Effects of Solar Radiation Intensity on Corn and Soybean Yield

INSIGHTS

• Shade significantly decreased yield in both crops.
• The impact of shade on yield was greater during reproductive growth stages compared to vegetative stages in corn.
• Methods to increase light in the lower canopy tended to have a minimal or negative effect on yield for both crops.

Photosynthesis

Figure 1. Shade cloth applied during vegetative stage in corn in 2024 trial.

The photosynthetic rate of both C3 (soybean) and C4 (corn) plant species is affected by numerous environmental and internal factors. Light, CO2, and water are the three substrates for photosynthesis and have a direct effect on photosynthetic rate and dry matter production. Light provides all the energy needed for photosynthesis. Both the duration and intensity of light striking the plant affects photosynthesis. As light intensity increases, the rate of photosynthesis also increases, but with decreasing efficiency, until the light saturation level is reached. At the point of saturation further increases in light no longer increase photosynthesis. C4 plants have a higher light saturation point than C3 plants, meaning C4 plants can utilize light up to a more intensive level. However, C3 plants have a lower light compensation point than C4 plants. The light compensation point is the lowest level of light intensity needed for a plant to begin photosynthesis. In other words, corn (C4 plant) requires greater light intensity to begin photosynthesis but can also increase photosynthesis at more intensive light levels compared to soybeans (C3 plant), which can begin photosynthesis under lower light levels but cannot utilize as intensive light.

Recent Smoke from Wildfires

In recent years, much of the Midwest has experienced smoke or haze in the air due to wildfires in Canada or the western U.S. A common concern from growers is whether smoke affects crop development and yield. This is a challenging question to answer because it depends on many factors such as smoke intensity, duration, crop stage, and other environmental stresses. Recent publications from Iowa State University¹, Ohio State University², and Purdue University³ highlight the potential effects of smoke in a few key points:

• Negative effects of smoke
  • Reduction in light, which can reduce photosynthesis
  • Increase in ground-level ozone, which can harm plant tissue during respiration
• Positive effects of smoke
  • Scatters sunlight, allowing light to penetrate deeper into crop canopy, increasing photosynthesis
  • Lowers leaf surface temperature and reduces transpiration, decreasing water stress
• Corn is more susceptible to smoke because it’s a C4 plant with a higher light saturation point than soybeans.
• Smoke during grain fill will affect yield more than during the vegetative period.

In summary, the consensus is that smoke from wildfires in recent years had minimal effects on crop yield. However, much more research is needed to better understand the impact of smoke on crop growth as wildfires continue to become more common.

Agronomy in Action Research Trials

In 2024, the Golden Harvest Agronomy in Action research team implemented trials in both corn and soybeans at Malta, IL, Slater, IA, and Waterloo, NE, to evaluate the effect solar radiation intensity has on yield. The main objective was not to quantify changes in yield potential based on specific levels of solar radiation but rather to understand which growth stages in corn and soybeans are most impacted by changes in light intensity.

Shade cloth was used to provide a 50% reduction in light (Figures 1 and 4). In the Midwest, smoke from wildfires has been shown to reduce solar radiation by up to 50%, however, it varies greatly by smoke intensity and other environmental factors. Even throughout the day at a given location, the level of solar radiation reduction from smoke can range considerably, making it difficult to mimic artificially. Frames were built, surrounded by shade cloth, and placed over the crop canopy during different timings throughout the season.

Mylar film was used in the field to increase the intensity of solar radiation (Figures 2 and 4). Mylar, typically used in greenhouses, can reflect 95% of the sun’s light. It was laid on the soil surface between the crop rows to reflect light up into the canopy at different growth stages.

Figure 2. Mylar applied between the corn rows designed to reflect light. Corn turned a lighter green during early vegetative stages (left). As the season progressed the mylar became dirty and became less reflective (right) in a 2024 trial.

Corn Trials

Two different trials were established in corn. One trial used mylar placed between the rows to increase light in the lower canopy at different timings.

1. Check – no mylar
2. Mylar V5 – placed at V5 and removed at black layer
3. Mylar Canopy – placed once corn canopied and removed at black layer
4. Mylar Pollination – placed 1 week prior to tassel and removed at R2 growth stage

The second trial focused on reducing solar radiation using shade cloth at different times. All timings had shade for 3 weeks.

1. Check – no shade cloth
2. Shade Vegetative – shade applied 4 weeks before silking and removed 1 week before silking
3. Shade Pollination – shade applied 1 week before silking and removed 2 weeks after silking
4. Shade Reproductive – shade applied 2 weeks after silking and removed 5 weeks after silking

Ten ears from the shade treatments were hand harvested, shelled, and weighed to measure grain per ear. One thousand kernels were weighed to determine average individual kernel weight and kernels per ear.

Corn Results

Mylar applied at the V5 and canopy growth stages reflected a tremendous amount of light into the canopy, causing the leaves to turn a light green color (Figure 2). It is likely the mylar increased the leaf surface temperature at these timings. As the inner rows became more shaded, less light was reflected and the leaves darkened up. Rain events caused water and soil to pool up on the mylar, making the surface dirty and much less reflective as the season progressed (Figure 2). It was also observed that the mylar helped reduce soil evaporation and preserve soil moisture.

Across the three locations, none of the mylar treatments had a significant effect on yield (Graph 1). All treatments tended to decrease yield at Malta, IL, while the early timings tended to decrease yield at Waterloo. Corn grown at Slater, IA, tended to increase yield when mylar was applied at V5 and the canopy timing. These yield trends were inconsistent across locations. The environment likely played a major role in how the corn responded to the increase in light intensity. If it was cool and cloudy the corn likely benefited from the additional solar radiation. But during hot and sunny days the increased light intensity may have stressed the plant, especially during the earlier timings.

