Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/29/2010
Publication Date: 2/22/2010
Publication URL: journalofdairyscience.org/article/S0022-0302(10)00083-4/pdf
Citation: Coblentz, W.K., Hoffman, P.C., Martin, N.P. 2010. Effects of Spontaneous Heating on Forage Protein Fractions and In Situ Disappearance Kinetics of Crude Protein for Alfalfa-Orchardgrass Hays Packaged in Large-Round Bales. Journal of Dairy Science. 93:1148-1169. Interpretive Summary: 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 (less hay to feed) and forage quality. Most livestock producers and nutritionists are familiar with how protein is damaged or lost when bales experience spontaneous heating; generally this occurs through formation of indigestible complexes of forage proteins and carbohydrates. However, bale packages today are generally larger and more susceptible to spontaneous heating than they were a generation ago, and concepts describing heat damage to forage proteins have not been evaluated thoroughly within this context. In a series of three harvests from the same research site, 96 large round bales of alfalfa/orchardgrass hay 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%. Concentrations of insoluble protein increased in curvilinear patterns that were closely associated with measures of spontaneous heating. These responses are consistent with expected reductions in protein digestibility. Within ranges of heating most commonly encountered under field conditions, estimates of in situ ruminal protein degradation rate and effective ruminal degradability (digestibility) declined sharply in close association with measures of spontaneous heating, which is consistent with past work. However, these expected trends did not continue within bales that heated excessively and sometimes exhibited visible charring. It remains unclear whether these unexpected responses could be demonstrated with other analytical techniques, or whether the severity of these heating increments exceeds the reliable limits for estimating protein degradability via in situ methodology. This research will aid nutritionists in the evaluation and appropriate use of heat-damaged hays for dairy cows and replacement heifers.
Technical Abstract: During 2006 and 2007, forages from 3 individual hay harvests were utilized to assess the effects of spontaneous heating on concentrations of crude protein (CP), neutral-detergent insoluble CP (NDICP), acid-detergent insoluble CP (ADICP), and in situ disappearance kinetics of CP and NDICP for large-round bales of mixed alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.). Over the 3 harvests, 96 large-round 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%. Internal bale temperatures were monitored daily during an outdoor storage period. The change in the concentrations of NDICP (poststorage – prestorage; 'NDICP) increased with heating degree days >30oC (HDD) in a relationship best explained with a nonlinear model [Y = 24.9 – (22.7*(e-0.000010*x*x)); R2 = 0.892] that became asymptotic at +24.9 percentage units of CP, thereby indicating that NDICP increases rapidly within bales that heat spontaneously. When maximum internal bale temperature (MAX) was used as the independent variable, the best regression model was quadratic, and the coefficient of determination was still relatively high (R2 = 0.716). The change in concentrations of ADICP (poststorage – prestorage; 'ADICP) also increased with HDD and was best fitted to a nonlinear model [Y = 14.9 – (15.7*(e-0.0000019*x*x))] with a very high coefficient of determination (R2 = 0.934). A similar quartic response was observed for the regression of 'ADICP on MAX (R2 = 0.975). Increases in 'ADICP as a result of heating (HDD or MAX) were paralleled by concurrent increases in hemicellulose at relatively low increments of heating, but the inverse relationship was observed as hemicellulose became reactive and concentrations declined in more severely heated hays. Changes in ruminal disappearance rate of CP ('Kd) were best fitted to cubic models for regressions on both HDD (R2 = 0.939) and MAX (R2 = 0.876), and these changes represented an approximate 50% rate reduction in severely heated hays relative to prestorage controls. Within ranges of heating most commonly encountered under field conditions, changes in rumen degradable protein ('RDP) declined in primarily a linear relationship with HDD or MAX. However, the mean 'RDP for the four most severely heated hays was only -2.6 percentage units of CP, which represents only a minimal reduction from prestorage controls, and is far less than the maximum of -7.9 percentage units of CP observed with less-severe heating. Interpretation of these results was complicated by poor recovery of NDICP from our most severely heated hays following machine rinsing of 0-h Dacron bags; theoretically, and by definition, this unrecovered pool of NDICP is assumed to be entirely degradable in the rumen. It remains unclear whether these responses could be corroborated in vivo, or by other analytical techniques, or whether the magnitude of HDD or MAX for our most severely heated hays exceeds the reliable limits for estimating RDP via in situ methodology.