Irrigation During Peak Crop Water Use

By Angie Gradiz and Lyndon Kelley

Effective irrigation management is crucial for maximizing crop yields, especially during the early to mid-reproductive stages of corn and soybeans. In the article “Early summer crop water needs” published by Michigan State University Extension, we emphasized when irrigation is necessary during vegetative stages to give a strong start to the season. Now that corn and soybeans are in their early to mid-reproductive stages, irrigation is crucial to meet the increasing crop water needs. During this period, crop water use exceeds precipitation, so precipitation does not satisfy water requirements, which can lead to water stress. 

The mid- to late reproductive stages in soybeans, occurring in late July and early August, include R3-R4 (pod development and elongation) and R5-R6 (seed fill). During these stages, soybeans are most sensitive to water stress and have a maximum water use of approximately 1.3 to 2 inches per week, depending on weather conditions. Failing to meet crop water needs during these stages can reduce the number of seeds per pod or seed size, thereby decreasing yield potential. 

For corn, the critical period is the week before and three weeks after tassel emergence. Water stress during tasseling and silking can significantly decrease yield potential. By the time corn reaches tassel emergence, it has achieved 100% of its effective rooting depth, about 3 to 4 feet. A 3-foot-deep reservoir of soil moisture can hold as little as 3 inches on sandy soils to as much as 6 inches on loam soils. Most typical irrigated soils, like sandy loams, hold between 3.5 and 4 inches in 3 feet of soil. 

To maximize irrigation efficiency during peak water needs, producers should aim to apply five to six days' worth of crop water per irrigation, typically ranging from 1 to 1.25 inches. Irrigation applications should align with crop water use, subtracting any rainfall, while leaving capacity for any future precipitation events. 

Larger irrigation applications are more effective. Typical water loss through evaporation from mid-season crop canopy and soil surface is about 0.1 inch per application, regardless of the irrigation amount. Fewer, large applications result in less evaporative loss and greater efficiency. For instance, two 0.5-inch applications provide 0.8 inches of effective water, whereas a single 1-inch application yields 0.9 inches of effective water. Fewer large applications also result in less disease potential. Producers with irrigation systems that apply water faster than it can infiltrate into the soil will be forced to make smaller applications to avoid runoff issues. 

To ensure optimal yields, it's recommended to maintain at least 50% of the available water in the root zone after the R2 full bloom stage in soybeans and from tasseling through black layer stage for corn. Soil moisture content can be accurately quantified using a soil water balance sheet or soil moisture sensors. More detailed information on utilizing these tools can be found in Fact Sheet #3 - Irrigation Scheduling Tools. 

If you are scouting your field, watch for visual indicators of water stress. In soybeans, stressed plants may rotate their leaves to expose the silver/gray fuzzy side to sunlight, reflecting light and reducing water use. These symptoms indicate severe stress and likely reduced yield potential. 

In corn, a natural defense mechanism causes the leaves to roll up to reduce transpiration and canopy temperature. During extremely hot days, corn may roll even with adequate soil moisture. Persistent leaf rolling into the early evening or mid-morning indicates water stress. Leaves may also turn a dark green/gray as chlorophyll dies. 

Additionally, a simple tool is the potential crop evapotranspiration model from Michigan State University (MSU) Enviroweather. This tool uses your crop growth stage and potential reference evapotranspiration (rPET) from the nearest weather station. MSU Extension also provides estimated weekly crop water use to aid in improving irrigation scheduling and efficiency.