Effects of Spontaneous Heating in Large Hay Packages On Fiber Composition, NDF Digestibility, and in-Situ Kinetics of Ruminal NDF Disappearance.

During 2006 and 2007, 96 large-round bales of mixed alfalfa-orchardgrass hay obtained from three harvests were used to assess the effects of spontaneous heating on concentrations of fiber components, 48-h NDF digestibility (NDFD), and in-situ disappearance kinetics of NDF within hay stored in large bale packages. Concentrations of all fiber components (NDF, ADF, hemicellulose, cellulose, and lignin) increased in response to spontaneous heating during storage. Changes in concentrations of NDF during storage (poststorage – prestorage; ΔNDF) were regressed on HDD using a nonlinear regression model (R² = 0.848) that became asymptotic after ΔNDF increased by 8.6 percentage units. While the specific regression model varied, changes in concentrations of ADF, cellulose, and lignin also increased in nonlinear relationships with HDD that exhibited relatively high R² statistics (≥0.710). Fiber digestibility, as determined by NDFD, was largely unaffected by heating characteristics, except within bales incurring the most extreme levels of HDD or MAX. In situ assessment of ruminal NDF disappearance kinetics indicated that disappearance rate (Kd) declined by about 40% within the range of heating incurred by these hay bales. The change in Kd during storage (ΔKd) was related closely to both HDD and MAX by nonlinear models exhibiting high R² statistics (0.907 and 0.883, respectively). However, there was no regression relationship between changes (poststorage – prestorage) in effective ruminal disappearance of NDF and spontaneous heating, regardless of which measure of heating served as the independent variable. The close regression relationship between ΔKd and measures of spontaneous heating indicates clearly that ruminal NDF disappearance was altered negatively by some direct or indirect aspect of spontaneous heating. However, it was equally apparent that these effects were offset by an expanding pool of DM recovered as potentially degradable NDF.