Potential Impact of Wildfire Smoke Across Midwestern U.S. Corn and Soybeans

INSIGHTS

• Many states across the Midwest experienced days of poor air quality caused by wildfire smoke in 2023.

• Studies show that prolonged extensive shading or light reduction is required to see higher levels of yield loss and it is unlikely that wildfire smoke haze caused excessive yield loss.

The Midwestern US has experienced air quality warnings and hazy days as smoke from wildfires moved over the region. This raised the question of the potential impact of smoke on growing crops. Available solar radiation from sunlight (in addition to temperature and precipitation) plays a strong role in crop growth and development.

Potential Impact of Smoke on Crops

Sunlight is an essential component in photosynthesis that results in production of carbohydrates used for plant development and grain production. Reductions of plant available light at key growth stages can have negative impacts on yield potential and possibly put plants at higher risk for late season stalk lodging. Corn utilizes the C4 carbon fixation pathway which has a higher light saturation point than soybeans (C3 pathway), making it more susceptible to solar radiation or photosynthetically active radiation (PAR) reductions.

Flowering and grain fill growth stages are expected to be more negatively impacted by reduced sunlight than vegetative stages. Yield response to solar radiation is dependent on the 1) crop stage when light is deficient, 2) length of time solar radiation is reduced, and 3) the severity of solar reduction. Low solar radiation has the biggest effect on corn yield potential during silking and grain fill periods.

Map showing Air Quality Index cause by wildfire smoke and other factors, Ozone and PM (PM 2.5 and PM 10) - June 26,2023; EPA AirNow - https://gispub.epa.gov/airnow

Influence of Crop Stage and Shading Intensity

- Shading corn to 50% light intensity reduced corn yield by:

• 12-20% when shaded during silking.
• 19-21% when shaded during grain fill.^1,2

- Less severe shading (85% light intensity) resulted in no yield loss in the same trials.

- Shading during silking often results in ear tip-back or fewer kernels per row whereas shading during grain fill results in decreased kernel weight from shallower kernels.

These studies show that dramatic shading or light reduction is required to see higher levels of yield loss potential. While there may be some yield loss in areas from reduced solar radiation caused by smoke, it is likely small and insignificant. Other factors such as temperature, precipitation, nutrients, etc. are also critical during silking and grain fill and will play a role in yield potential.

July flowering and August grain fill periods are the most critical times to evaluate for sunlight deficiencies as they are both periods that can highly influence yield and stalk strength. If planting was delayed into late-May or early-June, it may be more applicable to consider August and September solar radiation impact on corn since flowering and grain fill dates are later than normal.

Smoke haze over corn in Ohio, June 28, 2023. Image source, W.Looker

Other Considerations

Increased Ozone Levels: Wildfires produce large quantities of pollutants such as nitrogen oxides and organic carbons which can react with sunlight to make ozone(O₃). Ozone is a damaging air pollutant that may be harmful to plant growth. Elevated ozone pollution, if present near where plants are growing, has the potential to damage plant tissue during respiration causing plant stress.

Increased Light Diffusion: Smoke creates diffuse light which could beneficially lower leaf surface temperatures. Less direct light could reduce the amount of transpiration needed to cool leaves and be a benefit in drought/water stress conditions. Diffuse sunlight could potentially help improve photosynthetic efficiency since it has been found to be optimized at 50-67% of full sunlight intensity in some cases.³

Lack of precipitation in an area (and associated cloud cover) can create more days with adequate solar radiation, but can also reduce the solar radiation demands of the plant (due to reduced water uptake). The presence of smoke in drier regions may not always result in solar radiation deficiencies, even though less radiation was accumulated for the season.

Reference

¹Liu, W. and M. Tollenaar, 2009. Physiological mechanisms underlying heterosis for shade tolerance in maize. Crop Sci. 49:1817-1826

²Reed, A., G. Singletary, J. Schussler, D. Williamson and A. Christy, 1988. Shading effects on dry matter and nitrogen partitioning, kernel number, and yield of maize. Crop Sci. 28: 819-825

³Lindsey, A., L. Lindsey, and O. Ortez. 2023. How could the haze of wildfires affect crop growth? C.O.R.N. Newsletter. Ohio State University Extension.