I have written about irrigating corn over the years quite a bit.  Having advised on the crop since the early 80’s and watch corn growers go from a majority of dryland production to irrigating the crop, the value of irrigation to corn has continued to improve the more we learn how to effectively and efficiently use water.  The UGA corn production guide has a chapter in it that addresses corn irrigation.  Dr. Wes Porter, UGA irrigation specialist and ag engineer and I over the years have encouraged growers to actively adopt new irrigation technology for all our crops and learn how to effectively and efficiently use it to your advantage.  Commodity prices are low, and to remain in business, the return on your investment leaves no room for guessing particularly this year.  I am taking much of the following from that guide that we have refined over the years to help you with the idea of planning ways to improve your ROI.

Developing and having an irrigation plan is important to growing corn efficiently. The utilization of any irrigation scheduling method is typically better than no plan or method at all, particularly with corn.  A good plan pays dividends in terms of yield, water-use efficiency (WUE) and net returns.   In corn, irrigating too late causes yield loss while irrigating too much wastes energy, water, money and can leach or cause run-off of nutrients beyond the root zone.  Unlike other crops, the addition of too much water to corn does not directly reduce yields, but it can reduce net income due to the added costs of additional applications without equivalent yield benefits.  It is important to note that studies have shown that a lack of irrigation and rainfall during peak consumptive periods can deplete soil moisture particularly subsoil moisture, which is very difficult to replace via irrigation only.  Thus, caution is advised during high water requiring periods.

The most simple and practical way of scheduling corn irrigation is to use a moisture balance or checkbook method.  This method helps a grower keep up with an estimated amount of available water in the field as the crop grows.  The objective is to maintain a record of incoming and outgoing water so that an adequate balanced amount is maintained for crop growth.  You will require certain basic information to use a checkbook method.  This information typically includes the soil type of the field and/or soil water holding capacity/and infiltration rate, expected daily water use of corn, and a rain gauge or access to nearby rainfall information.  An example of a checkbook method calculation is presented in this entry.  The UGA Corn Checkbook was developed from a historical average of evapotranspiration.  This method is very conservative and most often errors on the side of over irrigating rather than under irrigating.  However, caution is advised when utilizing the checkbook method alone as it was developed from a historical average, and may not adequately address water requirements during extreme (either wet or dry) years.  This means that in years that are drier than average the checkbook method would tend to under-irrigate and during years that are wetter than average it would tend to over-irrigate.  The 2019 production season was a prime example of a year in which we had abnormally hot and dry weather.  Many irrigation scheduling and application issues were observed during 2019 because of this reason.  In most “average” years these problems are masked by supplemental rainfall, the lack of the rain during 2019 made these problems very prevalent across the state.      

        Estimated Water Use of Corn in Georgia**

**The relationship between reproductive stage development and GDU’s is more variable than that of vegetative development and GDU’s.  Reproductive development is identified based on appearances. This is highly influenced by changes in kernel moisture and dry matter. 

I encourage you to consider closely watching your corn fields and plan your irrigation in advance.  If you have a weather station on your farm or use moisture sensors, adjust your irrigation to the amount in your rainfall event or to what your sensors show.  Stay on schedule to prevent subsoil moisture loss. Water use reaches a peak during the early grain fill. Don’t let up.  Once you get over the hump, continue irrigating to black layer as it will help your test weight and prevent any losses.

The following example of the water balance or check-book method demonstrates how to determine the correct amount and how frequently to irrigate. 

Step 1.  The soil type of the corn field is a Tifton soil series.  In the next table, look at the average available water holding capacity in in/ft increments (1.1 in/ft).  Assuming a rooting depth of 24 inches (2 ft), the total available water is 2.2 inches (2 ft x 1.1 in/ft).

Step 2.  The corn crop is 65 days old.  From Table 10, the daily water use is about .31 inches/day.

Step 3.   Determine the irrigation by setting a lower limit of available water due to soil tension.  For this example, use 50% of allowable soil water depletion.  In other words, only half of the water in the root zone will be allowed to be depleted. Therefore, 1.1 inches of water will be needed to replace the soil water that was either used or lost.

Step 4.  Determine the amount of irrigation to apply by dividing the amount replaced by an irrigation efficiency.  Assuming 75% as the irrigation efficiency, the amount of irrigation required is 1.1/.75 = 1.47 or 1.5 inches.

Step 5. Determine the frequency of irrigation by dividing the amount of water replaced by water use per day.  An example of frequency of water (either rainfall or irrigation) need:

1.1 in /.31 in per day = 3.5 days.

Step 6. Therefore, it is necessary to apply 1.5 inches of water every 3.5 days to maintain 50% available water for 65-day old corn.

Examples of available water holding capacity of Coastal Plains soils

Many of you focus your attention on properly irrigating your crop.  I encourage everyone to look carefully at the growth stage of the crop and water according the need of the plant as it ages.  In addition, modify your application and take advantage of all rainfall events and use weather stations, and moisture sensors to “ground truth” (pun intended) your plan so you know that you are staying ahead of the crop’s need.  Temperature, wind,etc will modify evaportranspiration and adopting and using technology can help you increase or decrease irrigation amount to get your best ROI.