Authors: Ignacio Ciampitti, Mary Knapp

The most recent “Crop Progress and Condition” report from Kansas Agricultural Statistics (09/28/15) documented that grain sorghum maturity and harvest is ahead of last year’s pace. Will sorghum reach maturity before first freeze? As every growing season, 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) the crop’s life cycle progression through this season -- when the crop was planted, hybrid maturity, and the date of half-bloom.

Weather component

Wet conditions at planting time delayed sorghum planting in some areas of the state, delaying heading. The mean temperatures for August (Fig. 1) were 1 to 4 degrees cooler than normal, with the largest deviation occurring in southeast and south central Kansas. In contrast, mean temperatures were warmer than normal in September. The largest departure from the mean temperature for September was in north central and northwest Kansas.

A delay in flowering time could jeopardize yields if the crop is exposed to heat around blooming or if low temperatures occur during grain fill. Recent K-State research published by Prasad, Djanaguiraman, Perumal, and Ciampitti found that high temperature stress around flowering time (5-days before and after flowering) could impact sorghum’s final grain number. 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.

Figure 1. Departure from monthly mean temperature for August and September, 2015.

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 40 degrees F. Low temperatures at this time caused lower photosynthetic rates and the inability of the plant to translocate carbohydrates to the developing grains.

Crop’s life cycle progression

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 (seen as a “black-layer” near the seed base; Fig. 2) is about 1400-1600, with the shortest requirement in GDD for short-season hybrids. From maturity to harvest time, sorghum grain will dry down from about 35 to 20 percent moisture, but the final maximum dry mass accumulation and final nutrient content will have already been attained at maturity.

Figure 2. Black layer identification in sorghum. Differences in maturity related to the position within the same head should be expected. Pictures and infographic prepared by Ignacio Ciampitti, K-State Research and Extension.

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 3 below show accumulated GDDs up to September 30 for the current growing season at two different points: early August and early September. Lower GDDs are depicted with blue colors, while higher GDDs are represented in red colors.

If blooming occurred during early August the likelihood for maturing before freeze is close to 100% for the entire state. If blooming occurred during mid- to late-August, the sorghum in a small section of northwest Kansas will have a reduced chance of maturing (having accumulated less than 1400 GDDs) before the first freeze. The chances of reaching maturity before the first freeze is a bit less if sorghum was blooming around early September for the same area of Kansas (blue and light blue colors). In this case, the GDDs accumulated in September were low, less than 1100(. Still, the projections for this year are much better than the previous sorghum production year.

Accumulated GDDs from early August to late September

Accumulated GDDs from early to late September

Figure 3. Accumulated Growing Degree Days (expressed in degrees F) for August 1-September 30 and September 1-30. The maps show that for sorghum that reached half-bloom on September 1, the GDD accumulation as of September 30 is lower and prospects are less certain, especially in northwest Kansas. The darker the red, the higher the number of accumulated GDDs.

The lowest temperature in September was close to 32 degrees F in northwest Kansas. Even if most of the yield potential was realized by that time, the test weight will be affected (smaller seed size as compared with a full length of grain-fill) by temperatures that cold. For the northwestern area of the state, low temperatures translate into slower grain fill with smaller rates of dry mass accumulation in the grain; affecting final yield.

After a cool weekend, the Goodland NWS Forecast Office has a forecast of highs in the 70s and lows in the 50’s through Wednesday October 7. The 8-14 day outlook from the Climate Prediction Center, which extends out to October 15 calls for an increased chance of above normal temperatures across the state.

Figure 4. Map of the lowest temperatures recorded for September 2015. Dark blue refers to lower temperatures (31-41°F); dark red refers to high temperature (51-55 °F).

Management considerations

From a management perspective, the best way to mitigate this issue is to plan in advance. Recommended practices include the use of a different hybrid maturity and a different planting date:

If the sorghum is killed by a freeze before maturity, producers should first analyze the crop for test weight and yield potential before deciding whether to graze it or harvest the grain sorghum for silage.

For more information on this, see Harvesting Grain from Freeze-damaged Sorghum, K-State publication MF-1081:

Ignacio Ciampitti, Crop Production and Cropping Systems Specialist

Mary Knapp, Weather Data Library