Determining Silage Corn Moisture

August 11, 2021

A great way to provide value to our silage growers is to help them determine when to start silage harvest. The optimum harvest window for fermentation is when the whole plant moisture content is between 65 to 70%. I start checking moisture content when the grain is about 50% milk line (Figure 1). That is on the early side, but soil moisture, soil type, growing conditions, and plant health can impact whole plant moisture content. By checking a little early, this can provide a good estimate where the plants are for moisture content and give the growers a few more days to get ready. Under normal conditions, plants lose 0.5% moisture content per day. If it’s dry, plants can lose up to 1% moisture content per day.1

The process to check moisture is as follows:

  1. Depending on the field size, harvest two representative plants in 5 to 10 different spots. Cut the plants at least 6-inches off the ground or at the cutting height the grower uses. I use a two-handed pruner or lopper to harvest the plants.

  2. Keep each two-plant sample separate, and finely chop them. Personally, I like using a woodchipper (Figure 2) with a burlap bag to collect the samples. If the samples are not uniformly ground, run them through the woodchipper again. Uniform particle size is important for faster drying and accurate moisture content evaluation.

  3. After chopping the sample, weigh out a 100-gram sample to dry down. A 100-gram sample makes for easy math.

  4. There are multiple ways of drying your sample:2

    1. Dehydrator: lay your 100-gram sample evenly out on a drying sheet and let it dry overnight. By morning the sample should be dry.

    2. Microwave: (CAUTION: this method could produce a burning order like burnt microwave popcorn.)

      1. Microwave a paper plate for one minute (to remove moisture from the paper), then weigh for a tare weight.

      2. Add the 100-gram sample to the plate and place in the microwave.

      3. In the back corner of the microwave place an 8 oz glass of water to help prevent the sample from burning.

      4. Microwave the sample for 3 to 4 minutes and then weigh.

      5. Repeat step 4 until the weight change is less than one gram. Reduce the microwave cook time each time you repeat step 4 to prevent burning.

    3. Moisture tester (Figure 3) or an air fryer:

      1) Measure a 100-gram sample and place in the tray provided by either the moisture tester or air fryer. For the moisture tester, I usually let it dry for 20 minutes before I weigh the first time. Because air fryers vary by model, start out at 10 minutes to see how fast it dries.

  5. Keep drying and weighing until the weight change is less than one gram. After the sample is dry, it’s time for some math. The University of Wisconsin Extension has shown it is very difficult to remove the last 2 to 4% of moisture content; therefore, they recommend adding the residual moisture content to the final moisture content value.1 For example, using a final weight of 30 grams your calculation would be: 100 grams (starting) – 30 grams (final) = 70 grams or 70% moisture content plus at least 2% for the residual moisture content, which equals 72%.

    This moisture content value can help determine an optimum time for chopping to begin.

Nathan Falk

Technical Agronomist


1Peters, J. 2012. On-farm moisture testing for corn silage. Focus on Forage. Wisconsin Team Forage. University of Wisconsin. https://fyi.extension.wisc.edu/forage/files/2014/01/CSMoistTest-FOF.htm_.pdf.

2Trussing, T. 2021. Silage moisture testing tips. South Dakota State University Extension. https://extension.sdstate.edu/silage-moisture-testing-tips.

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