Lignin is a polymer that inhibits degradability of forage cell wall carbohydrates by herbivores and limits energy utilization. To understand the mechanisms involved in the inhibition, lignin must be chemically determined with acceptable precision and accuracy. However, there is not a consensus on the most suitable analytical method for lignin quantification. The UV acetyl bromide lignin (ABL) method may be an alternative. In this method, lignin is solubilized in a 25% acetyl bromide/acetic acid solution and read at 280 nm on a spectrophotometer. The phenolic nuclei of lignin are the responsible for the UV absorbance. A few studies were conducted to compare ABL and the gravimetric sulfuric acid lignin [lignin (sa)] methods with in vitro forage dry matter degradability (IVDMD) and neutral detergent fiber degradability (IVNDFD) assays. This presentation exemplifies a study conducted at University of Missouri where 73 grass and legume samples were used. In this study, the slopes and intercepts of regressions were declared different when there was an interaction effect among forages within each lignin method (SAS Mixed procedure). Regression curves of lignin (sa) 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. On the other hand, 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 lignin (sa) method has been attributed to grass lignin being more inhibitory to degradation than legume lignin. Similar and parallel curves of ABL method suggest 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 lignin (sa) procedure. We hypothesize that this loss is around 2.4, that is, the residual lignin (sa) multiplied by 2.4 would yield the actual grass lignin content. When we multiplied the grass lignin (sa) 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, but the magnitude of slopes differed with ABL. These data support our hypothesis and findings reached by other researchers relative to partial lignin solubilization, particularly in grasses, when conducting the lignin (sa) method. We can speculate that grass and legume lignins have about the same effect on the cell wall degradation and that ABL method seems a promising procedure for lignin quantification.