LSD (0.10) = NS for all locations.
Graph 1. Corn yield response to mylar applications during different timings at three locations in 2024.

Table 1. Effect of shade timing on corn yield, kernel number, and kernel weight averaged across two locations in 2024.

A wind event at Waterloo, NE tore the shade cloth, so yields were only recorded for Malta, IL, and Slater, IA. Corn grown at both locations responded similarly and results were averaged across both locations. Three weeks of shading significantly decreased yield by 43% at the vegetative timing and by 52% at both the pollination and reproductive timing (Table 1). Shade significantly decreased kernel number per ear during all timings with the largest impact coming around silking timing due to reduced pollination and kernel set (Figure 3 and Table 1). During the vegetative stages when ear length is being determined, shade decreased kernel number per ear by 313 kernels. Shade during the reproductive stages reduced the number of kernels per ear due to kernel abortion. Due to yield component compensation, both the vegetative and silking shade times had significantly heavier kernels compared to the check (Table 1), as there were simply less kernels the plant had to fill. Despite having a small reduction in kernels to fill, shade during the grain fill period significantly decreased kernel weight by 41%. Photosynthesis was limited and the plant could not produce enough assimilates to fill the ear.

Figure 3. Effect of shade timing on corn ear size. Order of shade timing in photo is check, vegetative, pollination, reproductive.

Soybean Trial

In soybeans, one trial was established at all three locations to evaluate the effect of both reduced solar radiation and increased light intensity on yield.

1. Check – no shade cloth or mylar
2. Mylar V5 – placed at V5 and removed at senescence
3. Mylar Canopy – placed once soybean canopied and removed at senescence
4. Shade R3-R4 – shade applied at R3 (beginning pod) and removed after R4 (full pod)

Figure 4. Mylar applied to soybean at the V4 growth stage (top). Shade cloth applied to soybeans at R3 and removed at R4 growth stage (bottom).

Soybean Results

Solar radiation treatments had a significant effect on soybean yield at all three locations. Shade during the R3 and R4 growth stages reduced yield by 44% at Malta, IL, 42% at Slater, and 14% at Waterloo, NE (Graph 2). A windstorm at Waterloo, NE destroyed the shade cloth and was removed 10 days earlier than the other locations which minimized the negative impact of the shading. Pod development is a critical time in the life cycle of a soybean plant in determining yield potential. Visually, plants under the shade cloth tended to stay green longer and mature later.

Mylar treatments did not significantly affect yield at Malta, IL, or Waterloo, NE, however, at Slater, IA, both timings significantly reduced yield. Mylar decreased yield by 8.3 Bu/A when applied at the V4 growth stage and 4.5 Bu/A at canopy timing (Graph 2), as soybeans cannot utilize as intensive light level as corn. Soybeans become CO2 limited before becoming light limited. It is suspected that the additional reflected light and heat was detrimental to crop growth. The longer the mylar was in the field, the more significant the yield penalty.

Different letters represent statistical difference at α=0.10 for each location.
Graph 2. Effect of solar radiation treatment on soybean yield at three locations in 2024.

Summary

Light is one of the most important components in plant photosynthesis. Manipulating light intensity using reflective mylar tended to have a minimal or negative effect on yield in both corn and soybeans. Any benefits from increased light in the lower canopy were likely negated from higher canopy and leaf surface temperatures. Mylar also became dirty and less reflective later in the season when the crop canopy was denser and may have been light limited.

Not surprisingly, shade had a significant effect on yield for both crops. During the critical period of pod setting in soybeans, yield potential was reduced by over 40% at some locations. Shade timing was also important when it came to corn yield potential. Shade during vegetative growth stages had significantly less effect on yield than shade around pollination or reproductive growth stages. Using shade cloth that provided 50% light reduced solar radiation far more than what has been seen with recent wildfire smoke. Shade cloth provided a more consistent and prolonged reduction in solar radiation than smoke created. However, it’s also valuable to understand what crop growth stages may be more impacted by smoke.

These studies demonstrate the impact a reduction in photosynthesis can have on yield. Any factor that causes crop stress or reduces crop growth will decrease photosynthesis. Drought stress, pest pressure, nutrient deficiencies, etc. all have the same effect on yield as shade. In most cases, a nutrient deficiency will not correct itself during the season and will affect photosynthetic potential all season long compared to smoke reducing solar radiation for a 2- to 3-week period. Focusing on crop management and minimizing crop stress for factors that can be controlled is key to maximizing yield potential.

Resources

¹Archontoulis, S., and M. Licht. 2023. Wildfire smoke impacts on crop production. Integrated Crop Management Blog. Iowa State Univ. Ext. https://crops.extension.iastate.edu/blog/mark-licht-sotirios-archontoulis/wildfire-smoke-impacts-crop-production

²Lindsey, A., L. Lindsey, and O. Ortez. 2023. How could the haze of wildfires affect crop growth? C.O.R.N. Newsletter. Ohio State Univ. Ext. https://agcrops.osu.edu/newsletter/corn-newsletter/2023-21/how-could-haze-wildfires-affect-crop-growth

³Quinn, D. 2023. How does wildfire smoke impact corn growth? The Kernel. Purdue Univ. https://ag.purdue.edu/news/department/agry/kernel-news/2023/07/2023-corn-wildfire-smoke.html