Authors: Ignacio A. Ciampitti, Mary Knapp

Based on preliminary readings, the lowest temperatures recorded in parts of Kansas this month could have an impact on grain fill and test weight of summer row crops -- especially sorghum and soybean. It is less likely to affect much of the corn crop since corn is close to 50% mature.

In the last 17 days, low temperatures have been below 35°F in several counties around the state. The county with the lowest temperature was Osborne, with 29 degrees F. In addition, Ellis, Trego, Ness, and Decatur counties had low temperatures around 30-31 degrees F (Figure 1).

Figure 1. Lowest temperature recorded for specific weather stations from September 1 to 18, 2014.

The coldest low temperatures in Kansas during this time period were in the northwest and north central regions. The central region had temperatures as low as 33 to 37 degrees; while the coldest low temperatures in southern Kansas generally were generally above 40 degrees (Figure 2). The lowest mean temperatures for the last 17 days were located in the northwestern and north central counties (Figure 3).

Figure 2. Generalized map of the lowest temperature recorded from September 1 to 18, 2014.
Figure 3. Generalized map of the average mean temperature from September 1 to 17, 2014.

The temperatures experienced in the last 17 days depart from the 30-year average for the entire state, except for Pawnee, Barton, Stafford, and Pratt counties (Figure 4). For the north central area, the departure was close to 7 degrees below the 30-year average, affecting primarily the counties of Smith, Phillips, Mitchell, Graham, Rooks, Osborne, and Trego.

Figure 4. Departure from 30-year normal mean temperatures (1981-2010).

Effect on crops:

A) Corn:

In most of the state corn is beyond the dent stage (50% mature). Corn is affected when temperatures are below or at 32 F. The colder it is below 32 degrees, the less exposure time it takes to damage the corn. At the dent stage, information from Wisconsin shows that a light frost (affecting leaves) will produce a 5% yield reduction; while a killing frost (affecting leaf and stalk) will reduce yields in 12%. Corn is not affected at all by freeze once it reaches the black layer stage.

Clear skies, low humidity, and calm wind conditions can increase freeze damage even with temperatures higher than 32 F. Any freeze damage at this point of the season may not produce any visible symptoms, but can impact the final test weight and potentially seed quality -- depending on the growth stage.

B) Soybean:

Soybean is now in the final reproductive stages (dropping leaves) in Kansas. Temperatures below 32 F can interrupt seed fill and impact yield through lower test weight and seed quality. Necrosis of the leaf canopy is a visible symptom of freeze damage in soybeans (Figure 5). 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.

Figure 5. Effect of late-freeze on soybean near Stockton (Rooks County). Photo by Lucas Haag, K-State Research and Extension.

C) Sorghum:

More than half of the sorghum in Kansas has turned color (14% mature). 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 (Figure 6).

Figure 6. Effect of late-freeze on sorghum (Osborne County). In the top part of the canopy, the leaves are all senesced. In the lower photo, the bottom part of the canopy still has green leaves. Photos by Lucas Haag, K-State Research and Extension.

Ignacio A. Ciampitti, Crop Production and Cropping Systems Specialist

Mary Knapp, Weather Data Library