Effect of cool temperatures (mild stress) on summer row crops
During the week of August 17-23, temperatures below 60 degrees F were recorded in several parts of Kansas, but specifically in the central and western regions (Figure 1). These temperatures could have a potential impact on summer row crops.
Figure 1. Number of hours with minimum temperature below 60 degrees F. Weather Data Library, Department of Agronomy, Kansas State University.
At this point in the growing season, low temperatures could present a bigger problem for sorghum and soybeans than for corn. Corn is transitioning out of the reproductive stages and getting close to harvest in many areas; thus, lower temperatures will have little to no effect on the crop. For corn, low temperatures are a problem when at or below 32 degrees F. Temperatures below 32 degrees F can produce equivalent or greater damage even when the exposure time is relatively short. The likelihood of experiencing at or below freezing temperatures at this time of the season is very small.
Similarly, 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.
However, for sorghum, temperatures below 50 degrees F (Figure 2) can potentially have an effect if the plant is at half-bloom, causing delaying on plant growth and potential yield reduction. A previous study has reported a high sensitivity of sorghum to low temperatures during flowering time (Maulana and Tesso, 2013), using 55 degrees F as the lowest night temperature.
Figure 2. Number of hours with minimum temperature below 50 degrees F. Weather Data Library, Department of Agronomy, Kansas State University.
There is a lack of sufficient data on the effects of cool temperatures (mild stress) on sorghum at flowering. One of the reasons for this lack of information is the infrequency of such events. For example, in the last 10 years, these low temperatures have occurred in 3 years for Belleville, 2 years for Manhattan, and 6 years for Colby. The higher incidence shown for Colby illustrates the difference of northwest Kansas relative to central (Belleville) and eastern Kansas (Manhattan).
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) such as planting date, plant growth rate, and date of half-bloom.
As weather changes develop in the coming weeks, stay tuned for more information about potential freeze injury on sorghum in future eUpdate issues.
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library