Careful drying can increase test weights
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Careful drying can increase test weights

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Grain bins with dryer

Editor’s note: The following was written by Emerson Nafziger, University of Illinois Extension agronomist, for the university’s Bulletin website.


One concern with late maturity and harvest of corn is low test weight, which in some cases can mean price dockage at the elevator.

Test weight is “bulk density,” or the weight of corn grain per unit of volume, expressed as pounds per bushel (one bushel is 1.24 cubic feet). The “standard” test weight for corn is 56 pounds per bushel, and this weight (not a volume of 1.24 cubic feet) is the standard unit for marketing corn grain.

Test weight is easily measured but is not easily understood: It’s a complex characteristic, affected by kernel shape, kernel density and the slipperiness of seedcoats, which affects how well kernels slide past one another. These are all affected both by genetics and by how the crop grows, matures and is dried for storage.

There are two main reasons for low test weight. The first is the premature end to kernel filling that can result from poor growing conditions, disease of leaves or ears, severe drought or frost that comes early or that occurs before late-planted corn is mature. Starch deposition in kernels starts at the crown of the kernel moving towards the base, and when the movement of sugars into the kernel stops before kernels are full-sized, the base of the kernel may be shrunken.

The starch deposited under poor conditions can also be less densely packed on the endosperm. The result can be kernels that don’t weigh as much as usual and that don’t fit together very well, both of which can lower test weight.

The market typically rewards sound, dense kernels, so shrunken kernels may be docked in price, with the amount of dockage tied to test weight. Test weight acts as a proxy for harder- to-measure things such as starch content or kernel density.

Kernels that don’t fill completely may also contain sugars that didn’t get converted to starch, and these can darken during heated-air drying, causing additional dockage due to kernel damage.

The last time we saw a substantial amount of this was 2009, when cool weather prolonged the season, and test weights were as low as the mid-40s in some fields. We don’t believe there was much of this in 2019, but some fields planted very late with normal-maturity hybrids might not have finished filling before freezing.

The other main reason for low test weights is having high grain moisture when test weight is measured. This is complicated — if all other kernel characteristics stay the same as kernels dry, loss of water weight might lower test weight. But loss of water from kernel starch usually causes starch granules to pack together more tightly, which increases kernel density and test weight.

We did a small experiment in 2017 to see how test weight responded as kernels dried down. We selected ears from several different trials, with different hybrids and management factors, including planting date, with the goal of starting with grain at about 30% moisture.

At the start and then every few days, samples were stirred, grain moisture was taken using a GAC tester, and test weights were taken using an official (funnel) test weight apparatus. Testing continued for two to three weeks, and ended once grain moisture dropped to the low teens.

While responses of test weight to grain drying varied some among sources of samples, trends were fairly consistent. From about 30% to about 27-28% moisture, test weight tended to drop by a pound or two, presumably as the kernel lost moisture (2 percentage points of moisture weighs about 1 pound per bushel) but little else changed.

From 25 down to about 15% moisture, test weights tended to rise more or less as a straight line. The increase in test weight as moisture dropped from 25 to 15% ranged from 5.3 to 7.7 pounds per bushel, and averaged 6.1 pounds per bushel. As moisture dropped below 15%, the rate of increase in test weight slowed, but stopped as moisture dropped to 10 to 11%.

I don’t know if corn grain delivered at, say, 28% moisture with a test weight (measured on wet grain) of 53 pounds per bushel gets docked in price, but if the grain is bright and sound, there is likely to be flexibility in assigning dockage amounts, especially if high-test-weight dried grain is being delivered as well.

Our results show that drying the grain (very gently, without heated air or mechanical disturbance) to 15% moisture should increase test weight of such grain by at least 5 pounds per bushel. Mechanical handling, such as moving grain through augers, rubs kernels together, which might increase test weight a little more.

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