August 9, 2012
How to Use a Microwave to Determine the Moisture Content of Forage
To test the moisture content of forage you’re planning to ensile or bale, use your microwave. It can take a few trials to become comfortable with the process, but it’s quick, easy, and can provide a good estimate. Other options include moisture meters and probes, which can be fast, although results can be variable. A traditional oven also can be used, but the process can take up to three days. If done properly, a microwave oven can provide a good estimate at any moisture level, sometimes in as little as 20 minutes.
The basic process to test for moisture content is relatively simple with a microwave oven. Collect the sample, weigh it, dry it in short intervals to a constant weight, and then calculate the moisture concentration.
- A microwave, preferably one with a turntable. If your microwave doesn’t have a turntable, be sure to rotate the sample each time you place it back in the oven.
- A microwave-safe plate or pan to hold the sample. (Don’t use a paper plate.)
- A gram scale accurate to at least 1.0 gram
- A sample of grain or forage representative of the entire lot to be tested. (If the sample is not to be tested immediately after sampling, seal it in an airtight container such as a glass jar or a good quality freezer bag to avoid changes in moisture content.)
Being as precise as possible when taking and recording the weights will help ensure accurate results.
- Prepare sample. Chop hay crop forages into lengths of 1-2 inches for ease of handling. Chop grain crop forages into lengths of ½ inch or less.
- Weigh the empty microwave-safe container and record the weight (Table 1).
- Place the sample in the container, weigh it, and record the number. (Using exactly 100 grams of sample will simplify calculations.)
- Spread sample evenly in the container (maximum depth is 1 1/2 inches), but leave a small depression in the center so the forage sample looks a little like a donut.
- Place the container with the sample into a microwave and begin drying at high power setting. The wattage of the microwave, evenness of heating (a turntable helps), and amount of material and its moisture content will influence how long it takes to dry the sample.
- Begin by using short drying periods — 2 minutes for haylage, silage, or fresh material; 1 minute for dry hay. As you gain experience with your microwave and forage, adjust times accordingly.
- Remove the container and sample, reweigh it, and record the weight.
- Mix the forage, rotate the container, and return it to the microwave. Continue drying for 30 seconds if forage is nearly dry or one minute if still moist. Reweigh the container. (It should weigh less than when you started.) WARNING: High moisture forage can get extremely hot and can burn. If forage gets too hot to mix comfortably by hand, use a lower power setting until forage is nearly dry.
- Repeat steps 6 and 7 until forage weight does not decrease by more than 1 gram from the previous weight. If forage starts to char or burn, use the previously recorded weight.
- Calculate the moisture and dry matter content.
Total drying time depends on several factors including the initial moisture content of the forage and the wattage of your microwave.
After drying, do not submit these same samples for nutrient testing as the heating may have affected protein and fiber levels.
Calculating Moisture Content
To calculate the moisture content of a sample, divide the moisture weight removed in drying by the fresh weight of the sample, then multiply by 100 to get a percentage.
If you have a scale which permits you to tare the container (initially adjust the scale to zero with the container on the scale), the percent moisture can be calculated simply by subtracting the dry weight in grams from 100 (if the original wet weight was 100 grams). The final weight is the dry matter content of the sample.
Extension Forage Specialist