ARS | Publication request: Spontaneous heating more likely with larger bales

Submitted to: Electronic Publication
Publication Type: Popular Publication
Publication Acceptance Date: November 10, 2008
Publication Date: December 1, 2008
Citation: Coblentz, W.K., Hoffman, P.C. 2008. Spontaneous heating more likely with larger bales. December 2008. Midwest Forage Association, Forage Focus. p. 4-5.

Technical Abstract: Spontaneous heating in hay is caused generally by too much moisture in the plant at the time of baling. This costs livestock producers in terms of dry matter losses and forage quality. With conventional small rectangular bales (80 to 100 pounds), a positive linear relationship between moisture content at the time of baling and heating has been clearly established. But today’s hay producers are using much larger round or square bales, and these larger hay packages have not been studied extensively with respect to spontaneous heating. Therefore, we conducted a study to determine if bale moisture and diameter (3-, 4-, or 5-foot bales) had an impact on spontaneous heating. Concentrations of NDF increased by as much as 11 percentage units as a result of spontaneous heating; from a practical standpoint, it is important to note that NDF is not really generated during the heating process. Increases in NDF concentrations occur because cell solubles (most specifically, sugars) are oxidized preferentially during microbial respiration. 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 energy density (TDN) of the forage. Surprisingly, results of our study suggest that NDF digestibility is not altered significantly by heating unless the heating is severe enough to cause charring, which appears as black or dark brown pockets within the bale core. When extreme cases of heating are excluded, there is little evidence that NDF digestibility and heating degree days are related statistically. In conclusion, larger bales are more susceptible to heat damage at lower moisture levels. Traditional guidelines defining heat-damage to forage proteins are reasonable, but concurrent reductions in energy density may be the most serious consequence of spontaneous heating.