Spontaneous heating in hay, generally caused by too much moisture in the plant at the time of baling, costs livestock producers in terms of dry matter losses and forage quality. Hay that has heated during storage will often appear brown or caramelized in color. Livestock will often like the flavor, but nutrients have been lost due to the excessive heating during storage.
With small rectangular bales, those weighing less than 100 pounds, there is a positive linear relationship between moisture content at the time of baling and heating. The wetter the hay, the greater the chance of heating.
These days, many producers use larger round or square bales. The U.S. Dairy Forage Research Center conducted several studies to determine if bale moisture and diameter (3-, 4-, or 5-foot bales) had an impact on spontaneous heating. Researchers also measured the amount of dry matter lost and the digestibility of the damaged forages.
Just like small rectangular bales, the study found that heating increased with the moisture content for each bale diameter. But as size increases, the larger bales are more likely to exhibit spontaneous heating at relatively low moisture contents, less than 20%, and to accumulate more heating degree days during storage. Similar to “growing degree days” used by agronomists, the heating degree day measurement aggregates the internal bale temperature over time. Larger bales also pose an increased risk of spontaneous combustion.
The increase in heating degree days with larger bales is due to two factors: Larger and/or denser bales contain more dry matter, and larger bales have less surface area per unit of forage dry matter, which impedes dissipation of heat and water.
Heat damage in forages is often viewed in binary terms, meaning that the forage is either damaged or not. In reality, the effects of heating on forage quality are better described as a continuum. Damaged protein is not necessarily the most negative consequence of spontaneous heating. Concentrations of neutral detergent fiber (NDF) increased by as much as 11 percentage units as a result of spontaneous heating. NDF measures structural carbohydrates representing the fibrous bulk of the forage. As NDF content of a feed increases, dry matter intake will decrease.
It’s important to note that NDF is not really generated during the heating process. Increases in NDF concentrations occur because cell solubles, specifically sugars, are oxidized first during microbial respiration. Therefore, the concentration of fiber components such as NDF and acid detergent fiber (ADF) increase because the concentration of cell solubles decreases. This is particularly important because sugars and other cell solubles are essentially 100% digestible, while fiber components are not. As a result, spontaneous heating decreases the forage’s energy density, expressed as total digestible nutrients.
Traditionally, the threshold moisture level for acceptable storage for small rectangular bales has been about 20%. To limit heating, this must be reduced to 16-18% for large round or square bales. For legume hays, these issues create a difficult management situation because lower moisture levels lead to greater leaf shatter and a drop in forage quality. It may not be possible to completely eliminate heating and optimize recovery of leaves in large legume hay bales without preservatives or plastic wrap to eliminate oxygen.
In storage, air movement around bales will help to dissipate both water and heat. Producers storing bales under roof should consider managing moisture at baling even more conservatively.
If heat damage is suspected, a feed nutrient analysis is recommended before diet formulation to determine the quality of the forage. Tests for heat-damaged protein in forages are expressed in many ways by commercial forage-testing laboratories, but the best definition is acid detergent fiber crude protein (ADF-CP). All forages contain some ADF-CP, which is largely indigestible by ruminants. Benchmarks suggest that if this fraction is less than 10% of the total forage crude protein, then minimal heat damage has occurred during storage.
We are often at the mercy of the weather when putting up hay under ideal moisture conditions. Monitoring hay moisture during baling and temperature at storage time is essential to having high-quality forages available for feeding livestock and minimizing storage losses.
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