The latest Crop Progress and Condition report from Kansas Agricultural Statistics, on September 30, stated that grain sorghum maturity was 60%, near 56% last year, and ahead of the average (50%). Harvest is underway at 17% this year, ahead of the 5-year average (11%).
The weather conditions experienced from early September to early October are critical for sorghum as they are related to the grain-filling rate and the determination of final grain weight. While there were periods of briefly cooler conditions, temperatures were, on average, warmer than normal during September (Figure 1). Temperature swings were 30-40°F daily with dry air in place. Precipitation was much more varied, with isolated portions of south-central and east-central Kansas receiving 4+ inches of rain. Despite the higher amounts in some locations, the intensity of the rainfall led to briefly increased runoff and poor uptake by the soil. Areas that missed the heaviest precipitation events only received 10-20% of normal for the month (Figure 1). Therefore, drier-than-normal conditions prevailed to end the month across the state, even where moisture was received.
Figure 1. Departure from normal temperatures (top map) and percent of normal precipitation (bottom map) from September 1 – 30, 2024. Maps by Kansas Weather Data Library.
In recent years, a common question from producers is related to the dry down rate for sorghum when approaching the end of the season. Based on previous information, the average dry down rate depends on the weather, primarily temperature and moisture conditions, but data from modern hybrids is limited. The weather outlook for the remainder of October and into November favors the continuation of below-normal precipitation for the state. With normal precipitation in November much less than in October, this could mean almost no moisture. This would favor a faster dry down rate than average, but any sorghum impacted by freeze (should an event occur) will present challenges in the dry down rate.
From a crop perspective, the overall cumulative GDD from flowering to maturity is about 800-1200 (based on 50 degrees F as the base temperature), with the shortest requirement in GDD for short-season hybrids. Before maturity, from the 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 2) 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). For further reference on sorghum growth and development please check: https://bookstore.ksre.ksu.edu/pubs/MF3234.pdf. Graphic by K-State Research and Extension.
Grain water loss occurs at different rates but with two distinct phases: 1) before “black-layer” or maturity (Figure 2), and 2) after black-layer. For the first phase, Figure 2 presents the changes in grain moisture from soft dough until the physiological maturity of sorghum.
To answer the rate of dry down question from many of our producers, a study was conducted to investigate the effect of the grain dry down rate from the moment of “black-layer” until commercial harvest grain moisture is reached. For the conditions experienced in 2019, 2021 and 2022 (from early September until Mid-October), the overall dry down rate was around 0.7% per day (from 31-34% to 16-17% grain moisture) – taking an overall of 30 days.
Figure 3. Grain moisture dry down across different sorghum hybrids for a study near Manhattan, KS (2019, 2021, and 2022 growing seasons). The graphic in the left panel illustrates the black layer stage of grain for sorghum. Graphics from K-State Research and Extension.
This dry down process can be delayed by:
- Low temperatures
- High humidity
- High grain moisture content at black layer (38-40%)
It is expected that the dry down rate will decrease to around<0.5% per day for late-planted sorghum entering reproductive stages later in the growing season. A similar decrease is also expected for sorghum that was exposed to late-season stress conditions (e.g., drought, heat, and freeze). Under these conditions, maturity may be reached with high grain water content, and the last stages after black layer formation could face lower temperatures and higher humidity. These main factors should be considered when the time comes to schedule harvest.
You can track temperature and humidity levels on the Kansas Mesonet website at http://mesonet.k-state.edu/weather/historical/ by selecting the station and time period of interest.
Ignacio A. Ciampitti, Farming Systems
ciampitti@ksu.edu
Christopher “Chip” Redmond, Kansas Mesonet Manager
christopherredmond@ksu.edu