Is it time for the last irrigation?
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EmailCrop stage, soil moisture and weather: the three pillars of your last irrigation decision.
One of the most common questions this time of year is, “When do I stop irrigating?” It’s also one of the hardest and most important decisions irrigators make. Ending irrigation too soon can leave crops short on water, risking yield loss. On the other hand, irrigating too long wastes water, energy and time.
The goal of the final irrigation is to carry the crop to maturity, maximize yield and leave the soil in good condition for harvest or cover crop germination.
Typically, late August and early September bring cooler weather, lower evapotranspiration, and higher chances of rainfall. That often reduces the need for late-season irrigation. However, in a dry year or with late-planted fields many crops may still have significant water needs into September.
Here are four key things to consider when deciding when to stop irrigating.
1. Crop stage and maturity
Crops use water differently throughout their growth stages. Once they pass peak water demand, water needs begin to decline because of lower transpiration and weather conditions.
For soybeans, water use continues until pods begin to yellow. At the R5-R6 stage, soybeans use about 0.10-0.15 inches per day. At maturity, water use drops to around 0.04 inches per day. From full seed to maturity, soybeans need about 3.5 inches of water. Several studies show that stopping irrigation too early during pod fill can reduce seed size, accelerate senescence and ultimately lower yields.
For corn, water use drops during the dent stage but still requires about 0.14 inches per day. Physiological maturity occurs at black layer. After this point, additional irrigation will not benefit the crop. From full dent to black layer, corn requires about 2.5 inches of water. When milk line is about 50-75% you should start questioning the last irrigation, depending on your soil moisture conditions. Research by Nielsen, 2025 shows that failing to meet water needs during grain fill can reduce kernel weight and grain quality.
2. Soil available water in the root zone
Think of your soil like a glass. Is it full, half-full or nearly empty? Simple methods include the checkbook approach using local weather data from Michigan State University's Enviroweather. Advanced soil moisture sensors give more precise readings.
A general rule is to keep at least 50% of your soil’s water capacity to avoid stress. For example, if your root zone holds a maximum of 3 inches of water (depending on soil type), aim to maintain at least 1.5 inches until the crop reaches full maturity. Follow the “Soil Moisture Sensor Factsheet” for more information. Michigan State University Extension provides different irrigation scheduling methods.
3. Weather conditions and forecast
Always factor in the weather forecast. A timely rainfall may reduce or eliminate the need for late-season irrigation. Leaving room in the profile for rainfall is usually a good strategy. However, hot and dry conditions may quickly increase crop water demand, making irrigation necessary.
4. Next season’s plans
If you plant seed cover crops, dry surface soil may prevent good germination. A light irrigation, 0.3 inches on sand or 0.4 on loamy sand, can wet the top 6 inches for better cover crop establishment.
As the season winds down, continue to monitor crop growth stage, soil moisture and weather conditions. Don’t make decisions based on neighboring fields; planting dates, varieties and soils differ, and each field has unique water needs.
Help us study irrigation’s impact on yields
This year, Michigan has seen wide variability in precipitation; some areas have had more than enough rain, while others have struggled with drought. To better understand how irrigation affects yield under these conditions, Michigan State University Extension is looking for farmer cooperators willing to share yield data from irrigated fields and non-irrigated areas.
Participants will need to provide:
- Yield data for both irrigated and non-irrigated areas (such as dry corners).
- Estimates of rainfall amounts and irrigation applied.
This information will be used strictly for research purposes. If you’d like to participate or get more information, please send a message to gradizme@msu.edu.
