Minimum air temperatures for October 7 dropped well below 30 degrees F in the north central and western regions of Kansas, which could pose a problem during the grain filling period of the summer row crops. The eastern and south central parts of the states did not get as cold on that date (Figure 1).
The risk of damage to summer row crops is a function of the current developmental stage of the crop and also related to the minimum temperature and the duration.
Duration of damaging temperatures
The coldest temperatures on October 7 were in the northwest corner of Kansas, where air temperatures were below 32 degrees F for more than 6 hours (Figure 2). The eastern and central parts of the state had temperatures close to 32 degrees F with minimum exposure to temperatures low enough to have a consistent impact on grain filling and final yields. Other areas had temperatures below 32 degrees F, ranging in duration from 2 to more than 6 hours (Figure 2).
Effect on summer row crops
In most of the state corn is mature (91% based on the most recent USDA Crop Progress and Condition report). Corn can be affected when temperatures are below or at 32 degrees F. The colder below 32 degrees, the less exposure time it takes to damage the corn. However, corn is not affected once the black layer is formed.
Soybean is now in the final reproductive stages (dropping leaves) in Kansas (56% dropping leaves based on the most recent USDA Crop Progress and Condition report). Temperatures below 32 F can interrupt seed fill and impact yield through lower test weight and seed quality (primarily affecting protein deposition). Necrosis of the leaf canopy is a visible symptom of freeze damage in soybeans. With soybean, absolute temperature is more important than the duration of the cold stress. The most severe injury occurs with temperatures less than 28 F. As the crop approaches maturity, the impact of a freeze event on soybean yields declines.
More than half of the sorghum in Kansas has already reach maturity (63% mature based on the most recent USDA Crop Progress and Condition report). The lowest proportion of sorghum mature across the state is in the north central (33% mature) and northwest (58% mature) areas of the state. Low temperatures will reduce seed growth and affect final test weight and seed quality, making the harvest process more difficult. A freeze will kill sorghum if the stalks are frozen, impairing the flow of nutrients to the grain. A freeze at the hard dough stage (before grain matures) will result in lower weight and chaffy seeds.
Ignacio A. Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library