Hay moisture probes aren't 100% accurate. Yet they can be valuable tools for operators who check probe readings against oven drying and gain experience in their use.

“Some probes could be off by as much as five percentage points,” says Oklahoma State ag engineer Ray Huhnke.

Ron Thaemert's two hay probes have always read differently. “One is about two percentage points off; I had to learn the accuracy of that probe,” says the Blaine County extension agent from Hailey, ID. He suggests oven-drying hay first tested by a probe to check its accuracy.

“I would never rely on a moisture probe as a sole source of information to make a decision,” adds Huhnke. “A person's experience is the best judge. The more a person uses that probe, the more he or she will understand in which conditions it will give reliable or more reliable results.”

A major reason for inaccuracies is that bales are made up of layers of inconsistent moisture and density. So the more probes made per bale, the more accurate the results. Meter readings can also be affected by bale temperature.

Hay probes use electrical conductivity between points on probe tips. The wetter the hay, the more electricity flows, causing higher moisture readings. If the surface of a bale being tested is wet with dew, that moisture can give a false reading, Huhnke says.

The meters, he adds, give an approximate moisture content, not an exact number.

Before probing, do some basic maintenance, Thaemert suggests.

“The batteries need to be checked, fully charged or in good charge. Keep tips clean from alfalfa juice buildup and contamination; the contact points need to be clean.”

The more dense the bale, the better the reading, Huhnke says. Probe at the ends of a square bale, or at an angle that goes across the stems.

“Round bales should be probed through their diameter. Make at least five probes per bale and take an average of the readings,” he says. “If the range is greater than three percentage points, probe more bales to improve accuracy.” Also probe several bales at different field locations to account for field variations.

The type of forage being tested affects probes as well. The moisture levels of leafy alfalfa, for instance, are more likely to read accurately in a hay probe than stemmy bales of sorghum-sudangrass, Huhnke adds.

Thaemert sees another reason for inaccurate moisture readings: “We don't allow enough time for the tool to adjust itself.” Pushing a probe into a bale quickly often causes friction, skewing results. “Allow those tips to cool down so you can get an accurate reading,” he advises.

Windrow Probing Works

Ron Thaemert prefers to moisture-probe hay in windrows. He says probing after it's baled is too late.

“What do you do if the bale's too wet? You set it off to the side and hope you can get it fed before it molds or burns,” says the Blaine County, ID, extension agent.

It helps that he has a handy-dandy way of probing, using his own Windrow Sample Gathering Tool. He simply rolls a windrow over, grabs a handful of the wettest-looking hay and twists it into a 2' × 2" ABS pipe until it's full. The pipe, capped on one end, is packed using a 2' × 1¼" capped PVC pipe.

Once the sample is compact, he pushes a 20" hay probe about 4" into it and takes a reading. He also takes readings at 8", 16" and 20". After averaging readings, he packs and probes other areas in a field.

Just as Thaemert cautioned with bale probing, don't overheat probe tips, clean them regularly and charge batteries before testing windrows. Double-check probes by oven-drying samples from the gathering tool.

“I have saved a number of haystack fires in my county because of my tool,” Thaemert says. “We try to push baling to the limit, because we're in a very high mountainous area where a rainstorm can come at any time.”

Growers in at least nine states — and Romania — use Thaemert's tool, which he will send to those interested for $10, plus shipping.

“They're also free to make their own. I'm very willing to share my pattern and dimensions.” Email him at: thaemert@uidaho.edu. Instructions are on the Web at: extension.oregonstate.edu/yamhill/pdf/sampling_the_moisture_content_of_alfalfa.pdf.