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

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improve recovery of lignin particles and the development of forage family and speciespecific equations for the estimation of the iNDF. The original AOAC lignin procedure for crucibles relied on the use of asbestos as a filtering agent, but the asbestos was rendered a health hazard and removed and another filtering agent was never instituted, thus the variation in the lignin assay is partially a function of the filtering step. Data in Figure 1 shows examples of the lignin/NDF relationships for conventional and bmr corn, grasses and alfalfas. The two types of corn silages and the grasses present differing ratios, while the alfalfas are characterized by a less variable ratio (avg=2.8), with later (4 th and 5 th ) cuts having a different behavior and larger values. More data are needed to allow specific equations to be used for groups of forages. The improved recoveries alter the previous lignin to NDF relationships primarily because of the increased recoveries of both lignin and fermented NDF and this does not invalidate the previous ratio of 2.4 but enhances our understanding to be more forage specific. More accurate and precise iNDF values will allow for better estimations of rates of NDF digestibility to be used in CNCPS. Table 2. Average increases in recoveries of ADL for different classes of forages using small pore size (1.5 µ) filter papers in Gooch crucibles. Class of forage % Increase in recovery Conv. corn silage 25 Bmr corn silage 36 Alfalfa 3 Grasses 9 Early cut grasses 28 NDFd, kd and Chemical Structure of Lignin Bonds Finally, when comparing different plants, the negative correlations observed between the extent of digestion and lignin content are relatively good for the same variety among stages of maturity; however, they are weaker within a family and poor when data from plant families are pooled (ie grasses and legumes). The action of lignins seems to depend not only on their amount but also on other factors like cross linking and because of the chemical nature of this heterogeneous compound, it is nearly impossible to extract lignin in any pure form – especially once it polymerizes into ADL. Lignin action on digestibility depends therefore not only on its content but its qualitative structural aspects. In general lignin is associated with carbohydrate (in particular with hemicellulose) through covalent bonds, hydroxycinnamic acids are attached to lignin and hemicellulose through ester and ether bonds as bridges between them forming lignin/phenolic-carbohydrate complexes. It is generally accepted that the association of phenolic components with carbohydrates presents the greatest barrier to their utilization. It is well established that hydroxycinnamic acids attach to arabinoxylans through ester bonds. A simple dimerization of these esters, even if there is no further attachment to lignin, may be enough to substantially reduce the biodegradability of digestible cell walls. Any further association (ether bonds) between this
hydroxycinnamic moiety and the lignin polymer that forms a bridge between a carbohydrate and lignin would cause a severe reduction in carbohydrates availability. These acids are mainly p-coumaric and ferulic acids. Because of their dual function (hydroxyl and carboxyl) these acids may form bridges between lignins and hemicelluloses. Figure 1. The ratio of lignin as a function of NDF (lignin * y)/NDF to describe the forage specific iNDF pool size (back-calculated from the residues of the NDF fermentations) regressed on the value of ADL/NDF for conventional and bmr corn silages, grasses and alfalfas. We are aware of the large genetic variability among cell types and within and especially among forage species and families because of the different speed of cell wall change and reproductive maturity. However our objective was to try to integrate recent findings relative to phenolic acids and nutritive value, focusing only on phenotypic correlations. Also, the possible correlations among cell wall components may prevent any type of cause and effect to be determined from these analysis but might lead only to a better prediction of fiber digestibility. We wanted to determine if the presence of measurable ester and ether linkages impact rates as well as extents of NDF digestibility and if these relationships were similar among forages. Thirty forages including conventional and bmr corn silages, alfalfas, immature and mature grasses were incubated in-vitro for 24hr and 96hr NDFD. Digested residues were analyzed for NDF, ADF and ADL and ester and ether linked para-coumaric acid (PCA) and ferulic acid (FA) were determined in those fractions. Three of the corn silages were chosen based on variable digestibility and fed to 6 fistulated cows for 3 wks in three iso-NDF diets. Diet, rumen and feces samples were taken every 3 hr for three days, 10d from the start of the study. Intact samples, NDF and ADF residues were analyzed for ester and ether linked PCA and FA. Extraction of phenolic acids was by 2N and 4N NaOH treatment, followed by HPLC equipped with a diode array detector using
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aNDF, NDFd, iNDF, ADL and kd: What have we learned?

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