You pull onto the scales at the local elevator, a probe takes a sample and you head to the corn dump pit to unload. While you are anxious to return to the field for another load, the elevator office staff records the moisture and grades your corn with a light test weight, which is less than the 54 lbs per bushel for #2 yellow corn. You knew you had some diplodia fungus in the field, and its damage to the kernels just pulled the profitability rug out from under your corn crop. Why is test weight so important?

Purdue’s Corn King, agronomist Bob Nielsen, says test weight is one of the 10 favorite discussion topics anytime farmers gather with a coffee mug in their hand, particularly on a rainy day during harvest season. His comments on test weight will help many producers understand the significance of what the grain elevator staff is trying to determine when it weighs and measures a sample of your corn.

The definition of #2 yellow corn requires that a volumetric bushel be 54 lbs. in weight. The 56 lbs that most farmers cite is for #1 yellow corn, which is the grade that usually originates on a Cornbelt farm in the fall. And Nielsen adds that US corn is marketed on the basis of a 56 lb. bushel regardless of test weight. Of course the elevator need not take an entire bushel to weigh, but will take a smaller sample that will be measured by its electronic moisture tester. Nielsen says, “These test weight estimates are reasonably accurate but are not accepted for official grain trading purposes.” While US grain standards include test weight, the moisture content has been removed, however the standard dry bushel cannot have moisture more than 15.5% or the buyer will impose a discount.

Test weight may be on the ticket, but it will show up primarily on the settlement sheet, because payment will be on the basis of 56 lb. corn. If kernel damage from diplodia or any other problem reduces the test weight, that 1,000 bushel truckload of corn quickly became 929 bushels of corn, since the test weight was 52 lbs. per bushel and not 56. Regardless of the volume of grain in the truck, its net weight will be divided by 56 lbs. for sale purposes. If your corn was more dense than the 56 lb. bushel standard, and a volumetric bushel weighed 60 lbs. then you would be paid for 1071 bushels, although the load was only 1,000 bushels by volume.

The density of the kernel is a function of many factors, one of which is the hybrid, but yield does not appear on that list of factors. And one hybrid can have one test weight in one field and a different test weight in another field. And it will vary from year to year as well, but its yield will not correlate with test weight, says Neilsen.

The test weight is an important factor for processors, since they will benefit from high test weight corn and their starch or oil output will be less efficient from a low test weigh corn. At least that is the conventional wisdom, whether or not research supports it, says Neilsen. One significant factor is that the quality of the lower test weight corn may be due to other factors that caused the reduction in test weight.

So, why did your corn this year have a low test weight? Neilsen says there were six potential causes of lower test weights for 2009 corn.
1) The wetter the corn, the lower the test weight. Corn weighs more than water, so dry corn weighs more than wet corn, and as corn dried in the field or the dryer it shrank, and picked up weight allowing more kernels to squeeze into that bushel volume. Therefore, when your wetter corn was tested at the scale house, it was destined to have a lower test weight.
2) Drought stress in a few spots around the Cornbelt this year.
3) Gray leaf spot and northern corn leaf blight.
4) Cooler than normal September temperatures that interfered with starch production in the kernel.
5) The early October freeze and frost which damaged immature kernels and halted the grain filling process.
6) Widespread damage from diplodia and other fungal ear rots, which deteriorated the quality of the kernel content desired by ethanol refiners, cattle feeders, and other end users.

Summary:
Test weight is an indicator of the density of the corn kernel and low test weights for many truckloads of corn in 2009 have resulted in farmers being paid less than they expected when delivering a truckload of corn. The same volume that may have measured 1,000 bushels in 2008 may have only measured 925 bushels in 2009 because of environmental factors that interfered with kernel development. Those may have included fungal issues, the early frost damage on immature corn, and the fact that wetter corn was being delivered, since wetter corn weighs less than dry corn.