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Predicting Corn Maturity and Grain Moisture (09/12/19)

Late planting and cool summer weather have raised concerns about corn reaching maturity before the first killing frost.

Late planting and cool summer weather have raised concerns about corn reaching maturity before the first killing frost. A related concern is how wet will corn be this fall at the time of normal harvest. Wet and cold weather in April and early May delayed the start of the growing season for most corn in the state. In fact, only 42% of the corn was planted by May 19th in North Dakota this year. The cooler-than-average weather in July and August further delayed corn development. To date, growing Degree Day (GDD) accumulations are running 150 to 270 GDDs behind normal (assuming a May 15th planting date) (see following table for information from a few locations). When you consider that GDD accumulations average less than 13 per day this time of the year, you get a sense of how late our corn crop might be in terms of calendar days. Planting date, hybrid relative maturity as well as weather are the main drivers of maturity. Digital tools that take into account these factors can be used to predict when corn is likely to mature. Of course, these tools use past weather data to predict outcomes, so predictions are subject to the uncertainty of future weather. The closer we get to the end of the season, however, the less uncertainty there is in making these predictions.

I use the U2U Decision Support Tools – Corn GDD for tracking corn development. This tool requires you to identify your location, your hybrid maturity and planting date. It generates graphic and tabular data on GDD accumulations and estimates the average date that the crop will reach physiological maturity (black layer) as well the range of possible dates based on the variability of past weather used in the model. I have included the output for a few scenarios in the following table.

Reaching maturity is important as it means that the crop has maximized the amount of weight that it has packed into its kernels. However, reaching maturity is only part of the issue, as the crop must dry in the field to the point that it can be harvested and be economically dried for storage and marketing. Predicting the rate of drying in the field is more complex than predicting corn development. Factors that affect the rate of field drying include: the initial moisture content of the grain, air temperature, relative humidity, rainfall, dew, wind speed and kernel characteristics. Recently, Iowa State University developed a corn dry-down calculator that uses historical weather data to predict the rate of dry-down. This calculator is available online at It uses a similar interface to the U2U GDD Decision Support tool but requires an estimated date of physiological maturity or the moisture percent of the grain at the date the simulation is initiated instead of information on hybrid and planting date. The last two rows in the following table contain data from simulations using the estimated date to maturity simulated from the U2U GDD tool as an example of how this tool might be used.

These simulations point to challenges this year for corn in reaching maturity and in drying sufficiently to be economically harvested, dried and marketed. These tools can help plan for these challenges. As indicated by these simulations, harvest will be later and more post-harvest drying will be required when compared to past seasons.

On the positive side, yield potential is quite good in many parts of the state this year and since there has been limited moisture stress, stalk quality is good. This should help keep the crop standing even through a delayed harvest is predicted.


Table 1. Growing Degree Day deviations from normal, and predictions for corn to reach maturity and its moisture level on November 1, for selected hybrid maturities and locations in North Dakota.







Deviations from normal GDDs






Hybrid RM used in simulations






Estimated date to maturity






Date to 20% moisture






Moisture on Nov 1st








Joel Ransom

Extension Agronomist, Cereal Crops

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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