There is a growing awareness of the negative correlation between high nighttime temperatures during grain fill periods and corn yields. The Climate Corporation performed an analysis of historical crop development rate, yield, and temperature data for the last 20 years for the states of Illinois and Indiana to quantify the impact of high nighttime temperatures on both grain fill length and corn yield.
Weather is a major factor in determining yields of all field crops and corn is no exception. Most corn growers are well aware of the ways in which hot days and dry weather can negatively impact the yield potential of a crop. But after years like 2010, and to a lesser extent 2011, many corn growers are becoming increasingly aware of just how important another weather event can be in determining yields: high nighttime temperatures.
In many parts of the country, corn yields were lower than expected in 2010 and 2011, and one of the drivers of low yields may have been high nighttime temperatures that the crop experienced during the grain fill portion of the development cycle. [1,2,3]
The analysis in this outlook report uses The Climate Corporation’s extensive database of historical US temperature data, along with public crop development and yield data from the National Agricultural Statistics Service (NASS), to quantify the relationships that exist between nighttime temperatures, the length of the corn grain fill period, and corn yields.
The mechanism by which high nighttime temperatures limit corn yield is fairly well known. When nighttime temperatures do not fall below approximately 72 degrees Fahrenheit, the corn plant doesn’t get a chance to slow down at night and instead keeps metabolizing sugars at a high rate around the clock. This high rate of nighttime metabolism, often referred to as ‘dark respiration,’ causes the plant to finish the grain fill period and reach full maturity faster than normal. Unfortunately the higher rate of dark respiration also causes the plant to put photosynthetic sugars into plant growth and maintenance that would otherwise be used to fill grain and create higher yields.  Every sugar that goes into plant growth and maintenance rather than into production of kernel dry matter represents lost yield potential for the plant.
In this analysis, we looked at quantifying two key relationships. First, we examined the historical relationship between nighttime temperatures and length of the grain fill period. Second, we analyzed the historical relationship between the average nighttime temperatures during grain fill and final yield.
In each case, we used historical state-level crop progress data and yield information from NASS, combined with state-level average minimum temperature values from The Climate Corporation’s extensive database of historical weather recordings for the relevant time periods in each historical year. We performed analyses for the states of Illinois and Indiana because of their unique situations as large corn acreage states with limited irrigation and moderate temperature patterns known to occasionally result in nighttime heat stress events. Any signal of the relationship between nighttime temperatures and length of grain fill or final yield would be difficult or impossible to detect in regions where irrigation is present or areas which either never or always experience qualifying high nighttime temperatures during grain fill.
The first finding from our analysis is that there is a clear relationship between nighttime temperatures and length of grain fill for both Illinois and Indiana. Using state-level NASS crop progress data , we identified the first week when at least 50% of the crop in the state had reached the silking stage and also the first week when at least 50% of the crop in the state had reached the dent stage for each of the last 20 years (1992-2011). The number of weeks between silking and dent was identified as the length of the grain fill period for each year . We then calculated the average of all the daily minimum temperature values for all weather stations in the state for the relevant grain fill weeks in each historical year. We plotted the relationship between average statewide daily minimum temperature during grain fill and the length of the grain fill period and fit a linear trend to the data and the plots.
As shown in Figure 1, the relationship between nighttime temperature and length of grain fill is consistent: Warmer nights lead to shorter grain fill periods. In Illinois the historical relationship indicates that growers can expect to lose one week of grain fill for every 2.6 degree increase in the average nighttime temperature during that time period, while in Indiana growers can expect to lose one week of grain fill time for every 2.8 degree increase in the average nighttime temperature during grain fill.
This relationship between the impact of nighttime temperatures and length of grain fill is a difficult one to study because of the lack of granularity in some of the data. First, NASS data only reports the length of the grain fill period at the state level, despite significant variation from north to south across each state. Second, grain fill length is only reported by NASS on a weekly basis, so it is not possible to distinguish differences in the length of the grain fill period from year to year when the difference was less than one week. However, despite these data challenges, the fact that a clear, quantifiable relationship is still visible through the noise shows just how important nighttime temperatures are to determining the length of the grain fill period.
The second finding from our study involved the yield loss a grower should expect to see if they experience high nighttime temperatures during grain fill. This second analysis compared de-trended annual state-level yields for Illinois and Indiana for each of the past 20 years (1992-2011) to the same annual state-wide average minimum temperature value that was calculated in the first part of the study. The expected relationship is clear in the graphics of Figure 2: higher nighttime temperatures during grain fill generally produce lower yields while lower nighttime temperatures during grain fill generally produce higher yields. But more important than validating this relationship between nighttime temperatures and yield, our analysis allows us to quantify the yield loss a grower should expect to experience for every degree of increase in nighttime temperatures during grain fill. In Illinois, growers have historically seen a loss of 3.6 bushels of yield for every one degree increase in average nighttime temperatures during grain fill. In Indiana, corn growers have historically seen a loss of 2.0 bushels of yield for every one degree increase in average nighttime temperatures during grain fill.
It is important to note that days with high nighttime temperatures are correlated with at least one other weather condition that may also drive some of the yield impact we see in Figure 2. The yield losses that we see correlated with high nighttime temperatures may, at least in part, be due to a lack of daytime solar radiation as opposed to increased nighttime respiration. Warmer nights tend to be associated with cloudy nights, as clouds act as a thermal blanket that blocks outgoing infrared radiation. Cloudy nights have some correlation with cloudy days, and the reduction in the amount of solar radiation available during cloudy days to drive production of photosynthetic sugars may explain some portion of the relationship we see in Figure 2.
The impact high nighttime grain fill temperatures have onfinal corn yield has not been well studied previously due to the difficulty in setting up relevant field experiments. And while the lack of granularity in NASS data can make it difficult to identify small data signals, we have been able to use the data to confirm the relationship between nighttime temperatures during grain fill and corn yield and have also been able to provide a quantification of the yield loss a grower in these states should expect to see if and when he experiences elevated nighttime temperatures during the grain fill period.