Sorghum development and potential freeze injury
The latest “Crop Progress and Condition” report from Kansas Agricultural Statistics, on October 6, stated that grain sorghum maturity and harvest are behind normal by 14%. Will remaining sorghum reach maturity before first freeze? The answer is, “it depends.” There are two main factors involved: 1) weather conditions and how they affected the development of sorghum during the season, and 2) crop phenology -- when was the crop planted, hybrid maturity, and the date of half-bloom.
Similar to 2013, the hot and dry conditions in some areas early in the season -- before flowering -- accelerated emergence and the vegetative progress, but delayed heading. Also, K-State researcher Vara Prasad and others found that high temperature stress after growing point differentiation (approximately 30 days after emergence) delays heading and decreases seed number and size, affecting final yields. Grain sorghum is even more sensitive to drought and heat stress around flowering -- 10 days before, and 10-20 days after flowering (Figure 1).
Figure 1. Pre- and post-flowering grain abortion associated with heat and drought stresses. Photo by Ignacio Ciampitti, K-State Research and Extension.
Sorghum is also sensitive to cold temperatures during most of its growth period. Temperatures below 40 degrees F will inhibit sorghum development. Previous K-State research by Scott Staggenborg and Richard Vanderlip documented an impact on grain weight during early grain fill when temperatures were below 30 degrees F. Low temperatures at this time caused lower photosynthetic rates and the inability of the plant to translocate carbohydrates to the developing grains, interrupting the flow of sugars from the plant to the head.
The amount of time between emergence and half-bloom depends on the planting date and the temperatures during this period. If sorghum was planted late and temperatures were cool early in the season, half-bloom will occur relatively late in the season. Hybrid maturity characteristics also make a difference in how rapidly the crop will go from emergence to flowering. Short-season hybrids have a shorter time from emergence to blooming; while full-season hybrids will need more degree days to reach flowering. The overall cumulative GDD from flowering to maturity, or “black-layer,” is around 1400-1600, with the shortest requirement in GDD for the short-season as compared to the full-season hybrid. From maturity to the harvest time, sorghum grain will dry down from about 35 to 20 percent moisture, but the final maximum dry mass accumulation has been already attained at maturity.
The likelihood of sorghum maturing before a freeze is related to all of these factors. When the crop flowers in late August or early September, it may not reach maturity before the first fall freeze in some parts of the state.
Probability of sorghum maturing before freeze for different flowering dates
The maps below show accumulated GDDs up to October 6 for the current growing season when half-bloom began at three different dates: mid-August, early-September, and mid-September. Lower GDDs are depicted with blue colors, while higher GDDs are represented in red colors.
Where blooming occurred during early- to mid-August the likelihood for maturing before freeze is almost 100% for all the state. Where blooming occurred during early September, the sorghum in the northern and central areas of Kansas will have a lower chance of maturing, having accumulated less than 1100 GDDs, before the first freeze. A worse picture is projected if sorghum blooming did not occur until mid-September. In this case, there is an extremely low probability of maturing before the first freeze (very low GDDs, < 900) almost everywhere in the state.
Half-bloom occurred in mid-August
Half-bloom occurred in early September
Half bloom occurred in mid-September
Figure 2. Accumulated Growing Degree Days (expressed in °F), as of October 6, for grain sorghum at three flowering dates: August 15, September 1, and September 15. The darker the red, the higher the number of accumulated GDDs.
As of October 6, the lowest temperature recorded for the season was close to or below the freezing point (32 F) for the northwestern area of Kansas, which includes the counties of Cheyenne, Rawlins, Decatur, Sheridan, Norton, Graham, Trego, Phillips, Rooks, Smith, Osborne, Greeley, Wallace, Sherman, Thomas, and Logan. Many of these low temperatures were reported in mid-September. These freezing temperatures could potentially affect sorghum fields. Even if most of the yield potential was realized, the test weight might be reduced.
Figure 3. Map of the lowest temperatures recorded from August 15 to October 6. The deeper the blue, the lower the minimum temperature (30-31 degrees F); the deeper the red, the higher the minimum temperature (37-44 degrees F).
The average day for the first fall freeze is earliest in northwest Kansas and latest in south central and southeast Kansas.
Figure 4. Average day of first fall freeze (32 F).
From a management perspective, the best way to mitigate this issue is to plan in advance. Recommended practices are:
- Use early planting dates for full-season hybrids, or
- When planting later, use medium- to short-season hybrids
If the sorghum is killed by a freeze before maturity, producers should first analyze the crop for test weight and yield potential before deciding grazing or harvesting for silage. Freeze damage may cause problems with drydown in sorghum.
For more information on this, see “Harvesting Grain from Freeze-damaged Sorghum,” K-State publication MF-1081: http://www.ksre.ksu.edu/bookstore/pubs/mf1081.pdf
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library