Harvest Delays – Light vs. Temperature

There has been a lot of discussion about the crop yields from 2023 in Ohio, from early reports of crop stress in May and June to greater than anticipated yield values for many producers this fall. Yield reports of >110 bu/ac wheat harvested in July were reported in parts of Ohio, and better than anticipated yields in some corn and soybean fields. Harvest progress of corn has been delayed from normal for many farmers.

Many questions have been raised on the role that haze from Canadian wildfires may have played on seasonal crop growth this year. Ohio experienced three major episodes of wildfire impacts on June 6-7, June 27-29, and July 16-17, with several more days throughout the two-month period of less intense smoke-filled skies. However, looking at 2023 compared to historical trends overall radiation availability was similar to the 10-year historical average for the three CFAES research stations of Northwest, Wooster, and Western (Figure 1). Light availability was higher than normal in May through mid-June, in part due to many clear days and below average rainfall. Light availability approached normal levels throughout June and July in part due to a slight reduction during the short period of haze, but recovered to mimic the 10-year patterns observed in recent past.

Despite the short haze periods, the photons available per heat unit accumulated (PTQ or photothermal quotient) were at or above the 10-year average (0-38% greater) aside from July at Western research station (6% lower) and September at Northwest (2% below normal). Generally, greater PTQ values suggest that more photosynthesis can occur in the same thermal period and could lead to greater yields.

Figure 1. Daily light integral (left) and accumulated growing degree days, base 50°F (right), and the 10-year averages for three Ohio locations of Northwest Agricultural Research Station in Custar (upper row), Western Agricultural Research Station in South Charleston (middle row), and the Ohio Agricultural Research and Development Center in Wooster (bottom row) in 2023.

Contrastingly, accumulated Growing Degree Days (GDDs) were below the 10-year average for every location this year (Figure 2). The same pattern that brought the frequent spells of wildfire smoke, northerly wind flow out of Canada, kept temperatures below average for the summer (Figure 2 – left). It is possible the cooler temperatures helped crop’s periods of water deficit better this year than in years past, but also can have contributed to the slow drydown experienced by many farmers this year.

Interesting to note, several folks have commented that this summer reminded them of the summer of 1992. Looking at that year’s temperature difference compared to average (Figure 2 – right), temperatures were cooler in 1992 than this past summer. Mt. Pinatubo erupted in June 1991 and is often pointed to as a main reason for cooler global temperatures in the year that followed. Volcanic emissions circled around the globe high in the atmosphere throughout the tropical and sub-tropical regions, reflecting and absorbing solar radiation and cooling the Northern Hemisphere surface temperatures by about 0.9-1°F.

Overall, the cooler temperatures and slower accumulation of GDDs can be the largest contributor to delayed corn harvest this year. Cooler overall conditions could have led to slightly higher than normal PTQ values for the season, which also may help explain the higher than anticipated yields in the wheat crop this summer.

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