MSU Extension Corn

Corn growth and development are strongly influenced by environmental temperatures. In general, corn develops more rapidly under warmer temperatures and more slowly under cooler conditions. For example, when corn is planted in late April in Michigan, emergence can take up to 18 days. In contrast, corn planted in mid-May typically emerges within a week. The difference in time to emergence is primarily due to normally warmer air and soil temperatures later in the spring season. Temperature also impacts the timing of other key phenological stages such as silking and black layer.

Growing degree days (GDD) is an index used to relate corn growth and development to environmental conditions, typically with observed air or soil temperatures relative to a given temperature threshold at which either growth is known to begin or stops. Using observed temperatures as input, GDD numerically approximate the amount of time an organism spends above (or below) a known threshold temperature.

Corn accumulates a specific number of GDD to reach different growth stages and seed companies use the amount of GDD to reach black layer to assign hybrid maturity rating. GDD can be calculated from either planting to black layer or from emergence to black layer, and seed companies use one of these approaches when assigning GDD ratings to corn hybrids.

When growers select a corn hybrid maturity for their field, they typically rely on relative maturity (RM) ratings assigned by seed companies. However, RM ratings have limitations in accurately reflecting growing season characteristics. First, RM is often misinterpreted as calendar days to maturity, but there is observational evidence to show that corn may require  more days in the field to reach black layer and harvest maturity than indicated by its RM rating. Second, RM ratings do not directly account for weather conditions during a given growing season such as temperature and management practices such as planting date.

Because of the limitations of RM rating, using GDD ratings for hybrid maturity selection is more meaningful, as it better reflects growing season length and conditions that corn experiences. Understanding seasonal GDD trends for Michigan can provide a useful guideline for hybrid maturity selection.

Seasonal GDD accumulation in Michigan

Figure 1 shows seasonal (May 1 – Sept. 30) GDD accumulation between 2020 and 2025 for Michigan. The GDD totals assume a lower base threshold temperature of 50 degrees Fahrenheit and an upper threshold of 86 F (the most common methodology used for this purpose). These data from the last six years provides current and reliable estimates of GDD across the state and can be used in selecting optimal hybrid maturity for a given location. Seasonal GDD accumulation has also been trending upwards over the last few decades, so using current GDD data also helps with optimal utilization of the growing season.

In the Upper Peninsula, seasonal GDD ranges from 1,200 to 2,100 units. Northern Michigan accumulates between 2,000 and 2,500 units, while the southern and Thumb regions accumulate between 2,400 to 3,100 units. Growers should match hybrid selection with the GDD estimates for their area to help ensure corn reaches maturity before a killing freeze in the fall season. For a more conservative approach, Michigan State University Extension recommends selecting a hybrid with a GDD rating slightly below the estimate for a given area to allow additional time for dry-down after maturity.

Impact of delayed planting on GDD accumulation

There is evidence that late planted corn matures quicker using fewer GDD compared to the same hybrid planted on an earlier date. This phenomenon is referred to as GDD compression. In Michigan, corn is planted over a wide window, largely due to variable field conditions including heavy spring precipitation events that lead to wet soil conditions and planting delays.

If planting is delayed, adjusting hybrid maturity becomes necessary as available seasonal GDD are now lower compared to data in Figure 1. However, GDD compression means that a given hybrid would also need less GDD under delayed compared to timely planting. Understanding GDD compression under Michigan conditions can be a useful resource to estimate GDD requirements and guide hybrid maturity selections.

To determine GDD compression in Michigan, we estimated the amount of GDD accumulated by different hybrid maturities planted between late April and early June from field trials conducted over 2021–2023 in Lansing, Michigan. We used five hybrids with RM ratings between 89–109 and GDD ratings between 2,225–2,725 units. Planting dates in our trials were April 28, May 10, May 30 and June 10.

Figure 2 shows the GDD accumulated to reach black layer by different hybrids planted from late April through early June. The downward slope indicates that each hybrid used fewer GDD to reach black layer with the delay in planting. We observed that around 25% of total GDD compression occurred by silking, indicating that majority of compression occurred from silking to black layer.

Table 1 is a summary of the daily GDD compression observed across all hybrids each year. On average, GDD needed to reach maturity decreased 6.4 GDD per day of delayed planting, with a range of 3.9 to 11.0. Practically, this means that for a delayed planting, you can estimate the GDD that a selected hybrid will potentially use to achieve maturity by multiplying 6.4 by the number of days of delay and subtracting that from that hybrid GDD rating.

Table 1. Growing degree day compression observed in each year from 2021–2023. Data is averaged across five hybrid maturities.

The magnitude of GDD compression differed among hybrid maturities. Early-maturity hybrids had lower GDD compression than mid- and late-maturity hybrids. In northern Michigan, where early-maturity hybrids are commonly planted, we recommend using a number slightly lower than the average GDD compression to adjust GDD requirements for a selected hybrid under delayed planting. In southern Michigan, where mid- and late-maturity hybrids are more common, use the average GDD compression or a slightly higher number.

Using online tools to estimate GDD

The web-based interactive U2U tool is an important publicly available resource that uses GDD accumulation to predict corn phenology. It is also a valuable tool for planning hybrid maturity selection. Nonetheless, U2U prediction does not account for GDD compression when planting is delayed. To improve prediction accuracy for late-planted corn, we recommend adjusting the hybrid total GDD requirement based on the planting date before entering it into the online tool.

For example, if a grower plants a hybrid rated at 2,475 GDD on May 30, multiply 6.4 by 29 days (May 30 compared to May 1), which equals 186 GDD. Subtract this value from the original rating (2,475 − 186 = 2,289 GDD). The adjusted value of 2,289 GDD should then be entered into the U2U for a more accurate prediction of the date of black layer.

Key takeaways

• Corn development is closely associated with environmental temperatures.
• Use climatological GDD available for a given location to guide hybrid maturity selection compared to relying solely on RM ratings.
• Use online tools like U2U to assist hybrid maturity selection
• Corn uses fewer GDDs to mature when planting is delayed, so use GDD compression estimates to adjust output from U2U under this scenario.