aNDF, NDFd, iNDF, ADL and kd: What have we learned?

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indicating where changes will most likely occur in the near future when estimating rates of digestion. This involves refinements in methodology and a more complete understanding of the extent of digestion. Sample Analyses Conventional and bmr corn silages, perennial ryegrasses, wheat straw, and alfalfa hays were analyzed for fiber fractions using the procedure of Van Soest et al. (1991). Sulfuric lignin was also used as the standard lignin procedure (Van Soest and Robertson, 1980). A small preliminary study indicated that fermentations in-situ for 16 days may not be enough to reach the maximum extent of digestion and this is still being investigated. Extended fermentations were also carried using the ANKOM in-vitro apparatus (DAISY III; ANKOM Technology Corp., Fairport, NY), because of the ease of re-inoculating the samples. This latter system allowed a better estimation of the intrinsic iNDF of the forage after 16 days. Fermentations were then carried for 16 days using the DAISY system and bags of polyester polyethylene terephthalate with 15 μm porosity and 8.5% open area (ANKOM Technology Corp., Fairport, NY). Jars were re-inoculated with medium every 4 days. Fermentations in-vitro were carried out for 6, 12, 24, 30, 36, 48, 72, 96, 120, 144, 168, 192 and 216 hours, in 125 ml Erlenmeyer flasks in a 39 °C water bath under constant CO2 in Goering and Van Soest buffer (1970), and with renewed medium after 96 hours. In-vitro flasks were inoculated with rumen fluid from the same cows used for the DAISY fermentations, fed hay and grain. Blank samples were run for both fermentations and used to correct for any contamination at each fermentation time. All samples at the end of the fermentations were analyzed for NDF. Further Evaluations of Rate Estimations for NDF Digestion – Comparison to a Non- Linear Model for a First Order Approach The earliest attempts to describe the kinetics of digestion have been reviewed by Mertens (1993a, 1993b). The term ―rate of digestion‖ appeared in the 1950s, but the assessments were mainly based on the visual interpretation of digestion curves. Waldo (1970) suggested that if the indegistible residue was subtracted, digestion of potentially digestible cell walls might follow first order kinetics. Two primary methods are used for fitting data to the first order kinetic models: linear regression on logarithmic transformations of undigested residues (ln-linear) and nonlinear estimation of parameters using multiple time points. The regression between the natural logarithm of the potentially digestible NDF (pdNDF) against time results in supporting the hypothesis that pdNDF follows the first order digestion kinetics. In this case, it is critical the estimation or measurement of the indigestible fraction (iNDF) is accurate in order to accurately describe the digestion kinetics. A further problem in describing NDF digestion and kinetics is that bias can also be associated with early measurement of digestion. Usually, digestion of the pdNDF fraction does not appear to start instantaneously at time zero. Instead there is a lag period during which digestion occurs slowly or not at all (Mertens, 1993a, 1993b) while the fermentation stabilizes and bacterial attachment and enzyme synthesis has
occurred. As a result the use of early fermentation points with the logarithmic transformation and linear regression may result in a biased estimate that is lower than the true rate. Problems associated with the logarithmic transformation-linear regression can be overcome by estimating kinetic parameters using non-linear least squares regression procedures (Mertens and Loften, 1980; Van Milgen et al., 1991; Ellis et al., 2005) but this would require a laboratory to run multiple timepoints. Nonlinear models assume an equal error at each fermentation time, whereas the ln-linear models assume that error is proportional to the size of residue at each time point. And since random errors are typically the largest for early and medium (8-48 hours) incubation times, neither of the approaches seems satisfactory since there is error associated with both. Therefore the only apparent discrepancy with the ln-linear method is during lag when fluxes and variation are low, but residue weights are high. Thus, it does not seem that the multiplicative error distribution associated with logarithmic transformation is a significant problem during parameter estimation. To evaluate the associated errors between the two approaches, values from in-vitro fermentations were used to obtain simultaneous estimations of rates of digestion (kdv), lag times (Lv), pdNDF (pdNDFv) and iNDF (iNDFv), through a non-linear first order decay model using PROC NLIN of SAS and the Marquardt algorithm. Initial values for the non-linear iterations were obtained using a linear transformation of the mentioned model (Mertens and Loften, 1980; Moore and Cherney, 1986). The model was: (1) NDFt = pdNDFv e –kd (t – L) + iNDFv where NDFt = concentration of residual NDF after t hours of fermentation when t > L and NDFt = pdNDF + iNDF when t < L; pdNDF = concentration of potentially digestible NDF; kd = fractional rate of pdNDF digestion; L = discrete lag time; iNDF = concentration of indigestible NDF. Estimations of rates of digestion with the approach by Van Amburgh et al. (2003) were then possible after calculating the pdNDF as NDF – iNDF. Estimates of the iNDF fraction were then found subtracting the 72% sulfuric acid lignin x 2.4 from NDF, according to Chandler (1980) (iNDF2.4), or using the residue after 216 hours of in-vitro fermentations (iNDF216) or using the residue of the DAISY fermentations (iNDFD). This resulted in respectively kd2.4, kd216 and kdD. Estimations were therefore done either calculating a lag or using an arbitrary fixed lag. The calculation of the lag was accomplished using 6 and 24 hours as the two time points needed, since a preliminary
Page 1: aNDF, NDFd, iNDF, ADL and kd: What
Page 5 and 6: The variation associated with using
Page 7 and 8: Lignin is generally accepted as the
Page 9 and 10: hydroxycinnamic moiety and the lign
Page 11 and 12: significance and with R 2 that vari
Page 13 and 14: Figure 5: Relationship between 96hr
Page 15 and 16: Take Home Messages � Description
Page 17: Van Milgen, J., M. R. Murphy and L.

aNDF, NDFd, iNDF, ADL and kd: What have we learned?

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