Posted on Oct 16, 2019
By Brent Bean, Ph.D., Sorghum Checkoff Agronomist
The unusually cool, wet weather throughout much of the U.S. this spring resulted in more grain sorghum acres planted later than normal. Late-planted sorghum, like any other summer crop, runs the risk of reduced yield and quality due to an early freeze prior to maturity. Fortunately, high temperatures in August and September across several regions helped late-planted sorghum mature quicker than expected.
Under normal conditions, once grain sorghum has flowered, it takes approximately 40-45 days to reach maturity. This process can be accelerated or slowed down depending largely on accumulated heat units. Drought stress can also slow down the maturation process.
The amount of yield loss will depend on the stage of the grain sorghum when it freezes. Only 30-50 percent of the grain's final weight has been accumulated at soft dough stage, when the grain can still be mashed between the thumb and forefinger. A freeze during this growth stage will drastically impact yield. However, 75 percent of the grain's final weight has been obtained once the hard dough stage has been reached. Normally, the grain will reach maturity in 10-14 days once hard dough has been reached. Yield loss will range from 2-27 percent at this stage, depending on how close the grain is to maturity. Every day with good growing conditions can greatly influence the crop yield late in the season.
Source: Staggenborg, Scott, A. and Richard L. Vanderlip. 1996. Sorghum grain yield reduction caused by duration and timing of freezing temperatures. Agron. J. 88:473-477
A light freeze may only kill the sorghum's leaves and not the stalk. The grain should continue to accumulate weight if the stalk has not been killed.
Freeze damage typically causes low test weight with small shriveled, grain that may be hard to thrash. Reduced test weight of freeze-damaged grain sorghum is due primarily to an increase in broken kernels and foreign matter. Any grain with a test weigh below 55 pounds per bushel likely will be discounted because it can no longer be sold as U.S. No. 2 grade sorghum. The discount amount will vary depending on the amount of the reduction in test weight.
Research has shown the feeding value of 40- to 60-pound test weight grain sorghum is similar. To avoid or reduce discounts, growers may choose to market the grain directly to a livestock feeder.
Alternatively, growers can improve the test weight by changing combine settings and/or by performing post-harvest cleaning to remove foreign material and damaged kernels. Both of these alternatives will lower the total yield but may improve the marketability of the grain. The Kansas State University publication “Harvesting Grain From Freeze-Damaged Sorghum” is an excellent resource that discusses options for dealing with freeze-damaged grain sorghum.