Klason lignin is a nutritionally heterogeneous fraction unsuitable for prediction of forage neutral detergent fibre digestibility in ruminants.

: Although lignin has been negatively correlated with neutral detergent fibre (NDF) digestibility (NDFD) in ruminants and used to predict potential extent of NDF digestion of forages, selection of an analysis, Klason lignin (KL) or acid detergent lignin (ADL), to describe the nutritionally relevant lignin has not been resolved. Dismissed as an artifact is the difference between KL and ADL (∆L). A question is whether ∆L influences digestibility of NDF. We evaluated the relationships of ∆L, KL, and ADL with NDFD in order to determine the nutritionally homogeneous or heterogeneous nature of KL. Data sets from 2 laboratories (DS1, DS2) were used that included ADL, KL, and in vitro NDFD at 48 h (NDFD48). DS1 contained 7 C3 grasses, 17 C4 maize forages and 19 alfalfas, and DS2 had 15 C3 grasses, 8 C4 forages, and 6 alfalfas. Mean ∆L was greater than ADL in C3 and C4 samples, and less in alfalfas. Within forage type and laboratory, ∆L was not correlated with NDFD48 (r = -0.34 to 0.49; all P>0.17). ADL was more consistently correlated with NDFD48 (r = -0.47 to -0.95; P <0.01-0.21) than was KL (r = 0.03 to -0.91; P <0.01-0.94). ∆L as a proportion of KL was correlated with NDFD48 in C3 and C4 samples (r = 0.44 to 0.76; P <0.01-0.08). The differing behaviors of ∆L and ADL relative to NDFD48 indicate that KL is a nutritionally heterogeneous fraction, the behavior of which may vary by forage type and ratios of ADL and ∆L present.