Our earliest planted corn has already reached black layer.  This is the abscission layer in which shuts out all nutrient and water movement into the kernel.  This is an important time to recognize, since it is a visual indication that the kernel has reached it final weight.  Much of the rest of the crop in the state, is rapidly progressing towards black layer.  If the crop has just entered into dent, it will take about 30+ days to progress from R5 stage (dent) to R6 stage (physiological maturity).  Why is it important to remember this tidbit?  First, corn moisture is around 60% at the beginning of R5 and declines to 35 to 36% moisture at R6.  Yield loss can still occur with water stress during the last 2-4 weeks prior to R6. Fortunately, widely scatter showers have occurred throughout much of the southeast over the last several weeks, easing the tension of dry weather. If your crop is not at R6, make sure to be prepared if soil moisture begins to decline prior to this period since the crop is still in danger of yield loss.  Focus on the moisture holding capacity of your soil to inform you on how to maintain enough to meet the daily need.  Corn will still use upwards of .2 to .25 inches of water daily all the way to black lay at which point the kernels are not subject to yield loss to dry weather. Make sure to look for that distinct black at the tip of the corn seed by scraping the tip to expose the abscission layer. 

Southern corn rust is widely scattered and still dangerous if your crop hasn’t reached the R5 stage.  For instance, if your crop has recently entered the R4 (dough) stage and you are seeing southern rust, current weather conditions will foster this disease and it can remove the ability to fill grain properly and reduce test weight.  Until the black layer forms, the individual kernels are gaining weight. It takes seven to ten days to move through the dough stage to early dent. You still have 30+ more days to move from R5 to R6. Unchecked disease such as southern rust will cost you considerable profit. 

Let’s look closely at this formation of black layer. Genetic maturity and/or the environment will force this stage to occur.  It is also 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 or disease can ultimately force the plant to prematurely shut down and, the kernels will lose weight.

Once this stage occurs, you will likely lose kernel moisture around .5% to .75% per day on average. It would be a good idea over the next 10 days to make sure your harvesting equipment is in working condition.  Parts and supplies are short around the country and I would not want to get caught needing a part for my combine when the crop is ready to harvest.  Overall, the southeastern states look to have a good corn crop this year.  I would not want to lose it at the end when everyone has worked so hard to achieve profitable yields.

Post publishing note: Thank you to a sharp-eye reader and great student of corn to catch that I first used V5 & V6 for R5 and R6. No matter how many times I proof read my material, my brain fails to catch everything. I welcome correction and comments. Thanks!