In general, research has shown many of the tools commonly utilized to visually determine moisture content of corn are not very accurate. However, we still mostly rely on visual evaluation to start harvesting. The following is a practical guide for determining harvest time for corn silage.

As the corn plant matures, changes occur in the plant components. The grain content increases and the percent of stalk and leaves decreases. Lignification of the stalk decreases the cell wall digestibility, and the starch content of the grain increases. This stage of maturity is just prior to physiological maturity. The rule then should be to harvest corn silage as close to physiological maturity as possible without allowing the stalk and leaves to lignify, therefore reducing the digestibility of the plant material. The whole dry plant matter levels should be greater than 30 percent at this time. The best way to measure whole plant dry matter is by harvesting a few plants, chopping and drying a sub-sample. Visual maturity tools can be used in place of this, but they are not as accurate.

The ideal harvesting time is when the corn plant has reached physiological maturity and is in the full dent. The maximum yield of dry matter per acre is achieved at this time. Dry matter yields are reduced about 1 percent for each day harvested prior to physiological maturity. At physiological maturity, the whole corn plant will have a dry matter of 32 to 38 percent. The corn kernel contains about 62 to 65 percent dry matter; ear corn contains about 55 to 60 percent dry matter. At this time, the black layer will be forming at the tip of the kernel.

Conditions at this time are also right for good fermentation in the silo and seepage losses will be low. Much of the corn silage in our area is harvested prior to black layer formation due to concerns of frost and traditional needs for silage. To maximize feeding values, corn silage should be harvested after early dent. At this time, the milk line should be one-third down the kernel.

Milk line formation proceeds down the kernel at the same time as the kernel proceeds through full dent. This line advances down the kernel towards the cob as the plant increases in maturity and drydown. When the hard starch line approaches the cob, the black layer will form. Ideally, most corn silage will be harvested from one-third milk line to black layer maturity.

Milk line
After kernels are dented, a milk line appears across the kernel opposite the embryo side. This line advances down toward the cob with maturity and drydown. When the hard starch line approaches the cob, the black layer will form.

Checking for the milk line
Remove the top one-half of the ear and view kernels on the broken end of the top half (side opposite to embryo). The accumulated starch above the line will be hard with the area below the soft line. Splitting the kernel may help to locate the line. Running a knife toward the dented end will also help locate the line. With experience, the line can be determined with accuracy.

Measuring the progress of the milk line
To chart the movement of the starch line, you will need a metric ruler. Measure the distance from the dent end of the kernel to the milk line. Also measure the total kernel length. Use these two values to express the position of the milk line (percent down the kernel). Calculate an average of 10 ears (one kernel from each).

When weather and growing conditions alter the rate of physiological maturity and require the harvesting of immature corn silage, the yield of dry matter per acre decreases and the energy levels of the resulting silage are lowered. Less grain is present in the silage and the silage is higher in moisture. Both of these factors alter the fermentation patterns in the silage. The fermentation is slower and seepage losses are generally higher.

If corn silage is frosted, the nutrient flow to the kernels is slowed then increased after five to seven days. Freeze-damaged corn should be harvested and ensiled seven to ten days after the freeze. This will improve the quality of harvested forage and minimize the field losses from dried leaves and stalk breakage. The quality of silage harvested in this manner will be equal to silage from non-frozen corn harvested at the same physiological maturity.

At times, silage is harvested at earlier stages to prevent freeze damage or when maturity is late because of cool weather. To prevent excessive seepage losses from corn silage harvested in this manner, other feeds may be ensiled with the silage to reduce seepage and improve the fermentation of the silage. The choice of dry feed additions should be related to the final diet fed to the producer’s livestock. Use feeds you would normally add to the silage at the time of ration formulation.

The greatest concern in corn silage making is the ability of the silage to ferment properly, thus providing highly palatable forage without an abnormal fermentation and a sour tasting, “off” smelling and mold- and yeast-infested silage. If the dry matter of corn silage is in the optimum range (30 to 35 percent dry matter), the resulting corn silage will be palatable and animal performance will not be influenced.

When weather conditions reduce the rate of physiological maturity of corn silage, producers should be aware of the implications of harvesting immature corn and take precautions to ensile the corn at the proper dry matter levels. This can be accomplished by mixing the immature high-moisture silage with dry feeds or by letting the silage field dry to the proper moisture levels.

Utilizing the proper tools like checking dry matter or observing milk line and stover height can help determine the appropriate time to harvest corn for silage fermentation. However, good planning and a good back-up plan for when unexpected weather conditions affect your original plan are indispensable to good and consistent silage preparation.  PD

—Excerpts from Utah State University Dairy Newsletter, Vol. 30, No. 5

Duarte E. Diaz
Extension Dairy Specialist
Utah State University

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