Rory Lewandowski, Extension Educator Wayne County
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 each day until bales reach the point where the
difference between internal bale temperature minus 86 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.
It is important to understand that
some heating loss occurs in any bale made at 15% or greater moisture during the
growing season, but heating losses increase as bale moisture content increases
and/or as bale size increases. Heating
losses can be minimized by baling small square bales at 20% or lower moisture
content, 18% or lower moisture content for large round bales and 16% or lower
moisture content for large rectangular bales.
For legume hay production, these lower moisture contents are problematic
because of increased leaf shatter and the associated forage quality loss. In that case, in order to minimize heating
losses and minimize leaf loss, it may be necessary to utilize a preservative or
plastic wrap to permit baling at higher moisture contents.
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