A website from UGA Cooperative Extension

by Rome Ethride


Corn Maturity


Corn is really moving as we are finding a lot of older, 90 days or so, corn that is beginning to Dent. So that’s R5 and the next stage is R6 or Black layer. That’s when the water can be cut off because the corn is MADE, done, finished, ready to harvest.  Some folks get the idea that when we hit Dent that the corn is done but we have 3 and a half weeks to go or so, depending on heat units and other factors. We can reduce watering a little but we need to keep it wet until black layer forms at the base of the kernel. At dent we still need 2 inches that week then we can slowly decrease water to 1.5 inches the last week before black layer.

Once the end of the corn kernel dents, the hardened part slowly goes down the kernel and when it reaches the base the black layer forms.


This is called the milkline, below the line, it is milky. When ¼ the way down we say its at “Quarter milkline” , then Half milkline and then Three quarter… you get the idea.


Generally it goes like this, ¼ milkline needs 3 more weeks and 1.8 inch water a week,


½ milkline needs 1 and a half weeks and 1.5 inch water a week,


¾ milkline needs 7 days and maybe one more watering if already wet at this point.

Quarter Milkline stage


It is useful to look at Growing Degree Units(GDU) or Heat units.  Heat units are calculated based on the day’s high and low temperatures. With a base (in Corn) of 50 and a high limit of 86. So yesterday, 89 was the high and 72 was the low.

So use the equation found in the UGA Corn Production Guide, , 86(high limit, can’t put in 89) + 72 = 158 divided by 2 = 79 – 50= 29 GDU


So we accumulating about 30 units a day now. We know corn is usually mature at 2800 Heat units (depending on the hybrid and other factors, but that’s close).


So a grower has an actual field I was just in, planted on March 9. It has accumulated 2203 GDU so far and is Denting now. If we continue to accumulate heat units at the current rate and it’s kept in good shape then we should hit black layer in 21 days.


We can go to the UGA Weather station site to figure this if we have the planting date. Pull up www.weather.uga.edu and go to “Calculator”, “Degree day”, choose your location, set it to 50 minimum (Base temperature) and 86 maximum (Disregard temperatures above), and put in planting date.


Notice it will automatically give you the last 3 years as well. Below is my screenshots using Donalsonville, Ga., yesterday.


This shows we’ve had more than normal heat unit accumulation in 2019. We are almost 200 units ahead of last year in Southwest Georgia. So that calculates to about a week ahead.







Southern Rust Found


First I’ve heard of this season thankfully, was found in Baker county this week. Dr. Kemerait has sent out info about it. Important info for younger corn, especially, fungicides are likely warranted.




In the Field


In corn fields this week we’re seeing some Garden Fleahopper damage. Causes a stippling of the leaves. Looks like a tiny cricket with big back legs. We see it in many crops now including peanuts and cotton.  Not really a problem here in corn since down low usually and minimal. If it started affecting the leaves close to the ear I’d be more concerned.  Reminds me of spider mite damage at first glance.

Underside of corn leaf with signs of the fleahopper and an immature light green one.


No Water


2 corn fields side by side, same planting date. Everything the same except the irrigation system broke down for 2 weeks where corn ears on the left were growing. Happened towards the end of pollination period. You can see the heat and dry effect on pollination and some kernel abortion as well.  System back up and going now but yield is affected of course.





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