Driving around the county the past week I could see that first cutting hay harvest is well underway. The goal in haymaking is to preserve forage quality in a form that can be used by livestock at a later date. Each year however, dry matter and forage quality is lost due to spontaneous heating in hay that is caused by baling at too high of a moisture content. Generally all bales left to air dry after baling at 15 to 20% moisture undergo some degree of heating beginning a couple of days after baling and continuing for a week to 10 days after baling. The heat that is generated is a result of plant respiration and microorganisms on the hay consuming carbohydrates (sugars and starches). In general, if temperatures do not exceed the 130 to 140 degree F range, quality damage is minimal. However, if internal hay temperature exceeds 175 degrees F, then combustion and fire can occur.
Wayne Coblentz at the U.S. Dairy Forage Research Center in Madison Wisconsin examined the relationship between moisture content at baling, size of bales and forage quality using a heating degree days concept. Heating degree days are calculated by subtracting 86 from the maximum internal bale temperature measured in degrees F, for each day of storage. The difference is summed until bales reach the point where the difference is zero. His research basically showed that small square bales, baled at 20% or lower moisture accumulated a low level (200 or fewer) of heating degree days. As bale size and diameter increased, baling at a moisture content of 20% resulted in more heating degree days accumulated and a higher risk of spontaneous heating leading to more significant quality losses.
Quality losses begin to increase dramatically as heating degree days exceed 300. Quality losses include an increase in fiber concentration and a decrease in energy concentration due to heating. For example, at 300 heating degree days the increase in neutral detergent fiber (NDF) was about 2 percentage units compared to the initial NDF concentration and the decrease in energy concentration was about 1 percentage unit. At 600 heating degree days the increase in NDF was 7 percentage units and the decrease in energy concentration was about 5 percentage units. Coblentz found that concentrations of NDF could increase by as much as 11 percentage points as a result of spontaneous heating. The increases in NDF are a result of cell sugars being oxidized during microbial respiration. So the fiber components increase because the cell soluble concentration decreases. The other consequence of heating is that energy density as measured by total digestible nutrients is decreased because the sugars and cell solubles are 100% digestible while the fiber components are less digestible.