High Night Temperature Effects on Corn Yield

Many agronomists and corn growers are aware of the general concept that above-average night temperatures during pollination and grain fill can be detrimental to corn grain yield. Average summer temperatures in much of the Corn Belt are commonly warmer during the day and much warmer during the night than those to which corn was originally adapted in its native region. The genetic lineage of corn can be traced back the Central Highlands of Mexico (Galinat, 1988), specifically the Tehuacán Valley and Balsas River Valley. Summer climate in this region is characterized by relatively mild daytime high temperatures, cool nights, and abundant sunshine (Figure 1). The first research in the U.S. Corn Belt that demonstrated negative effects of elevated night temperatures on corn yield was conducted in the late 1960s (Peters et al., 1971), and it has been generally known among corn producers and agronomists since then that warm nights can reduce corn yield.

What is less understood though, is how yield is impacted though effects on specific plant processes and yield components. Abiotic stress effects on crops can be complex, with the timing, duration, and severity of the stress all being important factors in determining the ultimate impact on yield. Until recently, very few studies had been conducted on this question, making it difficult to pin down precise effects of high night temperatures on corn yield and answer important questions regarding the degree and duration of heat stress that corn can endure before yield is affected. In recent years, however, a surge of new research in this area has brought more insights into how and why high night temperatures can affect corn yield.

The first experimental evidence that high night temperatures can have a detrimental effect on corn yield came from a field experiment performed by researchers at the University of Illinois in the late 1960s (Peters et al., 1971). In this study, small climate-controlled enclosures were constructed and placed over corn plants at night to alter air temperature. Nighttime temperature treatments were imposed at flowering and maintained through physiological maturity. In this study, corn grown with an average night temperature of 85°F yielded 40% less grain than corn grown with the average ambient night temperature of 62°F (Table 1).

Although the impact on corn yield was substantial in this study, the real-world insights that could be drawn from it were limited – it was a single year, single location study with only one high temperature treatment applied over the entire reproductive period. Corn yield was the only response variable reported, with no data on specific yield components. The elevated night temperature treatment applied in the study was also unrealistically high for the Central United States. So, while this study clearly demonstrated that elevated night temperatures could reduce corn yield, it provided little insight into the risk of yield loss associated with above average night temperatures within a range likely to be experienced under real world conditions.

Subsequent studies built upon the work done by Peters et al. and examined the effects of high night temperatures on corn yield components. Research conducted a decade later at the University of Guelph focused specifically on the effects of elevated temperature during the grain fill period (Badu-Apraku et al., 1983). In this study, corn plants were grown in pots outdoors and then moved into controlled-temperature growth chambers 18 days after silking. Results showed that grain yield per plant was significantly affected by temperature regime (Table 2).

The lowest temperature regime (77°F day, 59°F night) resulted in the greatest grain yield per plant as well as the longest grain fill duration. Increasing the night temperature to 77°F significantly reduced yield per plant. Increasing the day temperature to 95°F also resulted in lower yield per plant, regardless of nigh ttemperature. Since temperature treatments were applied after kernel set, yield reductions in this study were entirely attributable to differences in kernel weight.

A study conducted in Argentina in the 1990s showed that high night temperatures could negatively affect yield through reductions in kernel number as well (Cantarero et al., 1999). This study examined the effects of elevated night temperature (9°F above ambient) over a period extending from 1 week before silking to 3 weeks after silking. Results showed that kernel abortion in heated plots was 8% higher than in the control plots. Ears in the heated plots had an average of 34 kernels per row at harvest, compared to 37 kernels per row in the control plots.

Until relatively recently, the total body of research on the effects of high night temperatures on corn yield remained relatively sparse. A handful of studies had demonstrated that elevated night temperatures could significantly reduce corn yield and that those reductions could be a function of lower kernel number or lower kernel weight, depending on the timing of the heat stress. However, despite understanding the theoretical importance of night temperatures, it remained difficult to translate that knowledge into assessing real-world impacts. Temperature in the field is dynamic, and determining the timing, intensity, and duration of nighttime heat stress necessary to impact yield was difficult with only a few studies to go on.

