As the 2010 Illinois corn harvest gets under way, some farmers are expressing concern over test weights that are lower than they expected.

Emerson Nafziger, University of Illinois Extension agronomist, says, “Many people think test weights in the lower 50s are an indication that yield has been lost, and that there may be other problems they didn’t foresee. In many cases, kernels will seem sound (unlike those from many fields in 2009), even when test weights are 3 or 4 lbs. below the standard 56 lbs./bu.”

So what are these low test weights indicating?

Nafziger says it’s important to distinguish test weight from kernel weight. If yields are high, kernel weights can be normal, even if test weights aren’t. But if poor filling conditions result in shrunken kernels, such kernels may not fit together very well, and both kernel weight and test weight can be low.

Test weight, which is technically “bulk density,” is a complex measurement, including factors such as slipperiness of the seedcoat, kernel shape, endosperm density and even the size of the embryo. There are hybrid differences, but growing conditions also affect test weight. In general, correlations between yield and test weight are not very high.

“In 2009, starch filled very slowly, and in some cases didn’t fill to the maximum extent before freezing ended the process,” he says. “That’s not likely in 2010, except in some areas where dry weather could bring an early end to grainfill. It’s possible that the rapid filling in 2010 resulted in slightly lower density of starch ‘packing’ into the endosperm than normal. This directly lowers test weight, and may or may not result in lower kernel weights. And it’s kernel weights that determine yield.”

For the same reason, he believes endosperm density may not be quite as high as usual this year and that this might affect usefulness as food-grade corn. On the plus side, grain will dry down well in the field and farmers are not likely to see the high-temperature drying problems such as stress cracks and broken kernels that were problematic in 2009.

“Test weight affects pressure plate readings on yield monitors, so it will be important to calibrate yield monitors for this year’s conditions,” Nafziger says. “Instead of focusing on possible lost yield, focus on the big picture.

“Yield per acre is the product of kernel number times kernel weight. Yields are the only meaningful measure of the growing season. If kernels are sound and of normal weight, but test weights are several pounds below normal, we have little to complain about,” he says.