Authors: Ignacio Ciampitti, Mary Knapp

The last “Crop Progress and Condition” report from Kansas Agricultural Statistics documented that grain sorghum maturity and harvest are behind as compared with last year. Will 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.

Weather component

Similar to 2012 season, the hot and dry conditions during the early vegetative stages accelerated the 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) delayed heading and decreased seed set (number and size), affecting final yields. Grain sorghum is even more sensitive to heat stress around flowering (10 days before, and 10-20 days after flowering).

Sorghum is also sensitive to cold temperatures during most of its growth period. Temperatures below 40 degrees F will inhibit sorghum growth. Previous K-State research by Scott Staggenborg and Richard Vanderlip documented an impact on the grain weight early during the grain filling period when temperatures were below 30 degrees F. The low temperatures at this time caused lower photosynthetic rates and the inability of the plant to translocate carbohydrates to the developing grains.

Crop phenology

The amount of time between emergence and half-bloom will depend on the planting date and the temperatures (cumulative growing degree days) during this period. There are also hybrid differences in the amount of time it takes to 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 (visualized as a “black-layer” near the seed base) 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 in Figure 1 below show accumulated GDDs up to October 7 for the current growing season when half-bloom began at three different dates: mid-August, late-August, and early September. Lower GDDs are depicted with blue colors, while higher GDDs are represented in red colors.

If blooming occurred during early- to mid-August the likelihood for maturing before freeze is almost 100% for all the state. If blooming occurred during late-August, the sorghum in a small section of western Kansas (e.g. Greeley, Wallace, Logan, Thomas, and Rawlins counties) will have a lower chance of maturing (having accumulated less than 1100 GDDs) before the first freeze. A worst picture is projected if sorghum was blooming around early September for western area of Kansas (blue, green, and light green colors), which involves more than 20 counties. In this case, there is a very low probability of maturing before the first freeze (very low GDDs, < 900).

Accumulated GDDs from mid-August through October 7

Accumulated GDDs from late-August through October 7

Accumulated GDDs from early September through October 7

Figure 1. Accumulated Growing Degree Days (expressed in °F) for August 15-October 7, August 31-October 7, and September 7-October 7. The maps show that sorghum that reached half-bloom on August 15 should have accumulated enough GDDs by October 15 to have a good chance of maturing before the first fall freeze almost everywhere in the state. If the sorghum reached half-bloom on August 31 or September 7, the GDD accumulation as of October 7 is lower and prospects are less certain, especially in northwest Kansas. The darker the red, the higher the number of accumulated GDDs.

As of October 9, the lowest temperature recorded for this month was close to the freezing point (32 F) for Greeley, Wallace, Sherman, Thomas, and Logan counties. The freezing temperature registered during the last week could potentially affect sorghum fields in those counties. Even if most of the yield potential was realized, the test weight will be affected (smaller as compared with a full length of grain-fill). For the counties surrounding those previously mentioned, slow grain fill with smaller rates of dry mass accumulation in the grain can be expected with the temperatures recorded during the last week for western Kansas.

Figure 2. Map of the lowest temperatures recorded for the month of October 2013 for the entire state of Kansas. The deeper the blue, the lower the minimum temperature (30-34 degrees F); the deeper the red, the higher the minimum temperature (48-55 degrees F).

Management considerations

From a management perspective, the best way to mitigate this issue is to plan in advance. Recommended practices are just related to improve the use of different maturity for sorghum materials and also consider the planting date as another factor to strategize when planting sorghum:

If the sorghum is killed by a freeze before maturity, producers should first analyze the crop for the test weight and yield potential before deciding grazing or harvesting the grain sorghum for silage.

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