Minimum air temperatures during the October 5th to 12th period 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 summer row crops. The eastern and south central parts of the states did not get as cold during that time period (Figure 1).
The risk of damage to summer row crops is a function of the current developmental stage of the crop and the minimum temperature and the duration of the freeze events.
Figure 1. Lowest minimum temperatures measured from October 5 to 12, 2017.
Duration of damaging temperatures
The coldest temperatures from October 5 to 12 were in small pockets in the northwest, southwest, and north central portions of Kansas, where air temperatures were below 32 degrees F for more than 6 hours (Figure 2). The rest 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, including a small portion in west central Kansas, had temperatures below 32 degrees F, ranging in duration from 2 to more than 4 hours (Figure 2).
Figure 2. Total hours of air temperatures below 32 degrees F measured from October 5 to 12.
Effect on summer row crops
In most of the state, corn is mature (88% based on the most recent USDA Crop Progress and Condition report). The most sensitive areas, with higher probabilities of showing effects of low temperature impacts, are the northwest and west central parts of Kansas (<70% mature) but with a potential low impact on number of acres affected due to the advanced phenology (close or already matured). Corn can be affected when temperatures are below or at 32 degrees F. The further temperatures drop 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 currently in the final reproductive stages (dropping leaves) in Kansas (87% dropping leaves based on the most recent USDA Crop Progress and Condition report). Temperatures below 32 degrees F can interrupt seed fill and impact yield through lower test weight and seed quality (primarily affecting protein deposition). Necrosis (death) 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 freeze event. The most severe injury occurs with temperatures less than 28 degrees F. As the crop approaches maturity, the impact of a freeze event on soybean yields declines. The most affected soybean acreage will be related to late-planted and double crop soybean areas around the north central and western parts of the state.
More than half of the sorghum in Kansas has already reached maturity (60% mature based on the most recent USDA Crop Progress and Condition report). The lowest proportion of mature sorghum is in the north central (49% mature) and northwest (29% mature) areas of Kansas. 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