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Department of Agronomy

Harvesting Grain from Freeze-damaged Sorghum

Grain sorghum in Kansas is often subjected to a killing freeze before maturity. When this happens, test weights may be unacceptably low and producers must decide what to do with the grain.
It is crucial to make a test cutting and analyze it for test weight and yield potential before making any decisions. Once the information from test cutting is known, management decisions can be made. Options are:

  • Accept the test weight as harvested at normal combine settings and sell the grain at a discount without further cleaning.
  • Improve the test weight by adjusting combine settings to remove more foreign material and broken kernels.
  • Clean the grain after harvest.
  • Process and feed the grain to livestock on-farm.
  • Graze or harvest the sorghum for silage instead of grain.

 Other considerations:

  • How an early freeze affects grain sorghum
  • Discounts for low-test-weight sorghum
  • Methods for improving low test weights
  • Economics of cleaning and combine adjustments


How an early freeze affects grain sorghum
Freeze damage lowers the test weight of grain sorghum. In general, the less developed the sorghum is at the time of the killing freeze, the lower its test weight. Low test weights of combine harvested, freeze-damaged grain sorghum are due primarily to high levels of foreign material and broken kernels. A killing freeze prior to maturity stops the growth of the seed, creating small, lightweight grain, which may be shriveled and difficult to thresh. Signs of freeze damage include watery patches on plant leaves, followed by necrotic white lesions on the affected area. A light freeze may kill the leaves, but the grain may continue to fill until the stalk dies. Death does not occur until the stalks have been frozen, breaking the flow of nutrients to the grain.


Discounts for low-test weight sorghum
Grain sorghum with a test weight less than 45 lbs/bu is normally considered unmarketable at elevators and feedlots. Grain with test weights between 45 and 55 lbs/bu is marketable, although at a discount (Table 1). Few premiums are paid for grain grading better than U.S. No. 2, making test weight improvement above 55 lbs/bu unnecessary.

Table 1. Discounts for low-test weight grain sorghum, reported by a survey if Kansas elevators. Discount rates vary at different elevators and in different years.

Test Weight
(lb/bu)

Discount
($/cwt)
55 or greater0.00
54-54.90.02
53-53.90.05
52-52.90.08
51-51.90.11
50-50.90.16
49-49.9*0.19
48-48.9*0.22
47-47.9*0.25
46-46.9*0.29
45-45.9*0.34
Less than 45not accepted

*Some elevators may reject grain with test weights less than 50 lb/bu.

Methods for improving low test weights
The test weight of freeze-damaged grain sorghum can be improved by removing foreign material and broken kernels from the grain. This can be accomplished by either adjusting the combine for better threshing and foreign matter removal, or by cleaning the grain after harvest.

Research at Kansas State University tested these two methods on freeze-damaged grain sorghum. About two-thirds of the unmarketable samples, with a test weight of less than 45 lbs/bu, were improved to marketable condition by better threshing and foreign material removal. Nearly all samples were improved to marketable condition by cleaning with scalper screens and an air aspirator.

In freeze-damaged sorghum, combine adjustments and cleaning also will improve the visual quality of the grain. In contrast, sorghum that reaches maturity before the first killing freeze shows no appreciable increase in test weight or visual quality from combine adjustments or cleaning.


Economics of cleaning and combine adjustments
More aggressive combine settings and/or post-harvest cleaning will improve the test weight of freeze-damaged grain sorghum, but the amount of marketable grain will be reduced. Therefore, measures to improve test weight may or may not be economical. The profitability of improving test weight depends on the initial test weight and yield of the sorghum (obtained from a test cutting), the market price of the grain, and the value of the clean-out material when the grain is cleaned. Freeze-damaged sorghum should be harvested whenever the value of the grain exceeds harvesting costs.

Guidelines presented in Table 2 discuss the economics of the various harvest management options for handling and marketing grain from freeze-damaged sorghum. They are based on costs that include an average custom harvest rate of $17.45 per acre.

Table 3. Yields from test cuttings required to justify harvest using combine settings for better threshing and foreign matter removal.

Price per cwtInitial yield from test cutting (lb/acre)
$3.00750-870
$3.50670-750
$4.00600-670
$4.50540-600
$5.00480-540
$6.00420-480
$7.00370-420
$8.00320-370

Figure 1. Yields from test cuttings required to justify cleaning costs of
grain sorghum with test weights of 33-40 lbs/bu.*

*These guidelines are conservative in that they assume the clean-out has no value. Finding a market for the clean-out would decrease the yield requirement necessary to break-even.


Other alternatives
To avoid discounts, or the cost of cleaning or better combine settings, freeze-damaged grain sorghum may be fed to livestock on-farm. Low-test weight grain sorghum can make good livestock feed as long as it’s properly processed. Numerous trials in Kansas indicate that the feeding value of 40- to 60-pound grain sorghum is similar, on a weight basis.

Other options for freeze-damaged grain sorghum include cutting it for ensilage or grazing it. For more information, see KSU Forage Facts Publication, “Forage Sorghum Silage” and MF-1018 “Nitrate and Prussic Acid Toxicity in Forage.”

For future crops, producers can reduce the risk of freeze damage by using shorter season sorghum hybrids. KSU’s “Grain Sorghum Production Handbook” provides guidelines on management practices that will allow for optimum sorghum yields, even with shorter season hybrids. Producers must decide whether the occurrence of freeze in their area warrants possible yield limitations of shorter season hybrids.


Kraig Roozeboom, Extension Specialist – Crop Production, Cropping Systems
Leland McKinney, Extension Specialist – Grain Science and Industry
Grain cleaning and economic analysis based on: Sipes, J. 1993. Improving marketability of low test weight, prematurely frozen grain sorghum. MS Thesis. Kansas State Univ., Manhattan, KS.