Corn Test Weights

Corn test weight is a popular topic this year. It is a volumetric measurement; the official definition is “the amount of grain contained in 1.244 cubic feet of space”. The US Grain standard is 56 lbs. per bushel for No. 1 yellow corn, and 54 lbs. per bushel for No. 2 yellow corn. This weight is contingent on moisture content; the normal standard use in the grain industry is 15% or 15.5% moisture. I’m not sure how this system started hundreds of years ago, but my guess is that it was used because scales were not as accessible as today and it was just easier to measure the amount of grain in a wagon or corn crib than to actually take it over a scale.


Many growers associate high yields with high test weight grain, and low yields with lower test weight grain. The truth is that there is very little relationship between yield and test weight. We often see the same test weights across high-yielding and low-yielding environments. Higher test weights will allow you to get more lbs. of grain in a given volume of space, such as a truck or a bin.


What influences test weight?


The physical characteristics of the kernel, such as size, density, shape, and texture of the kernel surface, all come into play. Grain moisture is the most important factor to influence test weight. As grain dries down, it also shrinks, allowing more kernels to fit into a given space or bushel, in this case. This fall, we are seeing corn wetter than we have in many years, so it stands to reason that we will see lower test weights because of the higher moisture.


How much test weights will increase after grain drying varies based on hybrid genetics, condition of the grain, and the temperature that is used to dry it. The following table shows some general guidelines to use for calculations.

Some other factors that influence test weights are: 1) plant stresses during grain fill that could include disease, 2) ear rots and molds, 3) insects, 4) soil or fertility conditions, 5) hail damage, and 6) like this year, in many areas, frost hits before corn reaches Black Layer. Any of these events can disrupt the movement of nutrients and sugars to the kernels.


What if my corn didn’t make it to Black Layer?


Black Layer or physiological maturity signifies the end of nutrient accumulation in the kernel. If corn freezes before it reaches this stage, we can expect lower test weights. The University of Minnesota conducted a study on drying immature corn a few years ago. Their results are listed below (KM = kernel moisture, TW=test weight).

In this study, corn that was in the soft dough to early dent stage actually decreased in test weight after drying. Immature corn that was well dented to Black Layer all increased in test weight to nearly 56 lbs./bu after drying.
Once again, in a year like we have just seen, we can expect to see lower test weights because of higher grain moistures. The good news is test weights will most likely increase after drying, but the amount of increase will vary between varieties and their stage of development of the corn at the time of frost.


References:


Hicks, Dale. 2004 Corn Test Weight Changes During Drying. Minnesota Crop News. http://www.extension.umn.edu/cropenews/2004/04MNCN29.htm
Nafzinger, Emerson. 2003. Test Weight and Yield: A Connection? The Bulletin, Univ. of Illinois Extension. http://ipm.illinois.edu/bulletin/pastpest/200323h.htm
Nielson, R. L. 2009. Test Weight Issues in Corn. Corny News Network. Purdue University Dept. of Agronomy. http://www.agry.purdue.edu/Ext/corn/news/timeless/TestWeight.html
Rankin, Mike. 2009. Understanding Corn Test Weight. University of Wisconsin Extension Publication.