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CropTalk: Finishing the Season: Grain Fill Process in Corn

August 2022

Published on Monday, August 1, 2022

Most of the management of a corn crop is completed during the first half of the growing season. Establishing a consistent stand, managing weeds, and applying nitrogen and other nutrients are all tasks completed before tassel. We might apply a fungicide, insecticide, or foliar nutrition as needed after tassel, but unless irrigation is an option, August and September are usually spent preparing for harvest and praying for optimal weather – warm days, cool nights, and ideal rainfall.

Even though we may not spend as much time in the fields, a lot is happening to the corn crop as it completes its reproductive and grain fill growth stages. Here is a look at what is happening at each of these stages as we watch the crop progress toward harvest:


Tassel emergence (VT) and silk emergence (R1) usually occur close to the same time as the crop transitions from vegetative to reproductive growth. It is possible to check the progress of pollination by observing the tassel and silks. Pollen begins to shed from the center of the main spike of the tassel first. As pollination progresses into the second and third day, the entire center spike will have opened. The side branches of the tassel are the last to shed pollen. Looking for this pattern can tell you how long corn has been pollinating. You can also gently husk back the ear (from the butt end — try to leave all silks in place until the husk is removed), then shake the ear. The silks that fall off have already pollinated and the ones still attached have yet to pollinate. Pollination begins at the butt of the ear and progresses towards the tip.


After pollination is complete, the silks begin to turn brown and dry out and kernel formation begins. The white kernels look like “blisters” on the cob, and the silk attachment points are still intact, giving the ear a rough texture. At this stage, severe stress can easily abort kernels.

The R2 stage is when the plant begins remobilizing nutrients and moving them from the stalk and leaves to the ear. The plant also continues to take up nutrients from the soil, so if the soil is deficient in any nutrients, especially nitrogen, the plant will drain nutrients from the stalk much faster than normal, which can result in weaker stalk strength. Even if the soil has good fertility, drought conditions may limit the plant’s ability to take those nutrients in, leading to weaker stalks.


Kernel development continues, and the kernels begin to change to a yellow color. The contents of the kernels are still primarily liquid. I like to call this stage the “sweet corn stage” because the ear looks just like a ripe ear of sweet corn. Silks are completely brown and dried at this point. Stress can still cause kernels to abort at this point, but not as easily as at R2.

While the PFR Proven™ timing to apply fungicides has traditionally been shortly after tassel at VT, Beck’s PFR team wanted to take a closer look at that window and began testing a range of application timings. After the first year of testing, we found that it can be profitable to apply fungicides as late as the R3 growth stage (see results below.) In some cases, it may be more beneficial to wait to make an application later than VT/R1 if there is no disease pressure at that time. Making a fungicide application at R2 or R3 may better protect against late-season diseases like Tar Spot and Southern Rust.


Depending on weather and the relative maturity of the hybrid, the dough stage typically occurs three to four weeks after pollination. This is when the kernel begins to accumulate dry material, forming a dough-like consistency. The cob will change to a pink or light red color. The R4 stage is also important for disease management—any diseases that show up at R4 or later typically have little to no impact on final yield.


The dent stage is easily recognizable as dents begin to form in the kernels. This is the stage at which kernel size and density are finalized, which is the final determination of yield. The milk line forms at the dent stage, beginning at the outside edge of the kernel and progressing towards the tip. This line indicates the division between the dry and liquid material in the kernel and can be used as a gauge of maturity and drydown. Half milk line is an important stage for silage growers as that is typically when corn can begin to be harvested for silage.


This is the “finish line” as far as the plant is concerned. The milk line has disappeared at the tip of the kernel and has been replaced by a black line just inside the tip. This signals that the kernel and cob are no longer exchanging water or nutrients. The corn is also safe from frost at this point.

However, there is still time to wait before harvest. At black layer, corn is usually around 35% moisture. From 35% to 25%, it takes 25 to 30 growing degree units (GDUs) to remove 1% of moisture. Once the corn reaches 25%, dry down slows, requiring 40 to 45 GDUs per 1% of moisture.

The chart below provides an overview of how many GDUs to expect at different temperature levels. Sun, wind, and humidity levels also can accelerate or TREATMENTS slow drying rates in the field.

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Author: Mike Hannewald

Categories: CropTalk, 2022

Tags: CropTalk, corn, grain fill

Mike Hannewald

Mike Hannewald

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