In the past decade, however, there has been a surge of new research in this area (Hein et al., 2024; Kettler et al., 2022; Kettler et al., 2024; Niu et al., 2021; Wang et al., 2020). Understanding the real-world effects of warmer nights on crop yield has taken on increased importance due to the reality of rising global temperatures. As temperatures have increased around the world, night temperatures have increased at a faster rate than daytime temperatures (Davy et al., 2016). In the U.S., nighttime temperatures during the summer months of June, July, and August have increased by an average of 3.1°F since 1970 (Climate Central, 2025). Figure 2 shows summer night temperature increases for several U.S. locations in major corn-producing areas.

Figure 2. Change in average summer (June, July, August) minimum temperatures from 1970 to 2024 in several U.S. cities located within major corn-producing areas. All charts produced by and used with permission of Climate Central based on data from NOAA (ACIS).

Detrimental effects of high night temperatures have been observed in several crops, including wheat (Garcia et. al., 2015; Garcia et al., 2016; Hein et al., 2019), rice (Bahuguna et al., 2016; Welch et al., 2010), quinoa (Lesjak and Calderini, 2017), and barley (Garcia et al., 2015; Garcia et al., 2016). The prospect of widespread yield declines across multiple major crops due to rising night temperatures has generated concern about potential implications for global food security (Sadok and Jagadish, 2020).

Corn yield reduction from heat stress can be associated with reductions in both source and sink capacity. Impact on yield depends on the growth stage of the corn at the time stress occurs. The most critical period for corn yield determination is the roughly 4- to 5-week window bracketing silking when kernel number is set. Approximately 85% of total grain yield is related to the total number of kernels produced per acre (Otegui et al., 1995). Kernel number is closely associated with crop growth rate during this critical period. Any stress during this time that reduces the net photosynthetic rate and assimilate availability can reduce the number of kernels the plant sets and negatively impact yield. Even if the stress is temporary and the plant recovers, the damage to yield will be done because the plant’s sink capacity has been reduced.

Once kernel number has been set, stress can continue to impact yield through grain fill by reducing kernel weight. Stalk quality can also be impacted if the stress forces the plant to increase its reliance on remobilized carbohydrates to complete grain fill.

The effects of high night temperatures on plants are complex and can involve multiple physiological processes (Sadok and Jagadish, 2020). The primary physiological basis for the negative effect of high night temperatures on corn yield is an increased rate of cellular respiration during the nighttime hours, which increases carbohydrate consumption and reduces the amount of carbon assimilate available for translocation to the grain (Kettler et al. 2022; Niu et al., 2021; Sunoj et al., 2016; Wang et al. 2020). Increased respiration rates associated with high night temperatures have been documented in wheat (Impa et al., 2019) and rice (Mohammed and Tarpley, 2009) as well.

High night temperatures can also accelerate corn development rate, which can shorten the length of the grain filling period (Badu-Apraku et al., 1983; Cantarero et al., 1999; Niu et al., 2021). Other effects of high night temperatures on plants can include accelerated leaf senescence (Lesjak and Calderini, 2017), increased water stress (Sadok and Jagadish, 2020), reduced pollen shed and pollen viability (Wang et al., 2020), and reduced photosynthetic rates (Tombesi et al., 2019).

Increased Cellular Respiration

Cellular respiration is the process by which cells break down sugar to obtain energy for various cellular functions. Cellular respiration consumes carbon assimilated through photosynthesis to obtain the energy necessary to maintain and increase plant biomass. Respiration can be subdivided into growth respiration and maintenance respiration. Growth respiration is the expenditure of carbon that contributes to the growth of the plant. Maintenance respiration provides energy to processes that do not directly contribute to an increase in plant biomass or plant weight. The two are distinguished based on the relative growth rate of the plant; at a zero growth rate, all respiration contributes to maintenance.

