As your corn crop turns the last corner of the 2025 growing season and heads for the finish line and harvest, there is still a lot of potential yield to be made or lost in the process.
The R5 growth stage (dent) – the next to last growth stage for corn – is one of those key times in the season where your management practices and Mother Nature’s cooperation up to that point can influence harvest outcomes.
Dan Quinn, Purdue University Extension corn specialist, explains the reason: kernel dry matter content in a corn crop at the beginning of R5 is only at roughly 45% of the eventual final accumulation.
Another way to look at that – there is still up to 55% of the kernel dry weight left to be accumulated by the crop, starch that can contribute significantly to grain fill and yield.
“Therefore, if significant environmental stress (drought, nutrient deficiency, etc.) were to occur during beginning R5, significant yield losses can still occur,” Quinn writes in this online article.
Many of the newer hybrids seed companies have developed are designed to add bushels by increasing the amount of starch in kernels, according to Ken Ferrie Farm Journal Field Agronomist.
“That’s one way we obtain yield increases without raising populations,” Ferrie says.
Keying Into The R5 Growth Stage
Michigan farmer Nathan Baker addresses the importance of R5, in his most recent video, posted to YouTube on Monday.
In the video (starting at about 24 mintues), Baker is evaluating his early-April planted corn, which is starting to reach dent (R5). Of his entire 2025 crop, he says it is the most advanced field of corn he has, noting there is some disease pressure present.
“That is northern corn leaf blight. You can see it started with a lesion here, and it has spread. There’s another one. I don’t like to see that…but there’s nothing drastic,” Baker says, pointing to some damaged areas on a corn leaf.
Knowing the importance of late-season grain fill is a key reason Baker says he made the investment in applying a foliar fungicide some weeks earlier.
“That’s why it’s important that we … take care of these plants late in the season and keep packing that starch in there, giving it all the nutrients and the things that it needs. It’s why I still really, really want some rain, because we can still use it to help make this corn crop better,” he says.
Baker gives a shout out to his AgriGold agronomist, Wayde Looker, for the insights he learned about the R5 growth stage.
Look For The Milk Line In Kernels
As corn moves into the R5 stage, you can start to see a distinct line near the top of kernels, which is the milk line. This line indicates the division between the dry and liquid material in the kernel.
As kernels harden, the line moves from the top of the kernel down to where the base attaches to the cob. Keeping an eye on the milk line’s progression is useful to corn growers who cut crop for silage or are trying to determine when to stop irrigating. It’s also helpful for growers trying to determine how much time is left before the corn reaches maturity and will be ready to combine.
Quinn says corn moves through the early part of R5 quickly and then slows as it nears physiological maturity (R6, black layer). Overall, from the beginning of R5 to maturity is about 33 days:
3 days — from the start of R5 to the quarter milk line stage
6 days — from quarter to half milk line
10 days — from half to three-quarters milk line
14 days — from three-quarters milk line to black layer
Actual time to black layer may vary depending upon the hybrid and the environment. However, this is a guide that lets you know what to expect and help you plan for harvest, Quinn says.
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