By Mary Knapp
Low temperatures could present a bigger problem for sorghum and soybeans than for corn. At this point, corn is reaching maturity and the potential impacts of lower temperatures will have little (depending on the absolute value, the duration of the stress, and phenology of the crop) or no impact on expected corn yields. The main challenges of low temperatures will occur for soybeans in the coming weeks during the final reproductive stage, with the potential of impacting final seed weight (either affecting the rate of accumulation of dry matter on the seeds or by interrupting this process) if temperatures drop below 32 degrees.
Temperature effect on crops
Sorghum:
Wet conditions delayed sorghum planting in some areas of the state thus delaying heading. During August, cooler-than-normal temperatures dominated the state. 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. The long-season growing-degree-day (GDD) accumulation from August 1 to September 7 portrays a lower GDD accumulation for the north central and eastern parts of the state (Figure 1). The largest departure of GDD accumulation was recorded in the south central, southeastern, and northeast-north central portions of the state (Figure 2).
Figure 1. Accumulated Long Season Sorghum Growing Degree Days.
Figure 2. Departure from normal accumulation of long season Sorghum GDDs.
Low temperatures will reduce seed growth and affect final test weight and seed quality. Temperatures below 40 degrees F will inhibit growth. A freeze will kill sorghum if the stalks are frozen, impairing the flow of assimilates and nutrients to the grain. A freeze at the hard-dough stage (before grain matures) will produce lower weight and chaffy seeds.
The likelihood of sorghum maturing before a freeze is related to the following factors (as affected by weather and hybrid):
- planting date
- plant growth rate during the season
- date of half-bloom.
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. In the last week, the minimum temperature recorded across the state was between 32-40 degrees F for the northwest portion of the state (Figure 3). For the northwest, western, and north central areas, the question that will remain unanswered is if sorghum will be able to reach maturity before the first fall freeze? As emphasized above, the answer depends on factors such as planting date, date of half-bloom, hybrid (maturity), and more importantly, projected temperatures for the next coming weeks.
Figure 3. Map of the lowest temperatures recorded for September 1-7. Dark blue refers to low temperatures (32-40 degrees F); dark red refers to high temperatures (51-55 degrees F).
Corn:
Corn is affected when temperatures are below or at 32 degrees F. Temperatures below 32 degrees F can produce equivalent or greater damage even when the exposure time is relatively short. Clear skies, low humidity, and calm wind conditions increase freeze damage even with temperatures above 32 degrees F. Any freeze damage at this point in the season will hardly produce any visible symptoms, but can impact the final test weight and potentially seed quality - depending on the growth stage. Corn is not affected by freeze once it reaches the black layer stage.
Soybeans:
For soybeans, temperatures below 32 degrees can interrupt grain filling and impact yield, meaning lower test weight and seed quality. Necrosis of the leaf canopy is a visible symptom of freeze damage in soybeans. Absolute temperature is more important than the duration of the cold stress – especially if temperatures drop below 28 degrees F. The timing of the freeze effect will increase the likelihood of impacting yields. As the crop approaches maturity, the impact of a freeze event on soybean yields declines.
As weather changes develop in the coming weeks, stay tuned for more information about potential freeze injury on sorghum in future eUpdate issues.