Both processes are temperature-dependent – photosynthesis and respiration are slow at cooler temperatures, increase as the temperature increases, and cease when the temperature gets too high. The optimum temperature (Topt) for respiration is greater than that for photosynthesis. Net photosynthesis is a measure of carbon assimilated through photosynthesis minus carbon expended through respiration and has a Topt lower than that of gross photosynthesis. Grain yield is more closely associated with the rate of net photosynthesis (the red line in Figure 3).

Higher night temperatures increase the rate of respiration during the nighttime hours. If the daytime temperature remains the same and the nighttime temperature increases, the total expenditure of energy through respiration increases, while the input of energy from photosynthesis remains the same. The end result is that net photosynthesis decreases, and less assimilated carbon is available for grain fill. This concept is illustrated in Figure 4, which compares plant dry weight accumulation over successive days between warmer and cooler night temperature conditions, assuming equivalent temperatures during the day.

Accelerated Phenology

Research has shown that high night temperatures can reduce corn yields by accelerating phenological development resulting in a shorter grain fill period. Phenological development in corn is linked to the accumulation of heat units above a base threshold. For corn, the base level is 50°F and the upper threshold is 86°F. Growing degree unit (GDU) accumulation for a given day is calculated by the formula:

Higher temperatures increase GDU accumulation and increase the rate of thermal time that drives plant development. For example, a maximum temperature of 86°F and minimum temperature of 65°F results in a daily GDU accumulation of 25.5. However, a day with the same maximum temperature but a minimum temperature of 72°F results in a daily GDU accumulation of 29.

Accelerated phenology can impact corn yield in a couple of ways. First, it can reduce plant growth rate during the critical period around silking by reducing net photosynthesis relative to thermal time, which can reduce kernel set. Second, it can reduce the duration of the grain fill period. Shortening the length of time between silk emergence and maturity reduces the number of days that the corn plant is engaged in photosynthesis during grain fill, effectively reducing the amount of energy the corn plant can convert into grain yield. Based on long-term average daily minimum and maximum temperatures for Des Moines, IA, a 111 CRM hybrid that reaches 50% silk on July 10 would be predicted to reach physiologically maturity on September 2. A 2-week period following silking during which night temperatures are 5°F above normal would shorten the time to maturity by 2 days.

Multiple studies — particularly those in which heat treatments were applied over most or all of the grain fill period — have observed a reduction in the time to physiological maturity. Earlier leaf senescence and physiological maturity were both noted as outcomes of elevated night temperature in the initial University of Illinois study in the late 1960s (Peters et al., 1971). Research conducted by Badu-Apraku et al. (1983) showed that duration of the grain fill period and grain yield per plant were both significantly affected by temperature regime (Table 2). Niu et al., (2021) found a 1 to 3-day reduction in time to physiological maturity when temperatures were raised 4-5°F above ambient over the entire reproductive period.

Increased Water Loss

Another potential impact of higher night temperatures on corn is greater water loss due to increased evaporative demand (Sadok and Jagadish, 2020). Higher temperatures create a greater vapor pressure deficit (VPD) between the saturated leaf interior of plants and the ambient air. This causes the transpiration rate of plants to increase, placing a greater demand on soil water supply and potentially accelerating the onset of drought stress. VPD increases exponentially with temperature, so relatively small changes in temperature can substantially increase water demand, even though VPD at night is considerably lower than during the day.

It was long believed that stomata on the plant leaves were typically closed during the night, which would render any increase in nighttime VPD irrelevant, since transpiration could not occur if the stomata were closed. Recent research has shown this is not the case though. A study in wheat found that nighttime transpiration rates could be as much as 55% of daytime rates under high nighttime VPD conditions (Claverie et al., 2018). A study in corn found nighttime transpiration rates as high as 18% of daytime rates (Tamang and Sadok, 2018), demonstrating that nighttime VPD can have a non-negligible effect on water loss. Another corn field study found increased evapotranspiration and lower soil moisture levels with higher night temperatures, which exacerbated drought stress in one year of the study and led to earlier leaf senescence (Niu et al., 2021).

