Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2009
Publication Date: 5/15/2009
Citation: Coblentz, W.K., Hoffman, P.C. 2009. Effects of Spontaneous Heating on Fiber Composition, Fiber Digestibility, and In Situ Disappearance Kinetics of NDF for Alfalfa-orchardgrass Hays. Journal of Dairy Science. 92:2875-2895.
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; however, this is not necessarily the most important negative consequence. The loss of energy, due to the rapid oxidation of sugars during the microbial respiration that causes heating, significantly affects feed quality and value, too. This also increases the percentage of fiber in the forage; but little research has looked specifically at how spontaneous heating affects fiber components, composition, and digestibility. This study showed that: i) concentrations of all fiber components increased in response to spontaneous heating during storage; and ii) surprisingly, fiber digestibility was largely unaffected by heating characteristics except within bales incurring the most extreme levels of heating. Although the rate of ruminal fiber disappearance was decreased by as much as 40% by spontaneous heating, these effects were largely offset by an expanding pool of DM recovered as potentially degradable NDF. This research will aid nutritionists in the evaluation and appropriate use of heat-damaged hays for dairy cows and replacement heifers.
Technical Abstract: Hay from three individual harvests during 2006 and 2007 was utilized to assess the effects of spontaneous heating on concentrations of fiber components, 48-h NDF digestibility (NDFD), and in-situ disappearance kinetics of NDF for large-round bales of mixed alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) hay. Over the three harvests, 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, reaching maxima (MAX) of 77.2oC and 1997 heating degree days > 30oC (HDD) for specific combinations of bale moisture and diameter. 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; 'NDF) were regressed on HDD using a nonlinear regression model (R2 = 0.848) that became asymptotic after 'NDF increased by 8.6 percentage units. While the specific regression model varied, changes (poststorage – prestorage) in concentrations of ADF, cellulose, and lignin also increased in nonlinear relationships with HDD that were characterized by relatively high coefficients of determination (R2 = 0.710 to 0.885). Fiber digestibility, as determined by NDFD, was largely unaffected by heating characteristics, except within bales incurring the most extreme levels HDD or MAX. In situ assessment of ruminal NDF disappearance kinetics indicated that disappearance rate (Kd) declined by about 40% within the ranges of heating incurred over these hay harvests. The change in Kd during storage ('Kd) was related closely to both HDD and MAX with nonlinear decay models exhibiting high R2 statistics (0.907 and 0.883, respectively). However, there was no regression relationship (P > 0.085) between changes (poststorage – prestorage) in effective ruminal disappearance of NDF and spontaneous heating, regardless of which heating index was used as the independent variable. The close regression relationship between 'Kd and indices 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 DM recovered as potentially degradable NDF.