Microwave Drying to Determine Moisture Content in Forage
V. F. Colenbrander, K. D. Johnson, C. L. Rhykerd, and C. H. Noller
Departments of Animal Sciences and Agronomy, Purdue University
Cooperative Extension Service
West Lafayette, IN 47907
Knowing the moisture (dry matter) content of forage at the time of harvest and storage
is essential for making and preserving high quality silage and hay. Knowledge of dry
matter content is also important for accurate formulation of rations. Hay harvested and
stored at too high a moisture content (greater than 20%) will result in spoilage and
possible spontaneous combustion. Harvesting hay when it is too dry results in excessive
leaf loss; this reduces the feeding value and increases dry matter loss. Harvesting forage
for silage at too high or too low a moisture content results in improper fermentation,
which reduces feeding value.
A squeeze test is not an accurate method for estimating forage moisture content while
making hay or silage. Resistance-type moisture detectors are available for rapid moisture
determination of forage, but these devices are usually expensive and vary in accuracy.
They have questionable value. A portable electric drying unit is also available but
requires considerable time when determining the moisture content of forage. Under good
drying conditions the moisture level of the forage in the field may have changed
substantially before the test is completed.
Procedures for Obtaining Representative Samples
Proper sampling is essential if an accurate moisture content is to be obtained.
Procedures for obtaining representative samples from various types of forage are as
Forage in windrow (hay making). Care must be taken to obtain a representative
sample from windrows. Avoiding "slug" areas and "very thin" areas, cut
6-inch sections from several locations in the windrow. If the windrow is dry, extreme care
must be taken to avoid losing the leaves. Cut the sections into 1/2 to 1 inch sections for
use in the moisture determination.
Forage in windrow (silage making) or standing crop. If forage in the windrow, or
standing corn or sorghum is to be harvested with a forage chopper, a few hundred weights
of forage can be chopped from a representative part of the field. A small sample is then
collected from various locations in the forage wagon and blended thoroughly for use in the
Conventional square bales. A bale probe should be used to collect samples. The
probe should be approximately 12 to 18-inches long, hollow, and at least 3/8-inch in
diameter. Most probes are designed to attach to an electric drill or brace. Many Purdue
Cooperative Extension Service Offices in Indiana have a bale probe that may be obtained
for short-term usage by residents. Probes must be kept sharp so they will cut through,
rather than slide past stems. Known sources of probes include:
- * A cut-off metal golf club shaft. The handle end of the shaft is also cut off, a
plastic bag attached to the end of the handle with a rubber band, and the user drives the
golf club shaft into the end of a bale by hand.
- * Heavy-duty sampler. "California Belly Buster" is an 18-inch probe with
1/2-inch internal cutting diameter. Malm Metal Products, P.O. Box 4299, Santa Rosa, CA
- * Penn State forage sampler is an 18-inch probe with 3/4-inch internal diameter
available in hand brace or electric drill modes. Nasco Farm and Ranch Catalog, 901
Janesville Avenue, Ft. Atkinson, WI 53538, Phone: 1-800-588-9595.
- * Northwest Ag forage probe is a 12-inch probe with a 1/2-inch internal diameter,
equipped with sample collection box and electric drill mode. Northwest Ag, P.O. Box 238,
Culver, OR 97734.
- * Oakfield hay sampler is a 1/2-inch metal probe. Oakfield Apparatus, Inc., P.O. Box 65,
Oakfield, WI 53065.
- *Utah State hay sampler is a 1/2-inch electric drill, 15-inch steel barrel with a
1/2-inch internal cutting diameter. It contains a built-in sample collection box.
Information available from Dr. Jim Bushnell, Extension Agronomist, Utah State University,
Logan, UT 84322, Phone (801) 750- 2259.
- *Hay Chec. is a 16-inch machine steel probe, 7/16-inch inside diameter cutting edge. It
has a large body with handles for hand use. The probe fits at one end and a threaded
receptacle at the other end to receive sample jars. A.M. Hodge Products, P.O. Box 202005,
San Diego, CA 92120-0925, Phone 1-800-854-3565.
A minimum of 20 average-looking bales, representative of the lot, should be selected
for sampling. Steps in obtaining samples from these representative bales are as follows:
1. Take one core drilling from an end of each bale. Insert the probe near the center of
the bale's end at a right angle to the surface. (Figure 1).
Figure 1. Proper technique for sampling conventional square bales.
2. Place drillings in a clean plastic bucket.
3. Thoroughly mix drillings.
4. Put the mixed drillings in a sealed plastic bag until moisture determination can be
5. Repeat procedure for each lot, making sure not to mix drillings of different lots.
If a bale probe is not available, reach inside each sample bale and carefully remove a
handful of forage. With a pair of shears, cut each sample into 2-inch sections. Mix all
samples from a lot and again remove about one quart for moisture determination.
