Effects of bale moisture and bale diameter on spontaneous heating, dry matter recovery, in vitro true digestibility, and in situ disappearance kinetics of alfalfa-orchardgrass hays.

Alfalfa (Medicago sativa L.)-orchardgrass (Dactylis glomerata L.) hay was made in 96 large-round bales over 3 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 preset diameters of 0.9, 1.2, or 1.5 m and at moisture concentrations ranging from 9.3 to 46.6%. Internal bale temperatures were monitored daily during an outdoor storage period, reaching maxima of 77.2 degrees C (MAX) and 1,997 heating degree days >30 degrees C (HDD) for one specific combination of bale moisture, bale diameter, and harvest. Following storage, regressions of DM recovery on HDD and MAX indicated that DM recovery declined linearly in close association with measures of spontaneous heating. For HDD, slopes and intercepts differed across bale diameters, probably because the greater surface area per kilogram of DM for 0.9-m bales facilitated more rapid dissipation of heat than occurred from 1.2- or 1.5-m-diameter bales. Regardless of bale diameter, coefficients of determination were high (r(2) > or = 0.872) when HDD was used as the independent variable. Regressions of DM recovery on MAX also exhibited high r(2) statistics (> or = 0.833) and a common slope across bale diameters (-0.32 percentage units of DM/ degrees C). Changes in concentrations of IVTD during storage (poststorage - prestorage; DeltaIVTD) also were regressed on HDD and MAX. For HDD, the data were best fit with a nonlinear model in which DeltaIVTD became rapidly negative at <1,000 HDD, but was asymptotic thereafter. When MAX was used as the independent variable, a simple linear model (y = -0.23x + 9.5) provided the best fit. In both cases, coefficients of determination were comparable to those for DM recovery (R(2) or r(2) > or =0.820). Changes (poststorage - prestorage) in ruminal DM degradation rate (DeltaK(d)) and effective ruminal degradability of DM (DeltaDEG) were assessed similarly. Although the most appropriate statistical model varied, DeltaK(d) and DeltaDEG 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 DeltaK(d) and DeltaDEG; r(2) or R(2) statistics ranged from 0.788 to 0.921. Measures of spontaneous heating are consistently effective indicators of DM recovery following storage, as well as good indicators of concurrent changes in IVTD or in situ disappearance of DM for heated alfalfa-orchardgrass hays.