The smoke from the 2023 Canada wildfires was noticeable in Iowa, especially on June 28 and 29, and concerns about impacts on crop productivity were voiced. The smoke from the California wildfires (August 2021) was evident in Iowa too, however, this did not have a negative effect on crop production. On the contrary, 2021 was a record-yielding year for Iowa.
Overall, the effects of smoke conditions on crop productivity and health are complicated and not well-researched. There are both positive and negative aspects. Negative aspects include the reduction in sunlight and the increase of ozone levels which can lead to leaf photosynthesis reductions. In that case, the plant remobilizes carbohydrates from stalks to satisfy grain demand making stalks weaker and susceptible to lodging.
Positives includes the scattering of light and the increase of the diffuse fraction of photosynthetically active radiation. And because diffuse light can penetrate deeper into the crop canopy and thus increases whole canopy photosynthesis. The direct fraction is mostly absorbed by the top canopy leaves. It is beneficial for crop production that all canopy leaves contribute to photosynthesis, more than just the top-middle leaves that receive most of the direct sunlight.
Crops are more sensitive to cloudy-smoke conditions (reduced radiation and increased ozone levels) during reproductive stages compared to the vegetative stages. We believe that the smoke conditions observed at the end of June 2023 (vegetative period) had minimum to no impact on crop health and production potential. Furthermore, any reduction in sunlight caused by smoke conditions was small given also the dry (and cloud-free) conditions the crops experienced in Iowa in June 2023.