Corn Silage Approaching Maturity – Moving Past Thumb Rules

Corn Growth and Development

Corn growth is strongly correlated with the accumulation of growing degree days (also called heat units) throughout the growing season. Long-time producers will note that corn "just seems to do better" when temperatures are warm, and comment that "it's taking forever to grow" if temperatures are cool. For a short explanation of growing degree days (GDDs) and how they're calculated, read Understanding Growing Degree Days.

So, the question stands: If corn's physiological development (growth stage) is driven by the amount of heat units we acquire each day, why do we still try to harvest corn silage on x date of y month? Shouldn't we be able to use daily temperatures to help us monitor crop development, and predict when the crop is ready to harvest for silage?

Corn Silage Harvest and Growing Degree Days

An old rule of thumb told us that corn silage should be ready for harvest about 45 days after tasseling. Given the variability in each growing season's weather conditions, we must strive for a more accurate method of predicting corn silage maturity timing. Research in 2003 by Bill Cox at Cornell University's Aurora Research Farm (summarized in Table 1) observed corn growth, tasseling date, and whole crop moisture in relation to growing degree day accumulation. Over the course of 3 years, researchers found that the time allotted between silking and 68% whole-crop moisture could range from 31 to 45 calendar days. They also found that the number of growing degree days required in that time consistently ranged between 750 and 850 GDDs and had a stronger correlation to crop development than simply counting days on the calendar.

Table 1 - "Using the Number of Growing Degree Days from the Tassel/Silking Date to Predict Corn Silage Harvest Date", W.J. Cox, Cornell University. The years presented in this study saw varying levels of moisture and heat stress. For more information on the conditions present, you can read Record Silking/Tasseling Dates in Corn Fields.

So, producers looking for a consistent method of predicting silage maturity may simply record the date that their crop reaches tasseling (or is expected to reach tasseling) and track GDD accumulation after that date. Cox's research suggests that most hybrids under 100 RM require about 750 GDDs from silking to silage harvest, and most hybrids from 101-115 RM require about 800 GDDs from silking to silage harvest. An easy way to track growing degree days is with an online calculator, such as the Climate Smart Farming Growing Degree Day Calculator, a free tool developed by Cornell University to help track GDD progression. You can input a planting date for each field or farm you plant, update the GDD base to reflect the 86/50°F cutoff for corn, and plot when you would expect to reach the GDD target for 68% whole-crop moisture.

Figure 1. In this example, a 113 RM hybrid planted on April 20^th, 2024, would require approximately 1420 GDDs from planting to reach mid-silking, and an additional 800 GDDs are forecasted to accumulate by August 8^th, 2024. Note how the "season to date" line shows rate of accumulation was significantly faster than the 15- and 30-year averages.

Statewide Example: 2020-2024

Let's compare the GDD requirement for various hybrids planted across Pennsylvania, and how their rate of accumulation has varied over 5 growing seasons. This chart examines the GDD requirements from several commercially available corn hybrids and adds 800 GDDs to their mid-silking rating to reflect the date they reached 68% moisture.

Use of example seed brands, hybrids, and trait packages do not imply endorsement by Penn State Extension.

Year-to-year variability in GDD accumulation is obvious in this chart: Compared to the 2023 growing season, 2024 growing season weather projections places many of our heavy corn silage areas almost 10-14 days ahead of the previous years' silage harvest. In 2024, the Mount Joy location is about 5 days ahead of the 5 year "normal" harvest, but the Martinsburg location has little departure from "normal" harvest timing.

Other Considerations

I'd like to point out now that GDD ratings for most commercially available hybrids are developed in the Midwest, and Pennsylvania data has suggested that most corn hybrids require 150-200 fewer GDDs to reach physiological maturity. However, this reduction in GDD requirements does not translate perfectly to silage maturity. Silage timing is further complicated by severe stressors (such as drought, excessive moisture, or heavy rains late in the season).

The year-to-year variations highlighted in this chart do not account for other differences in management practices; farm-to-farm and field-to-field differences in planting dates, hybrid maturity, and premature death from disease can have a drastic impact on the timing of silage maturity within a single growing season. This model also does not account for delays in germination and growth initiation due to dry soils.

The target dates provided by the +750/800 GDD guide predict a whole-crop moisture content of 68%. As this is wetter than many farms may prefer, this target moisture is more of an "early-warning" system to give producers a chance to prepare for harvest. We can expect that on average, corn will lose 0.5% of moisture per day (or gain 0.5% of dry matter) as it approaches silage maturity. This dry-down rate can vary from 0.0-1.0% moisture loss in extreme weather conditions, so continue to monitor the crop moisture as you approach your targeted moisture level.

In severely droughty years, especially with temperature extremes, growing degree day accumulation is inaccurate for tracking crop development. Another common practice for estimating silage maturity (targeting 1/2-3/4 kernel milkline) can also be an inaccurate indicator of whole-crop moisture in extremely dry years. Review Managing Drought Stressed Corn for guidance on managing critically impacted corn crops.

Putting it to Practice

We should not use growing degree day accumulation as a hard and fast rule to start chopping, but rather as a guide for when it's appropriate to subsample fields and chip and dry them to test crop moisture content. Ideal moisture ranges for corn silage are based on storage structure type. A quick and easy way to test silage moisture on your farm can be found in How Do I Know When its Time to Harvest my Corn for Silage? You can also test moisture by sending a sample to a forage testing lab for a basic moisture test, or by utilizing a Koster tester.

When sampling your fields for moisture content, remember that a representative sample provides the most accurate glimpse into whole field conditions. Therefore, take plant samples that are representative of the entire field, avoiding any that appear excessively wet, dry, or damaged compared to the rest of the field. Generally, 3-4 stalks provides enough forage to test accurately, but larger or more variable fields may warrant extra subsamples to provide better representation.

Be cautious of starting silage harvest too soon in droughty conditions. Often, the whole-crop moisture will be much higher than it appears, leading to issues with seepage, nutrient losses, and poor fermentation quality. Remember that harvesting a crop that is too dry can lead to fermentation issues and spoilage from molds and yeasts and harvesting a crop that is too wet can lead to silo deterioration, seepage issues, and nutrient losses from leachate.

A comprehensive review of silage production practices can be found at From Harvest To Feed: Understanding Silage Management.

Be sure to check in with our weekly Field Crop News newsletter throughout this fall as educators across the state collect corn samples and dry them to track silage dry-down.