I published a portion of this topic last year and thought it appropriate to republish some of it due to current conditions. Growing degree units are accumulating at daily maximum and the heat (93 degrees+) is pushing the crop and it will be difficult to keep up with the daily water demand in corn.  If you have moisture sensors, then you can get an idea of how much water is used on a daily basis.  If you can apply 3.5 inches of irrigation per week (and I expect few can), then you will able to keep up easily with water demand from both plant and evaporation.  Less than that, and the crop will stress some particularly on the sandy soil.

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 with cotton and peanuts, 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 irrigation 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 such as now.

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 years, such as we are currently experiencing.  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**

Growth Stage	Days After Planting	Approx. growing degree accum. from planting	Inches Per Day
Emergence and primary root developing.	0-7 8-12	0———– ————245	.03 .05
Two leaves expanding and nodal roots forming.	13-17 18-22	245——- ———–375+	.07 .09
Four to six leaves expanding.  Growing point near surface. More leaves and roots developing.	23-27 28-32 33-36	375 —————————————–580	.12 .14 .17
Six to eight leaves expanding. Tassel developing. Growing point above ground.	37-41 42-45	580 ———————– 730	.19 .21
Ten + leaves expanding.  Bottom 2-3 leaves lost.  Stalks growing rapidly. Ear shoots developing. Potential kernel row number determined.	46-50 51-54	730————————-960	.23 .25
Twelve to sixteen leaves. Kernels per row and size of ear determined.  Tassel not visible but about full size. Top two ear shoots developing rapidly.	55-59 60-64	960————————1150	.27 .29
Tassel emerging, ear shoots elongating, (R1)-silking	65-69	1150—–1350	.31
Pollination	70-74 75-79	1350———————- 1470	.32 .33
R-2. Blister stage.	80-84	****	.33
R-3. Milk stage, rapid starch accumulation.	85-89	****	.34
Early dough stage, kernels rapidly increasing in weight.	90-94	****	.34
R-4. Dough stage.	95-99	****	.33
Early dent.	100-104	****	.30
R-5. Dent.	105-109	****	.27
Beginning black layer.	110-114	****	.24
R-6.  Black layer (physiological maturity).	115-119	****	.21

**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 possible.  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.