The spectroscopic acetyl bromide lignin (ABL) and the gravimetric acid detergent lignin (ADL) methods were compared with in vitro forage dry matter degradability (IVDMD) and neutral detergent fiber degradability (IVNDFD) assays of 73 grass and legume samples, and a conjecture was made with the lignin component of the Cornell Net Carbohydrate and Protein System (CNCPS) equations. The slopes and intercepts of regressions were declared different when there was an interaction effect among forages within each lignin method (MIXED procedure of SAS). Regression curves of ADL values with forage IVDMD (grass: y = −7.929x + 901.8; legume: y = −3.663x + 853.6) and IVNDFD (grass: y = −3.289x + 916.0; legume: y = −1.051x + 697.8) revealed different slopes, with steeper curves for grasses. Grass and legume samples assayed with the ABL procedure, exhibited similar slopes, with parallel lines for both IVDMD (grass: y = −3.847x + 886.7; legume: y = −3.638x + 789.8) and IVNDFD (grass: y = −3.636x + 1117.7; legume: y = −3.454x + 889.9) assays. Steeper inclination of curve for grasses relative to legumes in the ADL method has been attributed to grass lignin being more inhibitory to degradation than legume lignin. Similar and parallel curves of ABL method suggests that grass lignin is no more inhibitory than legume lignin. However, the steeper inclination may be attributed to partial loss of grass lignin during the ADL procedure. We hypothesize that this loss is around 2.4, that is, the residual ADL multiplied by 2.4 would yield the actual lignin content. This number is the same used in the Cornell equations to estimate the B2 and C carbohydrate fractions (NDF x Lignin x 2.4). When we multiplied the grass ADL values by 2.4, forage IVDMD regressions were: (grass: y = −3.690x + 934.7; legume: y = −3.663x + 853.6), which originated parallel lines and were strikingly similar to the ones obtained with the ABL method. After correcting IVNDFD, the regressions were: (grass: y = −1.744x + 962.9; legume: y = −1.737x + 805.8), also yielding parallel curves. At this moment we can speculate that grass and legume lignins have the same effect on cell wall degradation and that ABL method seems a promising procedure for lignin quantification.