in density between water and the grain kernel dry matter.
Gently dried grain will increase in test weight as it is dried. The amount of increase will vary depending on drying method and amount of kernel damage during harvest, but the increase generally will be about 0.3 to 0.5 pound for each percentage point of reduction in moisture.
A rule of thumb is the increase will be about 0.25 pound per point of moisture for corn dried in a high-temperature dryer. For wheat drying under good field drying conditions, the test weight increase is expected to be near 0.5 pound per bushel per point of moisture removed.
The composition of the kernels affects the test weight, Hellevang says. For example, low test weight can occur due to less starch in the kernel resulting from the kernel not maturing. This also will reduce the total pounds of grain harvested, so yield will be reduced. Low test weight is one indication of immature kernels.
If rewetting is enough to initiate the germination process, some of the kernel is consumed, resulting in less weight. Grain respiration and mold growth also can consume a little of the grain dry matter, with the loss being greater at higher moisture contents and temperatures.
An allowable storage time chart can provide an estimate of the time required for a 0.5 percent loss in dry matter. For example, the allowable storage time of cereal grains at 20 percent moisture content and 80 degrees is about seven days.
If some of the kernel is consumed, the pounds harvested will be lower and the test weight likely also will be reduced.
"This typically is not a factor in yield because normally the grain does not stay at high moisture contents during field drying," Hellevang says.