Are High Night Temperatures Associated with Decreases in Maize Yields in the U.S. Corn Belt?.

High nighttime temperatures may be negatively impacting yields of maize in the U.S. Corn Belt, and nighttime temperatures in this region are expected to increase with global warming. Decreased maize yields due to high nighttime temperatures may occur because of accelerated development, reducing radiation available to the crop by decreasing the length of the growing season and/or by diverting photosynthate to night respiration rather than growth. We analyzed irrigated maize yield data from yield contests (National Corn Growers Association) from 2005 to 2012 for Nebraska, Kansas, and Missouri and calculated cumulative incident growing season solar radiation based on planting date, air temperature and cultivar specific growing degrees days required to reach maturity. Heat stress factors included nighttime hours (7 pm to 7am) above threshold temperatures of 20,22,24,and 26 oC. A statistical model was developed to quantify the impact of the stress factors and cumulative solar radiation on yield, independent of management practices and underlying yield trends due to improved maize genetics. We found that night temperature, when examined independently, was associated with yield decreases but had no impact on yield independent of cumulative incident solar radiation when cumulative radiation was calculated using thermal-time defined intervals.. In addition, there was no trend of increasing yield losses associated with increasing nighttime temperature thresholds, suggesting that yield associations with night temperature are not directly related to heat stress. Our analysis supports the hypothesis that increased night temperatures are correlated with decreased maize yields by way of association with decreased cumulative radiation, most likely as a result of a shortened growing season due to an increased development rate.