Basic Irrigation Needs Of Corn, Beans And Grain Sorghum

May 15, 2015
By Angela McClure, Extension Corn and Soybean Specialist
Dry weather has already plagued some parts of the state.  We do have some likely rains in the forecast, but for those who miss rain once again this weekend and are thinking about turning on the irrigators, included are some basic irrigation amounts for corn, soybeans and grain sorghum. 
Corn water use using the checkbook method of irrigation (It is important to start on time, don’t get behind and try to supply most of demand between V-9 to 3/4 milk line stage.)
Growth stageImportanceEst. crop water use

Seedling corn

Early vegetative growth

<1″ per week

V9 – V12

Rapid vegetative growth


V12 – V17

Ear length determined


VT – R2 (blister)

Ear size potential


R3 (milk) – R6 (black-layer)Grain filling period


* Irrigation amounts may be reduced in latter part of grain fill.  Under hot, drought conditions producers may benefit from watering until black layer.  In wetter years, producers may stop irrigating after 3/4 milk line stage. 
Soybean water use using the checkbook method (It is important to not overwater soybeans.  Critical water needs are between R3 and R6 stages.) 
Growth stageImportanceEst. crop water use
V3 – R1 (begin bloom)Rapid vegetative growth1.0”/wk**
R1 (begin bloom) – R3 (begin pod)Pod count set1.4”/wk**
R4 (full pod) – R6 (full seed)Seed size set2.0”/wk
** Rainfall is usually adequate for soybeans until R2 and early season irrigation shows little benefit other than to ensure seed emergence and activate preemergence herbicides. The important thing to remember with soybeans is less is best during the early part of the season. 
Grain sorghum water use using the checkbook method.  (Grain sorghum is our most drought tolerant crop but does respond to water when weather turns dry. It is important to not overwater early season but try to irrigate prior to and at boot stage and at bloom if needed.  Late season irrigation is usually not recommended.)
Growth stageCommentsEst. crop water use
Early growthFirst 30 days after planting< 1 “ per week
Rapid vegetative growth30 to 60 days after planting0.7 to 1.4” per week
Preboot and BootWater  preboot and at boot stage if dry1.5”/wk
BloomWater during bloom if dry1.75 to 2.1”/wk
Grainfill 1.75 to 0.7”/wk (declining)


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