Does trying to get a grip on the concept of “test weight” all but drive you loony? Don’t feel bad. Even specialists acknowledge it’s a hard idea to wrap your head around.
“Test weight is an odd data point,” explains Mark Licht, Extension cropping systems specialist at Iowa State University. “Environmental conditions influence it, hybrids can influence, and grain moisture can influence it.
“In reality, we use it because it does an OK job of indicating the ‘quality’ of the corn and how long the corn can stay in storage. Higher test-weight corn stores better than lower test-weight corn.”
While all of that is true, that likely leaves you with questions about test weight and its relative, kernel weight. Two Midwest Extension corn specialists, Dan Quinn at Purdue University and Chad Lee at the University of Kentucky, provide insights:
Why don’t kernel weight and test weight for a hybrid correlate? Instead, some hybrids with high test weight have lower kernel weight and vice versa.
Lee: Test weight is a measure of weight per volume. It depends partly on how many kernels can pack into a bushel volume and how heavy each of those kernels is that packed into that bushel. Kernel shape, kernel size, kernel moisture and kernel coats all can affect how many of the corn kernels will pack in. You can have some very large kernels that might not settle into a bushel as well as some smaller kernels would. As grain dries, the kernel coats become slicker, and more kernels can pack into the bushel.
Quinn: Test weight and kernel weight measure different grain characteristics. Kernel weight is the average mass of individual kernels, while test weight is a volumetric measurement that reflects how densely kernels pack into a standard volume, defined as pounds per bushel. Factors such as kernel size, shape, density and moisture content influence how tightly kernels settle and pack together, thereby affecting test weight.
For example, a hybrid with small, round, dense kernels may have a high test weight even if individual kernel weight is low. Another hybrid may produce larger, flatter kernels that weigh more individually but don’t pack as tightly, thus resulting in a lower test weight. This is why crops like soybeans, popcorn and wheat each have different standard test weights based on kernel characteristics, not individual kernel mass.
Some lower-yielding hybrids have high test weight. Why don’t test weight and yield correlate?
Lee: Here in the U.S., we still measure yield as bushels per acre, which is the volume of grain per acre. Volume and weight are not the same thing.
I keep waiting for comedian Nate Bargatze and “Saturday Night Live” to do an entire “Weights and Measures” on agriculture. Why did we shun the British monarchy but decide that their measuring system was OK to use?
We base length off the “foot” of some old king. We kept their version of a bushel, which was a volume of produce believed to be easily carried by a horse or mule. We decided that 4 pecks equals 1 bushel was easier than 1,000 grams equals 1 kilogram. We decided that 43,560 square feet in 1 acre was much easier to remember and calculate than 10,000 square meters in 1 hectare.
Can’t you hear Nate Bargatze saying, “Down with the British and their taxes on tea. But, you know, I really like 43,560 square feet. We should keep that.”
“But, sir, each foot in 43,560 square feet is based on the size of King George’s foot.”
“You know, he does have nice feet. We’ll stay with feet.”
Quinn: Yield is a measure of total grain mass per acre, known as bushels per acre, determined by kernel number and weight per unit area. Test weight, on the other hand, reflects the density of harvested grain or how many pounds of corn fit into a bushel. That is a volumetric measurement.
A field can produce a high yield through a large number of well-filled kernels, even if those kernels are less dense or irregularly shaped, which would result in lower test weight. Conversely, a field with fewer but dense, well-packed kernels may show high test weight despite lower yield.
Since yield and test weight measure different outcomes, it’s common for them not to correlate, especially across hybrids or environmental conditions. Overall, yield tells us how much grain we produced. Test weight tells us the weight of grain within a defined volume.
Why do test weights vary among corn hybrids?
Quinn: Hybrid genetics influence kernel traits such as shape, size, hardness and density, all of which can affect test weight. Some hybrids may produce smaller, rounder, denser kernels that pack more tightly into a bushel, while other hybrids may produce larger, flatter or softer kernels that don’t pack as efficiently.
Additionally, different hybrids may vary in their balance between kernel number per ear and kernel size. A hybrid with many smaller kernels per ear may achieve higher test weight, while a hybrid with fewer but heavier kernels may yield similarly but exhibit lower test weight. This variation reflects each hybrid’s physiological characteristics rather than differences in performance or grain quality.
Lee: There is believed to be some genetic controls over it. Environment can influence it as well. People have spent years studying it. I have tried to explain it. But I think Nate Bargatze of SNL fame said it best: “No one knows.”