Our earliest corn is progressing through the V9-V11 growth stage. This is planted in what is usually our driest part of the county, so we must be on top of irrigation. With no rain in more than 10 days, keeping up with irrigation is critical, especially for corn around V6 growth stage where yield potential is being determined. Many growers may be using moisture monitors or other technology to determine when and how much to irrigate. If you do not use one of these methods, consider using the checkbook method for scheduling their irrigation events. Terrell County Agent Nick McGhee has put together tables from the “checkbook method” from the 2015 UGA Corn Production Guide:
Corn Water Use At Various Growth Stages
Water Holding Capacities of Coastal Plain Soils
Checkbook Method Example
This example shows how to use the two tables above and the “checkbook method” to determine when and how much to irrigate.
Step 1. The soil type of the corn field is a Tifton soil series. In Table 11, look at the average available water holding capacity in in/ft increments. 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 to 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.
We are also seeing some nitrogen burn and herbicide drift causing these spots on the leaves. Below is drift from herbicide and is common in all corn fields, but nothing to worry about.