The hay bales you stack in the shed this summer probably will look safe and secure on the outside. But danger may lurk inside the stack.

Hay is at risk of spontaneous combustion when baled at 18% or more moisture and immediately stacked inside a building, says Doug Overhults.

Overhults, a University of Kentucky extension ag engineer, addressed this topic during the recent Kentucky Hay Conference.

Ideally, he says, growers would leave hay unstacked for the first three weeks after baling. During that period, hay temperatures typically rise due to microbial activity and plant respiration — but usually no higher than 130°F. With stacked bales, if the heat cannot escape to the outside edges, the temperature may escalate and fire can result.

The good news: By checking hay temperatures daily those first few weeks after harvest, you can minimize the risk of fire. To monitor hay, buy a sturdy thermometer capable of reading temperatures up to 200°F. Accuracy within 5° is sufficient. Thermometers are available at most farm supply and hardware stores, or from heating and air-conditioning suppliers.

For accurate readings, probe the hay in several locations. If temperatures are on the upswing, check the hay frequently during the day.

In round bales, check the most tightly packed hay — where heat tends to build — typically 6-12" from the center. For this purpose, an 18-36" thermometer is adequate. To evaluate stacks, you must reach 5-10' down from the top or in from the side. You can accomplish this by inserting a hollow probe into the hay, then placing the thermometer inside it.

You can make an effective hay-temperature probe from ½" or ¾" steel pipe or electrical conduit. Close one end of the tube, either by squeezing the tube together or by welding a small bolt into its end. If the tube end is squeezed together, a rivet will keep it from separating. Then grind the end to a moderately sharp point.

Lastly, drill three or four small holes (¼" diameter) through the pipe near the pointed end. These holes will allow the thermometer to take a fairly accurate reading when inserted into the probe. If you like, attach handles to the other end of the probe with pipe and/or conduit fittings.

A long-stem dial thermometer (compost thermometer) works well in a probe. A simple glass thermometer, which can be lowered into the probe by a string or wire, also is effective.

One caution: Avoid using a mercury-filled thermometer, as the mercury will contaminate hay if the thermometer breaks.

Rake Design Doesn't Impact Hay Quality

The type of rake you use to windrow hay doesn't have much effect on its drying rate or quality. That's according to an Ohio State University study. The researchers compared bar, rotary and wheel rakes on an alfalfa-orchardgrass mixture and on pure alfalfa. Samples were taken prior to raking and 24 hours after round baling.

The raking and baling process lowered crude protein by 5% and raised NDF by 6.4%. But rake design had no significant effect on the dry matter content, crude protein or NDF of either type of hay.