Large round bales and stacks. Ten sampling bales or compressed stacks from each
lot should be randomly selected. The bale probe is used to make drillings from two
different locations from each selected bale or stack. Drillings should be made at the top
of each stack; bales should be probed from the sides (not the ends). Mix the drillings
from each lot and place in a sealed plastic bag until moisture determination can be made.
If a bale probe is not used, sample these large packages by hand as described for
conventional square bales, with the exception that two or three handfuls should be removed
rather than a single handful.
Loose hay. Samples should be taken from at least 12 random locations in the mow
or on the stack. To make bale-probe drillings, stand on the stack and insert the probe
vertically where the hay is compressed between your feet. If a bale probe is not used,
obtain samples by hand using the procedure suggested for large package hay.
Silage. Sampling a lot when filling the silo would be one way of getting
representative samples for moisture determination. However, this sampling method gives an
indication of moisture when the forage entered the silo, but it may not provide an
accurate assessment of its dry matter composition after fermentation. Follow suggested
silage sampling procedures:
- *Collect about 2 gallons of silage in a clean plastic container by taking handfuls at
random from 20 different locations on the exposed face or surface (avoid rotted silage on
top) or by passing the container beneath the chute several times (once a minute),
collecting one or two quarts at each pass while the silage is unloading (Figure 2). Mix
subsamples thoroughly and seal about one quart in a plastic bag for moisture
Figure 2. Obtaining a representative silage sample.
To identify different silage lots, several bales of straw, shavings, or shredded paper
can be fed through the blower when the last of each lot is ensiled. The presence of silo
gases should be of utmost concern when obtaining silage samples from the surface of
Technique for Moisture Determination
The following technique describes how a microwave oven can be used to determine
moisture content in forages in a matter of minutes with the accuracy close to that
obtained when using accepted research methods.
All that is needed is a small scale, paper plate, water glass, and a microwave oven
(Figure 3). The accuracy of the results are dependent upon the quality of scale used. A
diet scale measuring in grams will give reasonably good results, although a scale weighing
to 0.1 gram is preferred. This type of balance is available from most farm catalog supply
Figure 3. Moisture determination equipment.
The procedure for determination of moisture (dry matter) content with the microwave
oven is as follows:
1. Place a preweighed large paper plate (9-inch minimum) on the scale and weigh out
exactly 100 grams of the representative forage sample (Figure 4). A smaller sample can be
used but the 100-gram sample makes for easier calculations.
2. Spread the forage evenly on the paper plate.
3. Place an 8 oz. water glass, three-quarters full of water, in the back corner of the
microwave oven (Figure 5); keep water level constant during oven use. This will protect
the oven magnetron when sample moisture is low. The setting of adjustable microwave ovens
should be 80% to 90% of maximum power.
4. Initially, dry legume or grass samples in the 50% to 70% moisture range for eight
minutes. Then weigh and record sample weight. Mix the sample and place it in the oven for
two minutes; remove and weigh. If the weight has not changed more than one gram, use this
value. If the change is greater than one gram, continue drying using additional one-minute
intervals until the weight change is less than one gram. For greater accuracy, continue
drying until the dry weight is constant.
5. If corn or sorghum is to be utilized for silage, use a longer initial drying period
of 14 minutes, followed by a two minute period, and finally one minute drying intervals.
6. For forage with a moisture content of less than 30%, an initial drying time of four
minutes should be used. Then weigh the samples following the same procedure as for the
wetter forage above, using one minute drying intervals until the weight change is less
than one gram.
7. Be careful not to char the sample. If this occurs, it means the oven was set too
high, the drying time was too long, or the glass of water in the rear of the microwave
oven was omitted. Discard the charred sample and repeat the test.
8. Use the following equation to calculate the moisture content. Keep in mind, since
the wet and dry weights include the weight of the paper plate (unless scale can be tared
to zero with paper plate on scale), the weight of the paper plate must be subtracted from
the wet and dry weights before making the following calculation.
(wet weight) - (dry weight)
Percent moisture --------------------------- x 100
If you have a scale which permits you to tare the paper plate (initially adjust the
scale to zero with the paper plate on the scale), the percent moisture can be calculated
simply by subtracting the dry weight in grams from 100 grams (the assumed original wet
weight). The final dry weight is the dry matter content of the sample.
Cooperative Extension work in Agriculture and Home Economics, State of Indiana,
Purdue University and U.S. Department of Agriculture cooperating: H.A. Wadsworth,
Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30,
1914. The Cooperative Extension Service of Purdue University is an equal opportunity/equal