A common misconception regarding high night temperature effects on corn is that plants must expend energy to cool themselves. While transpiration of water does have a cooling effect on the plant, it is a passive process driven by physical forces that does not require any energy expenditure on the part of the plant. Any increase in energy use associated with high nighttime temperatures is unrelated to cooling the plant.

Research on the effects of elevated night temperatures have varied in their methodology, including the manner in which heat treatments were applied, as well as the timing, duration, and intensity. Some studies have involved growth chambers in which plants were subjected to fixed and constant day and nighttime temperature regimes (Badu-Apraku et al., 1983; Wang et al. 2020). Other studies involved semi-enclosed structures in the field used to maintain a dynamic temperature treatment at a specific level above the ambient temperature (Hein et al., 2024; Kettler et al. 2022; Kettler et al., 2024; Niu et al., 2021). Timing of high night temperature treatments has most commonly been targeted to the period bracketing or immediately after silking, although some studies have involved elevated night temperatures throughout grainfill, or even over the entire life of the plants. Although there is still much to be learned about the effects of high nighttime temperatures on corn, recent studies do provide insight into some key questions.

How Hot is Too Hot?

Heat stress effects on corn are incremental and cumulative, which makes it difficult to delineate a specific temperature threshold above which corn yield can be negatively affected. However, research suggests that conditions in which the overnight low temperature remains above 70°F are likely to be detrimental to corn yield. A field study in which corn was subjected temperatures 4-5°F above ambient over the entire grain fill period – increasing the average nighttime low from 66° to 70-71°F – found an average yield reduction of 8% over two years (Niu et al., 2021). Kettler et al. (2024) proposed 73°F as a threshold temperature for nighttime heat stress in corn based on previous research that found a significant increase in respiration above this level (Kettler et al., 2022).

How Much Can Yield Be Affected?

High night temperatures can significantly reduce corn yield, depending on the severity and duration of heat stress. Niu et al. (2021) found that even a relatively small increase in temperature (from 66° to 70-71°F) could significantly reduce yield (-8%) when it extended over the entire reproductive period. Hein et al. (2024) found a 13.8% reduction in yield, or 2% per °F.

Yield effects of heat treatments applied over shorter durations depend on the intensity of heat. A field study in which night temperatures 4-8°F above ambient (corresponding to an increase of nighttime minimum temperature from 68°F to 73°F) were applied for 15 days following silking found significant effects on respiration, crop growth rate, and kernel number, but no significant reduction in yield (Kettler et al., 2022). Badu-Apraku (1983) found that increasing the nighttime temperature from 59°F to 77°F over a period extending from 18 days after silking through maturity reduced corn yield by 18%. A two-year field study in Argentina found that high night temperature treatments applied for a period of 15 days immediately after silking did not significantly affect yield, but heating applied for a 30-day period did, decreasing yield by around 15% (Kettler et al., 2024).

Do Hybrids Differ in Response to High Night Temperatures?

Research has found that hybrids can differ in their response to high night temperatures and that hybrids adapted to temperate environments tend to be more susceptible to nighttime heat stress than tropical hybrids. Most studies have only involved one or two hybrids; however, a recent field study conducted in Kansas included 12 hybrids (Hein et al., 2024). This study involved an increase in night temperature of 7°F over the entire reproductive period (corresponded to an increase in the average nighttime minimum temperature from approximately 70°F to 77°F). Results showed an average of 8% lower kernel weight and 14% lower yield (Figure 7).

However, results significantly differed among hybrids, with individual hybrid yield response ranging from -28% to +4%, indicating the potential for selecting hybrids with a greater tolerance to high nighttime temperatures.

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