The test weight-high moisture connection
At harvesttime many farmers talk about test weight. It’s a ratio between weight and volume, and it’s measured as the weight of grain that fills 32 quart containers. There are 32 quarts in a bushel basket.
So the weight of grain in 32 quarts is test weight. It’s really not a fixed quantity or value. Corn is marketed here on the basis of 56-pound test weight per bushel. At 15% moisture, corn with 56 pounds per bushel is considered No. 1 yellow corn, and 54 pounds per bushel is called No. 2 yellow corn. You’re docked for corn with less than a 56-pound test weight, but aren’t credited if corn is higher than 56 pounds.
Test weight is highly related to genetics in corn.
Other factors can affect test weight, including grain moisture.
Test weight should increase once grain is dried.
Why test weight varies
Higher test-weight grain maintains its integrity better during shipping. It’s a highly inherited trait strongly influenced by hybrid genetics. Hybrids with higher test weights are most likely to maintain those values, and you have a lesser chance of getting docked.
Test weight can vary by year, field, date of planting and disease susceptibility. However, if you compare two hybrids in side-by-side test plots, the hybrid with higher test-weight values in your seed book from your seed company will remain higher than the hybrid with the lower test-weight value, unless other factors like disease resistance affect them differently. In other words, a hybrid with higher test-weight value hypothetically can become the victim of a leaf or ear rot disease, which adversely affects yield, moisture and test weight.
A lot of corn was planted late this year. Also, we had many cooler nights during this growing season, which affected maturity. Higher temperatures during late August helped most late-planted corn reach physiologic maturity, which is attained at 32% to 35% grain moisture.
However, lots of corn isn’t going to dry in the field to 15% moisture this year, and therefore may have lower test weight at harvest. But when it’s dried and the water content decreases, test weight will increase. Dry matter has a higher test weight than water. Dried grain shrinks and gives a higher test weight because you pack more kernels in a bushel basket.
Late-season leaf and ear diseases can decrease test weight because they interrupt grain fill.
Does it matter?
So how important is test weight in your hybrid selection? There’s no correlation between test weight and yield. However, if I have to decide between a hybrid with highest yield potential and average test weight of 56 to 57 pounds vs. a hybrid with highest test weight but average yield, I will pick the first one as long as both have similar tolerance to diseases, unless I’m growing food-grade corn and getting a premium for test weight.
Let’s hope for a good drying season so you can bring in a bumper crop of high test-weight grain!
Nanda is director of genetics and technology for Seed Consultants Inc. Email him at [email protected]
This article published in the November, 2014 edition of INDIANA PRAIRIE FARMER.
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