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K-State Agronomy eUpdates

Department of Agronomy

Kansas State University

1712 Claflin Rd.

2004 Throckmorton PSC

Manhatan, KS 66506



Extension Agronomy

Extreme temperatures: Impact on 2018 corn yields in eastern Kansas

How hot is it really this year? Working day after day in the heat can really get to you. High temperatures also impact crop growth and yield. Corn yields are directly affected by climate and weather (temperature and rainfall).

Growing degree days, or heat units, are a method of estimating the thermal time and can be related to the physiological growth of crops. Growing degree days (GDD) are calculated by subtracting a base or threshold temperature from the average daily temperature. For corn, that base temperature is 50 degrees F. The cumulative GDD50 also show year-to-year variability, but not as extensively as the cumulative rainfall.

Calculated from the corn crop insurance planting date of March 21, the 10-year average GDD50 received at Columbus, KS is 3436. For 110-day corn, it has been estimated to need 2650 GDD50 (Figure 1 top panel, dashed line) to reach black layer (Pioneer Hi-Bred International Sales Literature, 2001). The greater cumulative GDD50 is part of the reason for different rates of maturation of corn from one year to the next, with earlier harvesting in hotter years. Estimated black layer for 110 corn would vary from an early date of July 19 in 2012 to a late date of August 15 in 2014, with an average date around August 3. The cumulative GDD50 in 2018 were initially below average during the early spring growing period, but have since rapidly exceeded average. The cumulative GDD50 was reached on July 29, 2018 in Columbus, KS. Date of planting also impacts the rate of GDD50 accumulation, as later-planted corn will have fewer heat units accumulated than early-planted corn.

Figure 1. Growing degree days (top panel) and rainfall (bottom panel) at Columbus, Kansas.


Temperature determines the rate of physiological growth of corn, but moisture is also very important for determining the yield. Southeast Kansas receives the highest average yearly rainfall in the state, with totals ranging from 40 to 50 inches. About 20 inches of rain are received on average during the corn growing period from March through August (Figure 1, bottom panel), though the amount varies considerably from year to year. This year has been exceptionally dry, with current rainfall totals well below half of the growing-season average. This is comparable to rainfall received in 2014. In contrast to 2014, however, the 2018 spring/summer growing season has been warmer (Figure 1, top panel).

Not only the amount, but the timing of rainfall is important for optimal corn production. Corn is a determinate crop that flowers only once. The strong determinate nature of corn makes the flowering period (tasseling) very sensitive to environmental conditions during that one growth period, as the plants cannot flower again if flowering fails due to an adverse environmental period. Poor environmental conditions, especially low rainfall, can reduce the fertilization of ovules, resulting in unfertilized ovules (Figure 2). If adverse weather conditions continue, fertilized ovules may be aborted. High temperatures also limit grain filling (see “Effect of high night temperatures on corn yield” article from Issue 701).

Figure 2. Corn fertilization and grain filling are dependent on temperature and moisture. Low moisture during tasseling reduces the fertilization. Inadequate moisture and/or high temperatures impairs kernel development and grain filling. Photo by Gretchen Sassenrath, K-State Research and Extension.


The period of highest potential rainfall in southeast Kansas coincides roughly with the corn pollination window from mid-May to mid-June (Figure 3). Interestingly, although the total rainfall received in 2018 is well below average, the rainfall received during the critical corn pollination window from May 10 until June 20 was 4.23 inches in 2018, substantially more than was received during the drought year of 2012, which totaled only 1.01 inches. The second lowest rainfall year, 2014, received 5.6 inches of rain during this period. This timely rain, together with the mild temperatures, resulted in good corn yield in 2014.


Figure 3. Statistical rainfall probability for corn growing season in southeast Kansas. Graph from Xiaomao Lin, K-State Research and Extension.


Corn development and grain filling are especially sensitive to temperatures above 86 degrees F. As the number of days that the maximum temperature exceeds 90 degrees F increases, corn yield decreases (Figure 4). Comparing the number of days above 90 degrees F maximum temperature, 2018 is similar to the drought year of 2012 (Figure 5a). In contrast, the number of days that temperatures exceeded 95 degrees F in 2018 are nearly normal. So yes, it is a hot year, but not THAT hot!


Figure 4. Corn yield decreases as the number of days that temperatures exceeded 90 degrees F increases. Graph from Gretchen Sassenrath, K-State Research and Extension.


Figure 5. The number of days in 2018 that temperatures exceeded 90 degrees F (top graph) and 95 degrees (bottom graph). Data from K-State Mesonet.

This is how temperature and moisture both work to grow a crop.



Xiaomao Lin, State Climatologist

Gretchen Sassenrath, Crop Production Agronomist

Ignacio Ciampitti, Crop Production and Cropping Systems Specialist