By Mary Knapp
The latest “Crop Progress and Condition” report from Kansas Agricultural Statistics, on October 7, stated that grain sorghum maturity was 70%, ahead of last year and the average. Harvest was also ahead of last year, 16% for this year compared to only 9% in 2017. Nonetheless, wet conditions and mild temperatures will delay harvest in several regions across the state.
Will the 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 the crop was planted, hybrid maturity, and the date of half-bloom. Further details on sorghum growth and development can be found at: https://www.bookstore.ksre.ksu.edu/pubs/MF3234.pdf
Weather component
Wet conditions at planting time delayed sorghum planting in some areas of the state, delaying heading. During August, cooler-than-normal temperatures dominated the state, with the greatest departure in the northwest at 3 to 6 degrees below normal. The warmest conditions were in the eastern divisions but only a few pockets reached above normal temperatures (Figure 1, upper panel). In contrast, September mean temperatures were above normal across most of the state. Normal to below normal temperatures were concentrated in the central and southern divisions with the coolest areas having a departure of only 2.0 degrees below normal. In contrast, the eastern divisions had departures ranging from 3.5 to almost 5 degrees warmer than normal (Figure 1, lower panel).
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 the August and September 2018.
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 Staggenborg and Vanderlip documented the 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. From mid-August until this current week (Oct. 12, 2018), the lowest minimum were below 30 degrees F in a small area of the western section of the state but temperatures below 35 degrees F for the rest of the counties across the state.
Grain sorghum 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 is about 800-1200 (based on 50 degrees F as base temperature), with the shortest requirement in GDD for short-season hybrids. Before maturity, from beginning of grain filling (soft dough until maturity), grain moisture content within a grain will go from 80-90% to 25-35% where black layer is usually formed (Figure 2). From maturity (seen as a “black-layer” near the seed base; Figure 3) 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. Sorghum growth stages from half-bloom and grain filling (including soft dough, hard dough, and physiological maturity). Infographic representing changes in grain coloration and moisture content during grain filling period until black layer formation, maturity.
Figure 3. Black-layer identification in sorghum.
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 4 show accumulated GDDs up to October 11 for the current growing season, starting at two different points: mid-August and early September. Lower GDDs are depicted with blue colors, while higher GDDs are represented in red colors.
If blooming occurred during mid-August, the likelihood for maturing before freeze is high in most of the areas of the state that have accumulated 1100 GDDs (Figure 4). There are some areas of the state where sorghum GDDs accumulation was below 1100 (primarily related to light blue colors in Figure 4). Those areas will have a slight lower chance of maturing (having accumulated less than 1200 GDDs) before the first freeze. A worse picture is projected for the extreme northwestern area of the state (dark blue colors in Figure 4). In this case, there is a lower probability of maturing before the first freeze (low GDDs, <1000) but it will depend also on the hybrid maturity.
If blooming occurred during early-September, the likelihood for sorghum maturing before freeze is low for the southern part of the state (red color in Figure 4), presenting a cumulative GDD from early-September to early-October over 800 units; while the probability is extremely reduced for the northwestern section of the state, with a cumulative GDD below 650 units.
Figure 4. Accumulated Growing Degree Days (expressed in degrees F) for August 15-October 11 and September 1- October 11. The maps show that for sorghum that reached half-bloom on September 1, prospects are less certain especially in northwest Kansas. The darker the red, the higher the number of accumulated GDDs.
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 hybrid maturity and a different planting date:
- 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 the test weight and yield potential before deciding whether to graze or harvest the grain sorghum for silage.