Moisture stress early in the season has very little impact on yields when compared to similar stresses during the pollination and grain fill periods. Slightly dry, non-restrictive soil conditions can be favorable for deeper root development. When we see these brief periods of stress early but conditions are still favorable for plant growth, irrigation is not always necessary. Watering too early can have a negative impact later in the season. Keeping the soil saturated increases potential for leaching of Nitrogen below the root zone. But, if crops are under stress for a prolonged period of time, irrigation will usually be recommended.
Before starting any irrigation, a few considerations and calculations need to be made about the depth of the plant’s roots, available plant water from the soil, and the daily crop water use. Corn at the V6 to V8 will have a root system 18” to 24” deep and has access to about half of the plant available water that a full sized plant with a deeper root system will have. The soil structure along with percent organic matter will determine the water holding capacity of your soil, ranging from 0.60” of water/ foot of soil on fine sand to 2.00” water/foot of soil on silt loam and clay soils. It is recommended that irrigation systems maintain soil moisture levels between 60% and 80% on a silt loam soil to prevent stress to the growing crop while still having storage room available for rainfall. Calculating the available plant divided by the daily crop water use will provide an estimate on when irrigation must be completed to prevent crop stress. Subtracting the time it takes for your system to complete irrigation should give you a good idea of when to begin watering. Low capacity wells and irrigation systems on electric load control will need to start earlier and/or run more often to stay ahead of crop water needs.
Daily crop water use can be estimated from historical data, but calculating crop water use with an ET gauge will be more accurate and reflect current weather conditions. A hand probe of an experienced agronomist can estimate soil moisture very well, while soil moisture probe technology provides real time water readings via a website that producers may find more helpful. Moisture probes provide charts to better predict water use trends and help determine actual rooting depth.