By Ross Greene, UGA Extension ANR Agent – Evans and Candler Counties
As we move into the late summer stretch, many corn fields across Southeast Georgia are nearing the final stages of grain fill, making now the time to start watching for the right window to terminate irrigation. Knowing when to stop watering is a critical decision that can protect yield and reduce unnecessary water use.
The Importance of the Milk Line
One of the best ways to determine if irrigation is still needed is by watching the milk line on the kernel. Corn enters the dent stage (R5) about 31–33 days after silking. This is when a noticeable dent forms at the crown of each kernel.
Roughly a week after denting begins, a white milk line forms. This line marks the transition from milky liquid to solid starch inside the kernel, and it gradually progresses from the top of the kernel down toward the cob. While the plant’s water demand declines as this line moves, grain fill is still underway and must be supported.
When to Stop Irrigating
According to Dr. Michael Plumblee, Clemson University Corn Specialist, irrigation should continue until the milk line has reached ½ to ¾ of the way down the kernel, and soil moisture is adequate to carry the crop to physiological maturity (black layer, R6). At black layer, the plant has completed dry matter accumulation, and irrigation is no longer necessary.
In the Coastal Plain, where sandy soils have low water-holding capacity, shutting off irrigation too soon can be especially risky. Late-season drought stress can reduce yield and increase the risk of stalk lodging, as the plant pulls nutrients from stalks to finish grain fill.
Field Evaluation Tips
To make an informed irrigation decision:
- Pull 6–8 ears from representative parts of the field.
- Break the ears in half and examine the kernel cross-sections.
- Assess the milk line—aim to terminate irrigation when it’s around 50–75% progressed and your soil moisture is sufficient.
Always consider your current moisture levels, forecasted rainfall, and temperature trends before making a final decision.
What Happens If You Quit Irrigating Too Early?
The chart below shows potential yield and economic losses if irrigation is terminated before corn reaches physiological maturity:
| Milk Line at Irrigation Termination | Estimated Yield Loss | Bushel Loss (220 bu/ac potential) | Revenue Loss ($5.00/bu) |
|---|---|---|---|
| At Dent Stage (0% milk line) | 15–20% | 33–44 bu/ac | $165–$220 |
| 25% Milk Line | 10–15% | 22–33 bu/ac | $110–$165 |
| 50% Milk Line | 5–10% | 11–22 bu/ac | $55–$110 |
| 75% Milk Line | <5% | 0–11 bu/ac | $0–$55 |
| Black Layer (100% milk line) | 0% | 0 bu/ac | $0 |
Even a few missed irrigations during grain fill can have a significant financial impact, not just in yield, but also in quality and standability.
Final Thoughts
As we approach corn maturity, milk line monitoring and soil moisture assessments are key tools for smart irrigation management. Continue irrigating through the R5 stage, and terminate only once you’ve reached ¾ milk line progression and soil conditions are favorable.
If you have questions or want help evaluating your corn crop, contact your local UGA Extension office. We’re here to help you protect your yield, conserve water, and make sound decisions based on field conditions.
