Drought, wildfire smoke and GDUs at play in 2023 crop fields

By John Schoenhals, Pioneer Field Agronomist, Northern Ohio

There are many “adages” that fit all sorts of weather or farming situations, but one of my favorites came from my mom: “Normal is just a setting on the dryer.” Like most years, there has been nothing “normal” about the first half of the 2023 growing season.

Crop growth this year has been subjected to extremes in temperature, precipitation, and air quality/sunlight. The impacts of these extremes can be seen in the development of crops, and may continue having impacts on crops through the rest of the season.

Corn development is primarily driven by growing degree unit (GDU) accumulation. So far this year, GDU accumulation is near or below average. This is in contrast to several recent years in which GDU accumulation was above average for most of the growing season. While it may seem that the corn crop is “behind” where it should be based on calendar dates, most fields are at expectations based on GDU accumulation since planting.

While cool nighttime temperatures and few hot days have slowed growth, these conditions have also allowed the crop to efficiently use water and maintain growth, even with lack of rainfall. Several parts of Ohio tied or even beat the record for the longest stretch ever with no measurable rainfall (approximately 3 weeks).

The dry start to the growing season magnified any issues present in fields — starter fertilizer injury, seeding depth mistakes, herbicide carryover, compacted areas, etc. have been painfully visible. The return of rainfall in many areas occurred at an important time, as corn begins rapid vegetative growth and formation of ears and tassel in preparation for pollination. While severe drought stress in the early season can negatively impact yields, impacts early are modest compared to severe yield losses that can occur when drought stress occurs in the two weeks before and after pollination.

Compared to corn, soybean development is much less dependent on GDU accumulation. Instead, night/day lengths largely trigger key growth stages in soybeans. Soybean growth is slow early in the season, before rapidly advancing after plants reach about the V6 stage. With challenging conditions this year, soybean growth has moved even slower than normal. Many fields in dry areas seemed to grind to a halt before receiving beneficial rainfall.

Soybean nodulation is moving into high gear, thus jumpstarting nitrogen production and rapid growth. Under good growing conditions, soybeans add a new trifoliate node about every three to four days starting at the V6 growth stage (growth is much slower before this). Vegetative growth will continue until around the R5 stage. Soybeans are most sensitive to drought stress from R3-R6 (pod set and seed fill stages) but will likely compensate well for the early season stresses encountered this year.

A very unique aspect of the 2023 growing season has been presence of thick wildfire smoke/haze. There are three primary factors associated with wildfire smoke: reduced total solar radiation and elevated ozone, which are both negative, and increased diffusion of solar radiation, which can be positive. 

The most obvious effect of wildfire smoke in the atmosphere is a reduction in total solar radiation. Smoke reflects a portion of incoming sunlight, reducing the amount of light available to plants. Any reduction in light is potentially detrimental to crop productivity. Corn is likely more susceptible to reductions in solar radiation than soybeans.

In addition to reflecting a portion of incoming light, smoke also scatters it, making the light available to plants more diffuse. This can actually benefit plants by increasing their light use efficiency. The potential effect of more diffuse light on plant growth depends on the characteristics of the plant canopy, with taller, multilayer canopies likely to benefit more from diffuse radiation than shorter plants.

A study conducted in 2018 in the California Central Valley found that total photosynthetically available radiation (PAR) was only reduced by 3.6% on average due to wildfire smoke, while the diffuse fraction increased by over 34% during the study period. The predicted effect on corn was actually a 2.5% INCREASE in photosynthesis, as the positive effect of more diffusion exceeded the negative effect of reduced total light available to plants.

As a last possible impact, wildfires emit large quantities of chemicals that can react to create ozone in the atmosphere. Ozone may damage plants by entering stomata and oxidizing (burning) plant leaves during respiration.

The combination of rainfall, solar radiation, and temperature have a major influence on yield potential. During vegetative growth, the impact of these factors is often minimal. If extended periods of dry weather, smoke, or clouds persist during grainfill, the expected impact on yield will be much greater.