Effects of Spontaneous Heating On Dry Matter Recovery, In-Vitro True Digestibility, and In-Situ Kinetics of Ruminal Dry Matter Disappearancein Large Hay Packages.

Alfalfa-orchardgrass hays were made in 96 large-round bales over three harvests during 2006 and 2007 to assess the effects of spontaneous heating on dry matter (DM) recovery, in-vitro true digestibility (IVTD), and in-situ disappearance kinetics of DM. Throughout these harvests, bales were made at pre-set bale diameters of 0.9, 1.2, or 1.5 m, and at moisture concentrations ranging from 9.3 to 46.6%. Following storage, regressions of DM recovery on heating degree days > 30^oC (HDD) and maximum internal bale temperature (MAX) indicated that DM recovery declined linearly in close association with both measures of spontaneous heating. For HDD, slopes and intercepts differed across bale diameters, but r² statistics were high (r² ≥ 0.872) for all bale sizes. Regressions of DM recovery on MAX also exhibited high r² statistics (≥ 0.833), and a common slope across bale diameters (-0.32 percentage units of DM/^oC). Changes in concentrations of IVTD during storage (poststorage - prestorage; ΔIVTD) also were regressed on both HDD and MAX. For HDD, the data were best fit with a nonlinear model in which ΔIVTD became rapidly negative at <1000 HDD, but asymptotic thereafter. For MAX, a simple linear model (Y = -0.23x + 9.5) provided the best fit. In each case, r² or R² statistics were comparable to those for DM recovery (≥0.820). Changes in ruminal DM degradation rate (ΔKd) and effective ruminal degradability of DM (ΔDEG) were assessed similarly. Although the most appropriate statistical model varied, ΔKd and ΔDEG both became increasingly negative at low-to-moderate levels of heating, but generally stabilized thereafter. Both HDD and MAX were excellent predictor variables for both ΔKd and ΔDEG (r² or R² ≥ 0.788). Measures of spontaneous heating are effective indicators of DM recovery following storage, as well as changes in IVTD or in-situ disappearance of DM.