Analysis of Temperature Impacts during Early Growth, Anthesis/Silking and Grain-Fill on Yield of Rainfed Maize in the U.S. Corn Belt.

Recently reports have suggested that maize yields in the US Corn Belt are temperature limited. However, negative yield responses attributed to high temperatures may be due to correlation of high growing season temperatures with other yield-limiting climate factors, including crop available soil water, given that the linkages between other climate factors and high temperature were not well established. Our goal is to distinguish the overall climate conditions where high temperatures specifically affect maize yields in order to improve projections of climate change impacts on yield.   Rainfed maize yield data from yield contests (National Corn Growers Association) (2005 to 2012) for Nebraska, Kansas, and Missouri were analyzed. We calculated cumulative incident solar radiation, vapor pressure deficit (vpd) and heat stress factors (‘killing degree days’ (average daily temperature above 29 C); cumulative daytime and nighttime hours above reported threshold temperatures) seasonally and during critical phenological stages. We used statistical modeling to quantify the impact of the stress factors, cumulative solar radiation, precipitation and vpd on yield, independent of management practices and crop genetics. Vapor pressure deficit was significantly correlated with yield in all analyses. Heat stress factors did not significantly interact with yields when vpd was statistically controlled for. Yield was more highly correlated with vpd than with seasonal precipitation. Significant yield reductions were found above a threshold of approximately 2.0 kPa vpd. However, vpds above this threshold were associated with growing season precipitation less than 300 mm. Additional analyses assessing the importance of temporal variability in crop-available soil water storage on maize yield will be presented. These results indicated that, in rainfed maize, the impact of high temperature on yield may be due to strong correlation between high temperature and low moisture in the US Corn Belt, and not due to negative impacts of high temperature on maize physiology. 1