Hay Moisture Testing and Considerations

This year (2025) had a particularly wet spring, which complicated our hay cutting schedules. The information below is a compilation of hay moisture considerations, along with some more detailed information about why moisture matters so much for hay storage.

Why does Hay moisture matter?

The water in hay allows bacteria, fungi, and other microorganisms to grow. As microorganism populations grow, they respire and produce heat. The resulting heat can cause a chemical reaction in hay that reduces digestibility, and it can also increase the risk of fires in your storage area.

Use a thermometer to monitor the temperature of hay. Prolonged heated between 100* and 150F can reduce the nutritional value of hay, especially the digestibility of protein. Heating above 150F also increases the risk of fires.

Even after hay cools down, there can be lasting impacts on quality. The longer hay temperature is elevated, especially when it gets close to 150*F, the more protein digestibility is impacted. Severely heated hay may have protein value approaching 0%. The leftover effects of microorganisms from hay that got heated can also cause the hay to be dusty, which can lead to significant respiratory issues in horses.

What moisture level is safe in Hay?

Safe moisture depends on the size of your bales.

• Below 20% for small square bales
• Below 18% for round bales
• Below 15% for large round bales

How Do You Test Hay Moisture Levels

The most common option for moisture testing is a hay probe, but there are also moisture meters that can be mounted on implements for real-time monitoring. Follow the manufacturer’s instructions to make sure your moisture meter is well-calibrated.

If you don’t have a moisture meter, there are also ways to measure hay moisture using a scale and either a microwave or vortex dryer. Ag Proud has a great article about vortex dryers here.

Note that all those methods have some amount of error to them—typically in the range of 1-3%. Take multiple measurements of hay moisture and monitor how moisture fluctuates throughout the day while you’re baling. 

Other Hay Moisture Tips

• Hay will sweat regardless of its moisture content. But, dryer hay will heat up less during this sweating period, which helps retain digestibility of the hay’s protein. 
• If you’re using a moisture meter, make sure it’s calibrated according to the manufacturer’s instructions. 
• When storing hay, allow ample airflow and prevent wetter bales (that may be smaller or harvested using preservatives) from touching dryer bales.