Corn and Soybean Irrigation Reminders

Author: Angela McClure, Extension Corn and Soybean Specialist 1 Comment

Although most of Tennessee was blessed with an overabundance of rain this spring, warmer than average temperatures have increased water use by the corn crop and fields should be monitored now if irrigation is an option.

Author’s note:  This is a revision of an article originally posted on June 15, 2017.  It is important to consider crop stage, and pay attention to sensor data, if available, to make the best decision on when and how much water to apply.  Sensor technology has improved in recent years, allowing farmers who use sensors correctly to do a much better job not over-watering and potentially producing better yields while saving money.  Over-watering may also lead to nutrient leaching or loss if soil water level is kept too high for the infiltration potential of the field.  In mid to late June and beyond, water may be needed when available soil water falls below 50 percent in the crop root zone.

Corn benefits greatly from irrigation and pivots should be run early enough in the season to maintain adequate soil moisture.  For 2018, we have essentially no March planted corn, but mid to late April planted corn is between V9 and V12, which is the critical period of rapid growth and ear size determination. Note silking dates for each field over the next few weeks.  Water demand peaks between tassel emergence and blister stage (about 10 days after silking).  In average rainfall years, irrigate until ears reach the ¾ milk line (about 50 days from silking date).  If weather turns hot and continued dry in July, we may need to water until black layer (about 65 days from silking date).  Few irrigation systems can meet the maximum water demand for corn, therefore, it is important not to fall too far behind close to pollination and early grain fill.  Don’t get behind on watering!

Critical Water Demand for Corn: if sensors are not used, document rain received and estimate amount of water needed each week.

Growth stage Importance Max crop water use
V8 – V12 Kernel rows around ear set; Rapid vegetative growth 1.75”/wk
V12 – V17 Ear length determined 2.0”/wk
V17 – R2 (blister) Ear size potential 2.3”/wk
R3 (milk) – R4 (dough) Grain filling period 1.75”/wk
R5(dent) –

R6 (black layer)

Grain weight

 

1.25”/wk*

0.7”/wk

* Irrigation may not be needed past ¾ milk line. Under hot, drought conditions producers may benefit from watering until black layer.

For soybeans, irrigate to a stand or apply about 0.5” of water to incorporate residual herbicides in late planted beans.  Soil sensor use may benefit soybean growers by reducing the likelihood of overwatering a field.  Soybeans should be irrigated for crop stage, but overwatering, regardless of stage, can cause lodging, more disease and lower yield.  Producers who struggle with lodging and lower irrigated bean yields often plant varieties that grow too tall under irrigation or apply too much water after bloom for soil conditions, so soil sensors would be helpful to manage water levels more accurately.

  • Recent work at Milan (good rainfall years of 2014-2015 on a silt loam soil) showed that delaying irrigation on schedule until R3 produced yields similar to initiating irrigation on schedule beginning at R1.
  • In 2016, Milan was droughty in May and early June, and beans watered on schedule starting at R1 retained more pods per plant at harvest.
  • 2018 has been fairly wet up to now, and fields should not need irrigating until beans move into reproductive stages.  Early bloom occurs about 30 to 40 days after planting.
  • Terminate irrigation at R6.5 or full seed stage (More than 50% of plants have touching seed in at least one of the top 4 nodes).  Adding water beyond R6.5 may reduce yields based on 3 years of data from Milan.

Critical Water Demand for Soybean: if sensors are not used, document rain received and estimate water needed each week; monitor soil runoff and avoid over-watering:

Growth stage* Importance Est. crop water use
V3 – R1 (begin bloom) Rapid vegetative growth 1.0”/wk*
R1 (begin bloom) – R3 (begin pod) Pod count set 1.4”/wk
R4 (full pod) – R6 (full seed) Seed size set 2.0”/wk

* Rainfall is usually adequate until R3 unless fields suffered dry spring or are planted late into dry conditions.

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