Determining Forage Moisture Content

Rory Lewandowski, Extension Educator, Wayne County;
Jason Hartschuh, Extension Educator, Crawford County;
and Dr. Mark Sulc, OSU Extension Forage Specialist, The Ohio State University

Stored forage is an important component of many livestock operations. Stored forage quality is dependent upon several key factors, including forage maturity at harvest, forage moisture content at harvest, and forage storage conditions. Stored forages are produced and fed primarily as either dry hay, baleage, or silage. After forage maturity, the quality of a stored forage is greatly dependent upon moisture content at harvest. Given that moisture content is so important, what tools and methods are available to help producers determine forage moisture content to make good harvest decisions?

The primary methods used to determine forage moisture are either some type of hand grab, twist, or squeeze test or the use of some instrument/tool. By far, the most common method of determining forage moisture content is some variation of the hand test. For forage that will be baled, grab a sample from the windrow and tightly twist it. Depending upon how quickly the grab sample springs back or “untwists” determines if the forage is too wet, too dry, or ready to be baled. Table 1 shows guidelines published years ago in Hoard’s Dairyman for estimating moisture ranges in hay that will be baled. For a chopped forage, a hand sample is grabbed and squeezed into a ball. Upon release, the sampler watches how quickly the ball falls apart to determine if the forage is ready to be ensiled. 

Table 1. Sensory assessment of moisture content of drying hay.




Leaves begin to rustle and do not give up moisture unless rubbed hard. Juice easily extruded from stems using thumbnail or knife or with difficulty by twisting with hands.


Hay rustles – a bundle twisted in the hands will snap with difficulty but should extrude no surface moisture. Thick stems extrude moisture if scraped with thumbnail.


Hay rustles readily – a bundle will snap easily if twisted, leaves may shatter, a few juicy stems remain.


Swath-made hay fractures easily, snaps easily when twisted, juice difficult to extrude.

Reproduced from Hoard’s Dairyman, Vol. 132, 1987.

There is as much art as there is science to the various hand methods and experience plays a role in the “calibration” process. There is no denying that a lot of good quality stored forage gets made with these methods of determining forage moisture, so it can’t be pooh-poohed and written off. However, for those who may want more certainty and an actual forage moisture percentage number, there are some other tools that can be considered.

Tools available to determine forage moisture include a microwave oven, commercial forage moisture testers, hand-constructed vortex dryers, moisture probes, and moisture sensors built into harvest equipment. Each has some advantages and disadvantages, but each used with the proper knowledge and protocol can help the forage producer more accurately determine forage moisture.  Each of the following tools/testers requires that a good representative sample is collected to produce a reliable result. As with most sampling systems, taking an average of several samples increases the confidence level of the moisture reading. When sampling windrows, be sure to sample the entire cross section (top, middle, and bottom) from multiple areas of the field.

The microwave oven is commonly available and can provide forage moisture determination to within 1 to 2% of actual forage moisture in about 20 minutes. The biggest drawback to using a microwave is that the forage material can catch on fire if the sampler is not being careful as the forage dries down to its endpoint moisture. The procedure involves weighing out 100 grams (fresh weight) of a representative forage sample that has been cut into pieces no larger than one inch in length. On a microwave safe plate, place a paper towel. Weigh and record this “plate” weight. Add the forage sample, spreading it as evenly as possible over the paper towel on the plate.  Weight the plate, towel and forage and record this initial weight. Place a 10- or 12-ounce coffee mug of water inside a corner of the microwave, place the plate and forage sample in the center, set the microwave setting on high and “cook” the sample for about 3 minutes. Remove the plate and sample, weigh it and record the weight.  Change the water in the cup. Replace the water cup and sample in the microwave and cook it for another 2 minutes.  Again, remove the sample, weigh it and record the weight.  Repeat this process until the weight does not change by more than one gram. Record that weight as the final weight. Important note: As the forage gets closer to its final moisture content, run the microwave in shorter time intervals and change water as necessary to avoid igniting the sample. To calculate the forage moisture of the sample, use the following equation:

Moisture % = [(Initial weight – Final weight) / (Initial weight – Plate weight)] x 100

Commercial forage moisture testers use either heat or electrical conductivity to come up with a moisture reading. Heat type testers include a heating unit and a fan. Like a microwave, they need to be plugged into a power source to work. They determine moisture content by forcing heated air through a forage sample of known weight. Unlike a microwave, they do not require constant supervision/monitoring. Depending upon the initial forage moisture, the process takes 25 to 35 minutes. Accuracy is within 3% of oven dried samples. Forage samples should be in pieces that are 1 to 2 inches in length. The advantage over a microwave is that the risk of setting fire to the sample is reduced. For many producers, the biggest disadvantage of this type of tester is the cost. The Koster forage moisture tester is a familiar name and can be purchased for approximately $350 to $400, depending upon the model and need for a scale, from many sources, including NASCO. The “Best Harvest” silage, hay crop moisture tester brand is another example, with a cost of $400 to $450.

Forage moisture testers that use electrical conductivity include probes and sensors mounted on balers/harvest equipment. The advantage of these types of testers is that they provide near instantaneous moisture readings. Cost is a factor. Depending upon the model of the tester probe, they can cost anywhere from $180 to $300 or more. Sensors on your baler or harvesting equipment may add $275 to $400 to the price tag. For the most advanced systems that can mark hi-moisture bales, the cost can be over $1000, or when systems also manage variable rate preservative application, cost can be over $2500. Depending on the system, the accuracy may only be ±5%. There are some other factors with these testers that affect their reliability. Probes generally do not do a good job of measuring moisture of forage in a windrow. The electronic moisture testers are most accurate in densely packed forage. To increase reliability, grab a representative forage sample from a windrow and pack it into a PVC tube.  Insert the probe into the densely packed forage material and take various forage moisture readings at different depths in the tube (a description of this method is available in an article at Sensors on balers and harvesting equipment begin to lose accuracy and reliability as forage material over time leaves gummy residues on the sensor. Periodic cleaning and calibration of the sensor is needed to maintain accurate readings.

A final option that combines the accuracy of a heat drying tester with a more reasonable price tag is the vortex dryer. Penn State Extension developed the vortex dryer. It uses a common hand-held hair dryer, some CPVC tubing, some galvanized steel, a furnace filter, window screen, and some plywood. Cost will be approximately $50 to $60 to assemble in the farm shop. Forage sample size in the vortex dryer is 200 grams. The procedure is basically the same as with a microwave oven, starting with an initial weight and drying the sample until the weight does not change more than one gram.  Accuracy is within 1% of a drying oven. Hay samples will take 20 to 25 minutes to dry, while forage silage/corn silage samples can take 40 to 60 minutes to get a final moisture determination. More information about the Penn State Vortex Dryer, including a list of materials, how to assemble, and how to use the dryer are available online at

Determining forage moisture can help producers make more accurate decisions regarding timing for baling and ensiling to ensure a safe and stable final product. There are tools available to help with that decision.