Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2010
Publication Date: 6/24/2010
Publication URL: http://journalofdairyscience.org/article/S0022-0302(10)00347-4/pdf
Citation: Coblentz, W.K., Hoffman, P.C. 2010. Effects of Spontaneous Heating on Estimates of TDN for Alfalfa-Orchardgrass Hays Packaged in Large-Round Bales. Journal of Dairy Science. 93:3377-3389. Interpretive Summary: Due to greater ease of harvest and transportation, large-round and large-square hay bales are being used more extensively now than hay packaged in conventional (100-pound) bales. However, these larger bales are more likely to heat spontaneously than conventional bales during storage, and much of the feed value is lost when the hay is heated. In severely heated hays from large-round bales, estimates of the energy available in the hay, expressed as total digestible nutrients (TDN), declined by as much as 13.0 percentage units during storage. Our objectives for this project were: 1) to assess the relationship between estimates of total digestible nutrients (TDN, a common energy estimate) and spontaneous heating; and 2) to describe any important differences in energy estimates that may result specifically from two methods of estimating truly digestible fiber (part of the TDN estimate). Results showed that greater estimates of TDN were obtained when using the product of protein-corrected neutral detergent fiber (NDF) and 48-h NDF digestibility to estimate truly digestible fiber compared to the acid-detergent lignin option. Discrepancies between these estimates also were made worse when NDF rather than protein-corrected NDF was used to estimate truly digestible fiber. As such, any use of the NDFD option for determining truly digestible fiber, and subsequently TDN, should utilize protein-corrected NDF if there is any other evidence suggesting that hays were heated during storage. This information will help dairy nutritionists to better balance diets for dairy cows and heifers, and to properly discount the energy content of heated hays.
Technical Abstract: Large-round or large-square hay packages are more likely to heat spontaneously during storage than hay packaged in conventional (45-kg) bales, and the consequences of this phenomenon on the resulting forage energy estimates for these hays can be severe. Our objectives for this project were to assess the relationship between estimates of total digestible nutrients (TDN) and spontaneous heating, and to describe any important differences in energy estimates that may result specifically from two methods of estimating truly digestible fiber (TD-Fiber). Using the summative approach to estimate TDN, TD-Fiber can be estimated from: i) inputs of protein-corrected NDF (NDFn) and acid-detergent lignin (TD-FiberLIG); or ii) inputs of NDFn and 48-h neutral-detergent fiber digestibility (TD-FiberNDFD). Throughout 2006 and 2007, mixed alfalfa (Medicago sativa L.)-orchardgrass (Dactylis glomerata L.) hays from three individual harvests were obtained from the same 8.2-ha research site near Stratford, WI. Both options for estimating TD-Fiber (TD-FiberLIG or TD-FiberNDFD) were then utilized independently via the summative approach to estimate the total TDN concentrations (TDN-LIG or TDN-NDFD, respectively) within these hays. Estimates of both TDN-LIG and TDN-NDFD then were related to heating degree days > 30oC accumulated during storage by various regression techniques. Changes (poststorage – prestorage) in TDN-LIG that occurred during storage ('TDN-LIG) were best fitted with a nonlinear decay model in which the independent variable was squared [Y = (11.7 * e-0.0000033*x*x) – 11.6; R2 = 0.928]. For changes in TDN-NDFD ('TDN-NDFD), a quadratic regression model provided the best fit (Y = 0.0000027x2 – 0.010x + 0.4; R2 = 0.861). Generally, 'TDN-LIG estimates were 2.0 to 4.0 percentage units lower than 'TDN-NDFD estimates when heating exceeded 500 HDD. For regressions on maximum internal bale temperature, both 'TDN-LIG (Y = -0.38x + 16.3; r2 = 0.954) and 'TDN-NDFD (Y = -0.25x + 10.2; r2 = 0.848) were best fitted by linear models with heterogeneous (P < 0.001) slopes and intercepts. In both cases, coefficients of determination were high, suggesting that simple measures of spontaneous heating are excellent predictors of energy losses in heated forages. Regardless of method, reductions in TDN were associated primarily with losses of non-fiber carbohydrate, which is known to occur via oxidation of sugars during spontaneous heating. For heated forages, some discrepancy between TDN-LIG and TDN-NDFD existed because the relationship between NDFD and spontaneous heating was shown previously to be very poor, resulting in minimal changes for estimates of TD-FiberNDFD as a consequence of heating. Discrepancies between TDN-LIG and TDN-NDFD were exacerbated further when NDF rather than NDFn was used to estimate TD-FiberNDFD. In contrast, TD-FiberLIG declined in close association with heating, largely because TD-FiberLIG was sensitive to changes in concentrations of both NDFn and acid-detergent lignin. Estimates of TDN declined by as much as 13.0 percentage units within severely heated hays, and this is a serious consequence of spontaneous heating.