Oct. 29, 2010

Corn test weights are the pride of farmers some years and their humility the next. So, what does it mean to have high or low test weights? The answer can be somewhat surprising. Corn test weight is simply a measurement of volumetric grain content expressed in pounds.

What does this mean? U.S. No. 1 yellow corn has an official allowable test weight minimum of 56 lb/bu and U.S. No. 2 yellow corn has a minimum of 54 lb/bu. A bushel, by definition is the dry volume equivalent to 8 gallons. In translation, we expect corn to be sold at 7 and 6.75 lb/gal, respectively, for No. 1 and No. 2 grade corn. However, this weight must be for a dry volume.

In U.S. markets, grain sold at moistures greater than 15% or 15.5% will be discounted at the sale to account for drying cost. Additional moisture in the grain translates into higher weights/volume, but this is not truly reflective of a bushel because a bushel is based on the dry volume.

More important is corn yield. Corn yield is

weight/kernel x number of kernels/ear x number of ears/plant x number of plants/acre

While test weight is partially related to the weight/kernel, that is only half the story. The other important factor is how many kernels fit into a specific volume. This is affected by the kernel size, kernel shape, and other seed characteristics.

Although test weight is often thought of as weight/kernel, this is not necessarily true. Depending on conditions during the growing season, test weight and weight/kernel can vary significantly.

For example, in Figure 1a, pretend the kernels are 0.001 lb each for a total of 0.022 lb. In Figure 1b, each kernel is 0.00275 lb for a total of 0.022 lb. The volume they occupy is exactly the same and there is no room for additional kernels in either one.

While this is an over simplification, you can clearly see that seed size and test weight are not correlated because the total weight on each side and the total volume on each side are exactly the same. In this example the weight/kernel is drastically different.

The thing to keep in mind is that, unlike in this example, seed size, seed weight, and seed shape will all vary from kernel to kernel as you harvest your field. Test weights will be a representative sample of all of the kernels you harvest.

Environmental Causes

Environmental conditions contribute significantly to yield, as most farmers know, and also can contribute to great variances in test weights. One example of this would be a killing frost that prematurely causes the corn to black layer. Much like the conditions that were observed in Nebraska last year from early snows on corn plants that weren’t completely mature, the test weights would be much lower than what would have been seen if the corn had a chance to naturally mature.

Other factors such as high moisture, late season disease pressure, ear diseases, and late season drought can cause low test weights. This year, in much of Nebraska, high test weights have been reported. This is likely due to the fact that we had timely rainfalls, especially late in the summer, warm days which helped corn mature timely, and relatively low incidence of widespread late season diseases.

Recommendations

While there is often little or nothing that you can do to affect test weights, particularly if they are affected by late season drought stress or cold conditions prior to maturity, it is important to be aware of how they are affected. If you are continuously seeing low test weights due to cold conditions, try using a shorter season hybrid. This may help increase test weights.

Most importantly though, focus on yields. While test weights may be slightly lower than you like in some years, focusing on yields will help make the most difficult decisions a little easier.

Greg Kruger
Extension Cropping Systems Specialist
Jennifer Rees
Extension Educator
Mark Hinze
Extension Educator