Ohio State University Extension

Ohio State University Extension
Department of Horticulture and Crop Science
2021 Coffey Road, Columbus, Ohio 43210-1044

Using a Microwave Oven to Determine Forage Moisture


It is important to the forage producer to know the moisture content of forage in order to perform the harvesting operations of ensiling or baling and storing. A crude procedure for estimating the moisture level is to squeeze or twist and to feel the forage before making a "guesstimate" based on past experience.

Another method involves drying a weighed sample of the forage in a conventional oven or oven-like device for a prescribed time. This is time-consuming. Beyond this, the moisture content of the forage in the field could have changed substantially by the completion of the drying of the sample and could be greatly different from the computed moisture. This procedure has been the most accurate of those used in recent years.

A microwave oven, however, can be used to estimate moisture levels of forage in just a few minutes. Many farm homes now have microwave ovens which make this procedure possible. A small kitchen, dietary or postage scale is also necessary, preferably one which weighs in grams.

Select a representative sample of the forage to be tested. As with all analysis, selecting a representative sample is a difficult but very important chore. Obtain different samples of several pounds each from the field. These should be collected by taking several handfuls at random from across the field. Subdivide these to obtain the samples to be tested.

Clip or chop the forage subsamples into pieces 1-2 inches long. Accurately weigh 100-150 grams of the forage. This is the wet weight. It is very important that this forage be a representative sample because it is only a fraction of a pound (454 gms = 1 lb).

Place this sample on a paper plate and spread in a thin layer leaving an open area in the center. Thus, the sample would be somewhat doughnut-shaped on the plate.

Place an 8 ounce glass of water (3/4's full) in back corner of the microwave. Keep water amount somewhat constant during microwave use. This is as prescribed by the microwave manufacturers.

If the sample is estimated to be 50-75 percent moisture, place sample in oven for 4 minutes. If the sample is 35 percent moisture or less, heat for 2 minutes. Remove from oven and if forage feels almost dry, weigh and record weight. Stir forage and rotate plate and place in microwave for 1 minute if sample was originally 50-75 percent moisture or one-half minute if less than 35 percent originally.

Continue this procedure until the drying does not reduce the weight more than 1 to 2 grams and/or forage begins to char. If any charring has occurred, use the previous weight for calculating the moisture content.

The dry weight is the last weight recorded after which the sample does not decrease more than 1 to 2 grams and charring has not occurred.

To calculate the percent moisture, subtract the dry weight from the wet weight and divide by the wet weight. Multiplying this by 100 will produce percent of moisture.

Percent Moisture = (Wet weight minus dry weight) divided by Wet Weight X 100

Example: Wet wt. (150 grams) minus dry wet (111 grams) = 39 grams

39 grams divided by wet wt. (150 grams) = .26.
26 x 100 - 26% moisture of this example

Power of microwave ovens will vary slightly and thus there may be variations in drying times. With a little experience the procedure can be quickly established and moisture percentage determined in several minutes. Of course this does not include the sample collecting and preparation time.

Moisture suggestions for storing forage:

Prepared by:
Donald K. Myers, Extension Agronomist Emeritus

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181

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