I get this question several times every year as corn growers are looking to shift their water resources to other crops once they feel the corn crop is “made”.  The question is most often rooted in getting the best return on their investment (ROI) particularly with limited resources. There are lots of ways to come up with an answer but I will focus on the cost to corn in “yield” with an example of an average price per bushel and average price of irrigating.  Use the example as a method to critically examine your own situation and draw conclusions.  You should finish the question by looking at the value of shifting the irrigation to the other crop vs the lost to corn to get the complete ROI answer.

There are many studies on how irrigation affects corn growth and development.  Studies as far back as the mid 1970’s and 1980’s when supplemental irrigation of row crops were expanding, focused on yield as affected by timing and ultimately, soil types and moisture holding capacity.  That expanded over the years to explain the impact on various plant components responsible for yield and yield losses. In other words, there are lots of studies that give us clues on the impact of irrigating corn.  To be clear, the impact is affected by soil type and slope, atmospheric environmental conditions, plant genetics, plant health and age, previous management and others that I tend to forget which some you will rightfully point out.  One other factor in this equation is that black layer (R6) is often a moving target as it is determined by genetics and environmental interactions.  I will focus on the “average” to get us into the ball park.

Most all studies agree that yield loss in the last few weeks of kernel development is due to loss in kernel weight and not kernel numbers.  For today’s entry, let’s look at the effects from loss of water.  In general, we can say that it takes between 60 to 65 days in corn from R1 (silking) to reach R6 which is declared as physiological maturity. This is visually determined by the formation of a thin black layer of cells (abscission layer).

This formation can be affected by many stress factors shutting down or reducing the physiological demand for carbohydrates (sugars) which begins to change the balance of hormones/phytochemicals (ethylene, abscisic acid, and auxins).  This change results in an ethylene increase in the cells closet to the kernel base to begin to lose cell contents increasing abscisic acid which results in the formation of the abscission layer (I am really generalizing here). This leads to the darkening of this layer and ultimately to a thin layer of base cells that cause a separation of the kernel from the cob and therefore, water and carbohydrates.  Stress in the leaves, roots, stems and cobs caused by soil moisture depletion ultimately forces the plant to prematurely shut down and, the kernels will lose weight.

How much loss will occur if we prematurely quit irrigating? One of the first studies I ever reviewed suggested an average of 8% yield in the last 14 days of R5 which would be about ½ milk line (roughly 14+days prior to R6).  It also suggested a 15% to 20% loss if stress occurred at late R4(very early dent) which is roughly 30+ days before R6.  Now all of this depends on how much water is available in the soil from the last irrigation and when water stress begins.  Let’s say you are only seeing a ”little” dent, then you have about 30 days of kernel weight gain potential.  If you see full dent and the middle kernels show ½ milk line and your plants are healthy, then you have about 14 days left to R6.

On average you should still be using about .27 in. of water per day (some studies suggest that moisture use may be ~ .225 in. per day at this time). As an example, if your soil is a Tifton sandy loam soil, it has about 1.1 inches of moisture holding capacity per foot. We assume that at this point, in Georgia, corn roots are extracting moisture from only 2 feet of soil due to our difficult to penetrate clay subsoil.  We recognize that not all moisture is available and therefore determine that below 50% moisture, water is unavailable to the plant in which the plant begins to stress.  Given the information in the sentences above, you will need to replace 1.1 in. of water in 4.1-4.8 days to maintain at least 50% availability of soil water. If not replaced, corn can begin to enter into water stress.

At mid R5, ½ milk line, you still have about 2 weeks of kernel weight gain left. To get a target loss, I suggest that you count kernels, rows, ears, etc and see what your theoretical yield will be.  Let’s say it looks like you have a potential 225 bushel per acre yield, then according to the research, the rough potential loss is about 18 bushels if you cut your irrigation prematurely.  A cash price for delivery in south-Georgia in Sept. to Central States at the time of this entry is around $3.84. That could translate to a $69.12 net loss if you stopped irrigating at ½ milk line. To finish till R6, it will take approximately 3 inches of water over the next two weeks to reach black layer.   Assuming no rain, and at a cost of $7 to $9 per inch of irrigated water, the total cost will range from $21 to $27 dollars per acre to potentially capture the total yield.  Your net gain would be $42-$45 per acre ($69 -$27 or $69-$22) by continuing to water your corn crop to black layer.  The net value of each 1 inch per acre irrigation in corn then is between $14-$15 ($45/3) per inch.  Would those 3 inches of water return you greater value in another crop with-in the 2 weeks’ time?   Some thing to think about before shutting your pivots off and moving to another crop.

Feel free to comment or send me a direct email at deweylee@uga.edu.