Moisture testers for hay can be a useful tool, as long as they're calibrated with experience.
Bruce Anderson, University of Nebraska Extension forage specialist, points out in one of his recent and insightful Hay and Forage Minutes newsletters, "…it can be very risky to blindly believe values displayed by the tester… This doesn't mean that hay testers can't be trusted. What it means is that you as the operator need to know more about how the tester works, what causes values to vary, and how to use the tester most effectively."
Anderson explains that testers measure the electrical resistance of the hay between two sensors. "The more moisture, the less resistance," Anderson says. "Then they convert this resistance to a value associated with average moisture content. However, other hay characteristics influence how testers sense moisture. For example, tight or dense bales, or even areas within a bale, will give different results than looser hay. Wiggling the probe or inserting it at an angle vs. straight into the bale changes the value. And hay with dew moisture gives much different values than hay without dew even when overall moisture content is the same."
So, it's more a matter of individual producers learning what a particular reading in a particular situation means. Even with modern gadgets, it seems that determining the optimum time to bale is part art, part science and plenty of experience.
I'm reminded of one cagey cow-calf producer and cattle feeder who followed one of his sons through a hay field, having him churn out a round bale even as it began to rain, then shoving a probe into the bale to find the reading was at least as high as the son expected; the son obviously believing it was nuts to even try once the rain started. After a few bales, each reading higher than the last, Dad told the son maybe it was a little too wet. As his son hit high gear taking the baler home, his dad smiled and told me, "They worry too much. I'll mark those bales and keep track of them; they’ll be just fine."