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NE MO & W IL | Exactly...kernel weight is not entirely the same as "test weight". Test weight is a volumetric measure. I talk to sooooo many growers that cannot grasp this concept. Hopefully, this article and Corn Guy's articles can help explain it. This is one of the most misunderstood concepts among growers today.
October 2009 Insights Newsletter – Heartland South
Factors Influencing Test Weight
I’ve been taking several calls dealing with test weight as the 2009 corn harvest has just begun. Before we can truly understand factors influencing test weight or if lower test weights are a big concern, we have to understand what test weight is or specially, what it measures.
The test weight concept was developed many years ago by the grain trade as a means of accounting for the varying densities of grain that could cause increased storage or transportation costs. Therefore, test weight is a quality measure of how many pounds of corn will fit into a certain volume (1.25 cubic feet or 1 bushel). If the weight is lower than the acceptable range (usually < 54 lbs/bushel for No.2 yellow corn), the sale is "docked" on a percentage or per bushel basis. When we define the scientific development of “good test weight”, we are specifically talking about the accumulation of dense starch in the kernel. Higher test weights mean better filled kernels with a higher percentage of hard endosperm making them denser and allowing more bushels fit in a given volume.
Test weight is basically an indicator of grain quality that is influenced by plant genetics and the environment. Test weight or “dense starch development” is most often influenced by stresses that occur during the grain-filling period of the plant. Factors that decrease the rate or duration of grain fill can result in lower test weights at harvest. These factors include inadequate AND/OR excessive soil moisture, nutrient deficiencies, lack of sunlight, foliar leaf diseases, insect damage to leaf and stem tissue, frost, and hail. There are also genetic differences in hybrids and varieties, some producing harder endosperm kernels. Although test weight may be a consideration when selecting hybrids or varieties, don’t make it the only one at the expense of other important characteristics such as yield or disease resistance.
Another reason test weights may be perceived as lower than normal is the fact that we are harvesting wetter than normal corn due to late plantings and/or poor fall drying conditions. Grain moisture and test weight are inversely proportional; as moisture increases, test weight decreases. For example, corn at 20% moisture will have a test weight that is 2 pounds lower than the same corn dried to 15.5% moisture. It doesn’t matter whether the drying is done naturally in the field or artificially in a bin. As moisture decreases, the kernels shrink and pack together tighter allowing for more kernels to fill a volume bushel. More kernels mean more dry matter or dense starch in that given volume which increases test weight. Research at the University of Illinois and Iowa State University showed mature corn increased in test weight when dried to 15% kernel moisture (see chart below).
Yield is neither improved nor minimized by test weight. Test weight is a volumetric quality measure of grain density and yield is completely a weight measurement of grain harvested per acre. Grain merchandisers buy grain based on a standard weight (56 lbs/bu) and reduce the purchase price if test weight, moisture and/or grain quality are below minimum standards. Falling below standards or buying some extra truck fuel is how “low” test weight can hurt your bottom line, not through lower yields. I would much rather have 200 bu/ac corn (11200 lbs/ac) at 55 lb/bu test weight than 190 bu/ac corn (10640 lbs/ac) at 60 lb/bu test weight. Sure, I might have to make a few extra trips to town, but I’m netting more dollars per acre, right or not?
Edited by agger802 11/20/2009 22:27
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Bu. weight on corn revisited. This article was posted
