Corn grain moisture at physiological maturity (black layer) typically ranges from 30–35%, but it can vary from 25–40% depending on the hybrid and environmental conditions. We collected corn ears from a relative maturity study at the Boone Agronomy Farm, Iowa State University, to determine current kernel moisture levels. The study was planted on May 6, 2025, with several hybrids of varying relative maturities.
Ear samples collected on August 28 from selected hybrids showed an exponential relationship between grain moisture and hybrid relative maturity (Fig. 1). Short-season hybrids (<100-day) had grain moisture at or below 30%. Hybrids with 110-day relative maturity, the most common in this region, averaged around 38% grain moisture. Late-season 120-day hybrids had grain moisture of about 45%.
Fig. 1. Corn grain moisture on August 28, 2025, as a function of hybrid relative maturity. The trial was planted on May 6 at the Boone Agronomy Farm, central Iowa.
From May 6 to August 28, corn accumulated 2,427 °F-days, which is 55 °F-days above the 30-year average. Typically, a 110-day hybrid requires about 2,650 °F-days to reach maturity. In this case, an additional 219 °F-days, roughly 7–12 calendar days, are needed to reach physiological maturity. During this final period, kernel dry matter accumulation is relatively small.
Overall, corn is rapidly approaching physiological maturity this year, particularly in fields planted before May 1. After reaching physiological maturity, kernels begin losing moisture until they reach harvest maturity. The ISU Dry Down Calculator can be used to estimate corn dry-down rates between September 1 and November 20.
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