Harvesting Corn Silage at the Right Moisture

Factsheet - ISSN 1198-712X   -   Copyright Queen's Printer for Ontario
Agdex#: 120/50
Publication Date: 09/13
Order#: 13-051
Last Reviewed: 09/13
History: Replaces OMAF Factsheet of the same name, Order No. 07-047
Written by: Joel Bagg - Forage Specialist/OMAF and MRA; Greg Stewart - Corn Specialist/OMAF and MRA;Tom Wright - Dairy Cattle Nutritionist/OMAF and MRA

(PDF - 188 kb)

Table of Contents

  1. Introduction
  2. Importance of Correct Moisture
  3. "New Silage" Slump
  4. Correct Moisture & Silo Type
  5. Silking Date
  6. Kernel Milk Line
  7. Moisture Testing
  8. Milk Line Triggers
  9. Resources


Corn silage quality and animal performance are significantly affected by the whole plant moisture and maturity of the corn at harvest. Not only is it important to minimize fermentation dry matter losses and spoilage, it is difficult to compensate for poor forage quality with ration balancing.

Don't get caught harvesting corn silage outside the range of optimum moisture and maturity. Focus on harvesting at the correct moisture.

Importance of Correct Moisture

Ensiling at the correct whole-plant moisture level and optimum stage of maturity is critical. Corn maturity is very dynamic. As corn matures from dent to black layer, yield increases, starch increases, starch digestibility decreases, and fibre and fibre digestibility decrease.

Too Wet

Harvesting corn silage at moisture levels above 70% will not only yield less but will result in seepage and a very undesirable clostridia fermentation. Clostridia bacteria are very inefficient and convert forage sugars and organic acids into butyric acid, carbon dioxide and ammonia. This silage will have high dry matter losses, high levels of foul-smelling butyric acid, with a higher pH, and poor feed quality, palatability and intake potential.

Laboratory fermentation profile analysis is available to determine pH and relative amounts of lactic, acetic, butyric and propionic acids, all of which affect quality.

Seepage results in a loss of nutrients and can be harmful to the environment. Very wet or frozen silage can be difficult to unload in the winter.

Additional information is available on the OMAF website at www.ontario.ca/omaf. Search for Silage Fermentation Problems.

Too Dry

Harvesting corn silage at moistures that are too low will result in poor packing, inadequate air exclusion, poor fermentation and heating. This will mean higher dry matter losses, greater spoilage and poor bunk life.

Kernel processors can be used to increase starch digestibility. Low-moisture corn silage that is not harvested using a kernel processor can be lower in starch digestibility. Kernels that are too dry will become hard and pass through the cow undigested. Fibre digestibility has been found to decrease by over 10% as moisture decreases from 70% to 58%.

"New Silage" Slump

Nutritionists have described the "new corn silage slump" where cows do not milk according to the expected results of the balanced ration when being fed newly fermented corn silage in the fall. This is attributed to the reduced starch digestibility of hard-textured, dry kernels in newly fermented, unprocessed corn silage. This can be very frustrating for milk producers trying to fill fall quota. This problem is usually reduced after 3 months, when the kernels have a chance to absorb silage moisture, become softer and fracture more easily.

Correct Moisture & Silo Type

The best silage fermentation usually occurs when whole plant moisture is 65%-70%, producing optimum feed for livestock performance. This corresponds well to horizontal and bag silos, but silage may have to be a bit drier in tall tower silos to prevent seepage. Variability between fields and within fields can make this even more complex. Recommended moisture contents for corn silage are as follows:

Horizontal bunker silos
65% - 70%
Bag silos
60% - 68%
Tower silos
62% - 67%

Silking Date

The silking date can be used as an indicator to give you an estimate of timing for corn silage harvest, usually 42-47 days after silking. Of course, the optimum harvest time will be affected by crop heat units (CHUs) during that period and may come earlier or later, depending on temperatures. It can be useful to predict which fields are ahead of others.

Kernel Milk Line

The kernel milk-line has often been used to determine when to harvest corn silage. This is done by breaking a cob in half and looking at the kernels. After denting (which shows as 0% milk-line), a whitish line can be seen on the kernels. This line is where the solid and liquid parts of the kernel are separated while maturing and drying. This line will progress from the outer edge of the kernel towards the cob. When this milk-line has reached the cob (100% milk-line), a black layer is visible. The traditional recommendation has been to harvest when the milk-line is between one-half and two-thirds.

There is considerable variation in the percent kernel milk-line and the moisture percent of the whole plant. University of Wisconsin data over many years show a range in whole plant moisture at one-half milk-line of 52%-72%, with an average of 63%. This will be too wet for some and much too dry for others. The two most significant causes of the variation are weather and hybrid differences.


When extended dry weather results in abnormal corn plant development, percent moisture estimates based on the kernel milk-line are quite variable and not very accurate. Corn plants severely stressed by dry weather without cobs do not have kernel milk-lines to use as estimates, but plants are typically much higher moisture than they appear. Similarly, it can sometimes be difficult to accurately estimate whole-plant moisture from kernel milk-lines in frost-damaged corn.


Hybrid differences also affect the accuracy of using kernel milk-line to estimate moisture level. Corn hybrids have varying degrees of "stay-green" characteristic. More stay-green means there is faster grain dry-down relative to stover dry-down. This is desired in a grain hybrid, because, as the grain dries, the stalk stays green and healthy, and is less likely to have broken stalks and lodge in late season.

Some hybrids are designed only for use in silage and have less stay-green, so that the grain will have higher moisture relative to the whole plant. In other words, hybrids with higher stay-green ratings will have milk-lines that are more advanced relative to whole plant moistures. Silage-only hybrids that have less stay-green characteristic will likely be ready to harvest at a less advanced milk-line.

Check with your seed company representative for historic milk-line recommendations for estimating moisture levels in a given hybrid.

Moisture Testing

The most accurate method for determining when to harvest is to measure the moisture content. This takes a little more effort but is well worth it when you consider the impact on your animal performance that feed containing corn silage harvested at the correct moisture will have.

Sample at least 10 plants from the field, avoiding the headlands. Watch for moisture variability within fields. Chop a sample using a harvester or yard chipper. Use a Koster Tester, microwave or laboratory to determine percent dry matter.

Be aware that samples have residual moisture that is not removed when dried with a Koster Tester or microwave but will be removed in laboratory ovens. The most accurate option is to send a sample by overnight delivery to a forage laboratory for oven drying. Results can be e-mailed immediately.

The Miner Institute has estimated that Koster Testers and microwaves underestimate moisture by about 3%. A 68% moisture sample reading is actually about 71% moisture. In a typical year, that 3% is equivalent to almost a week in harvest time. When using a Koster Tester or microwave, taking the time to carefully dry the sample is important. The finer the sample is chopped, the easier it will be to dry and the more accurate the result.

Milk Line Triggers

Because of the variation between milk-line and whole plant percent moisture, the current recommendation is to determine a whole plant moisture shortly after denting when the milk-line is about 20%. This can be done by sampling, chopping, drying and measuring as described above. Experience shows that in a typical year, corn silage at this stage dries approximately 0.5% per day. Therefore, if the sample was 70% moisture, and 65% moisture is the target, harvest about 10 days after the corn was sampled. In dry years, the drying rate will be more rapid. During wetter years, the drying rate will be slower. Check moistures again, closer to harvest.

It is important to recognize the importance of harvesting corn silage at the correct moisture. Don't let the harvest window sneak up on you. Walk your fields, monitor whole plant moistures and be ready to fill silo.


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