One of the most common mistakes farmers make each fall is harvesting corn for silage before it’s at the ideal stage for yield and quality. Many farmers make a practice of looking at the ear to see if the kernels are dented, and if they are, they figure it’s ready to chop. This method is quick and easy, but often results in silage with dry matter levels several points too low. This problem has added importance on CAFO farms since management of silage effluent is a required part of nutrient management plans (as well it should be). Following are four tips to help decide whether it’s time to harvest your corn crop or if you should wait a few days or more.

Dented doesn’t mean done

Most corn hybrids will begin to dent (the indentation forming at the end of each kernel) well before the whole plant is at the desired stage of maturity. Furthermore, genetic differences can result in corn becoming fully dented at widely varying whole plant dry matter levels. This is due mostly to what seed companies call “staygreen,” which is the ability of the plant to remain alive as the ear matures. This is a big advantage for grain corn farmers, who want their plants to remain alive and healthy as long as possible as the kernels mature, because dead corn plants are much more susceptible to stalk rots and lodging. The more mature the kernels get while they’re still on the plant, the lower the grain farmer’s drying costs. But, this isn’t as much of an advantage for farmers harvesting their corn as whole-plant silage. If they use stage of dent to guide harvest, corn hybrids with high staygreen ratings—8 or 9 on the normal 1 to 9 scale—can have fully dented kernels while the whole plant is still well under 30 percent dry matter.

Tip #1: Don’t rely on stage of kernel dent to make final decisions on harvest. It’s a sign that harvesttime is getting close, but it’s impossible to determine whole plant dry matter based on kernel dent.

Kernel milk line

Corn kernels mature from the outer cap to the tip that connects the kernel to the cob. As the kernel matures and starch formation becomes complete, it takes on a dry, chalky appearance. There’s a definite line between this and the still-developing starch; this is called the milk line. You can see this line on the whole kernel, but I prefer to cut the kernel in half, from cap to tip, which makes it much easier to see the milk line. We used to tell farmers that the ideal stage of harvest was when the kernels were at one-third milk line—the fully-developed starch was one-third of the way down the kernel. However, hybrids differ greatly in when the kernels reach one-third milk line. (Not surprisingly, growing conditions also play a part.) Years ago, we planted a corn hybrid at Miner Institute that had a high staygreen rating, and made the mistake of starting to chop when the kernels were at one-third milk line. We got a shock when our first dry matter test was 26 percent DM. A second test confirmed the first, and we stopped chopping in that field. Milk line is still useful, but only as a guide as to when to start checking whole-plant dry matter. Under most conditions start checking dry matters when the milk line is about one-quarter of the way down the kernel.

Tip #2: Use milk line as a general guide or as an “early warning system,” but it should not be the final determinant.

Whole plant dry matter testing

Start with a representative sample. Avoid the outside few rows of the field, and try to ensure that the sample is as close as possible to what’s in the whole field. There are two common ways to determine dry matter content in fresh-chopped corn: a microwave oven, or drying by forced hot air using a Koster Tester or similar dryer. If you opt to use a microwave oven, don’t make the mistake of using the one in your kitchen. Dried corn forage doesn’t have an offensive odor, but the only way to determine if the sample is fully dry is to repeatedly “nuke” the sample until it stops losing water weight. The difference between “completely dry” and “incinerated” isn’t much. A better alternative is to buy a basic microwave at a discount store or at a yard sale and set it up in the shop or farm office.

I much prefer a Koster Tester, which dries forage samples using forced hot air. At Miner Institute, we plugged our Koster Tester into a heavy-duty timer, making it much more convenient to dry samples as the silos were being filled. Through trial and error we’ve learned just about how long it takes to dry the various forages harvested on the farm; fresh-chopped corn forage takes longer than fresh-chopped grass, for instance. Whichever method you employ, use an electronic postal scale that reads in grams. We’ve found it to be more accurate than the old spring scale that sometimes comes with the Koster Tester.

Tip #3: Heat drying is the only accurate way to determine the dry matter of whole plant corn forage.

What’s the idea dry matter content for corn silage?

This depends to some extent on two variables: the type of storage structure and whether the crop is harvested using a chopper equipped with a silage processor. Some field crop handbooks state that 28 to 35 percent DM is the normal range for corn silage, but 28 to 30 percent DM is not ideal. I much prefer 32 to 34 percent DM in most cases, though a percentage point or two higher than 34 percent DM is OK for processed corn silage since whole-kernel passage through the cow’s digestive tract isn’t nearly as much of an issue. Total dry matter yields will be higher at 32 to 34 percent DM than at 28 to 30 percent, and silage quality will be better. Stalk quality changes little as the plant matures from 28 to 34 percent DM, while starch content increases greatly.

Tip #4: Aim for 32 to 34 percent DM for most harvest and storage situations.

Ev Thomas has worked as an agronomist in New York for 42 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has a written our Forage column for over 10 years and has been an expert contributor on a number of other topics. Comment or question? Visit and join in the discussions.