Posted on Sep 17, 2018
By Brent Bean, Ph.D., Sorghum Checkoff Agronomist
Late-planted grain sorghum can suffer from injury and reduced yields when an early fall freeze occurs. Typically, a temperature of 26 degrees or lower is required to kill a sorghum plant, but damage to the grain can occur at higher temperatures. The grain is susceptible to injury until it has reached physiological maturity, which usually occurs 40 to 45 days after flowering, depending on heat unit accumulation.
Little injury to the grain occurs at temperatures just below freezing. However, growers can expect yield reduction and lower test weights once temperatures drop to 28 degrees or lower for two hours or longer, even though the plant may not die. These temperatures reduce the sorghum plant's ability to move carbohydrates to the developing grain. Reduction in photosynthesis by the plant due to damage to the leaves also plays a role in reduced yield.
The amount of yield loss depends on the growth stage of the sorghum, how low the temperature drops below freezing and the length of time the temperature remains below freezing. In a study conducted at Kansas State University, when plants were exposed to 28-degree temperatures for four hours, grain weight was reduced between 6.5 and 27 percent in hard dough grain and between 27 and 52 percent in soft dough grain.
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.
Once sorghum reaches the hard dough stage, which is when the grain has turned color and can no longer be mashed by squeezing it between one’s fingers, the kernels have achieved approximately 75 percent of their final weight. For this reason, yield reduction seldom is reduced more than 20 percent by an early freeze.
Reduced test weight of freeze-damaged grain sorghum is due primarily to an increase in foreign matter and broken and shriveled grain. Any grain with a test weight 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.
To avoid discounts, growers may choose to market the grain directly to a livestock feeder or feed the grain to their own livestock. Research has shown that, on a weight basis, the feeding value of 40- to 60-pound grain sorghum is similar.
Alternatively, growers can improve the test weight by changing combine settings and/or by performing postharvest 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.