Comparison of acetyl bromide lignin with acid detergent lignin and Klason lignin and correlation with in vitro forage degradability

The spectroscopic acetyl bromide lignin (ABL) and two versions of the gravimetric sulfuric acid lignin method [Lignin (sa)] were compared for their correlations with in vitro forage dry matter (IVDMD) and neutral detergent fiber degradability (IVNDFD) assays of 73 grass and legume samples. The two versions of Lignin (sa) were the acid detergent lignin (ADL) and Klason lignin (KL) methods. ABL and KL methods employ crude cell wall as the fibrous preparation while ADL uses acid detergent fiber. In vitro forage measures of degradability were negatively correlated with almost all lignin values, but the ABL method had the highest correlation coefficients for both grasses and legumes. Regression curves of ADL values with grass and legume IVDMD and IVNDF revealed different slopes, with steeper curves for grasses. With KL, grass and legume IVDMD and IVNDFD slopes were statistically similar and parallel. The results observed in the ADL and KL methods testify to the distance between the lines and data points, that is, these methods are poor predictors of NDF degradability. Grass and legume samples assayed with the ABL procedure, exhibited similar slopes, with parallel lines for both IVDMD and IVNDFD assays. Steeper inclination of curve for grasses relative to legumes in the ADL method may be attributed to partial loss of lignin during the procedure. This shows that ABL can accurately and consistently measure the inhibitory effect of lignin on degradation of structural carbohydrates of both grasses and legumes. The ABL method provides an accurate, easy and fast procedure for quantifying lignin in plant materials. A step-by-step laboratory procedure of the ABL method is described.