An accurate method for estimation of lignin concentration is important for prediction of the digestible energy content of livestock feeds. The accuracy of lignin concentration estimates based on the Klason lignin and acid detergent lignin methods was compared. Ten diverse forage samples were analyzed for protein, carbohydrates, lipids, organic acids, ash, lignin (by both methods), and gross energy. The accuracy of the two lignin concentration estimates was examined by comparing the measured forage gross energy to a gross energy value calculated from the compositional analysis. Use of the acid detergent lignin estimate in this gross energy calculation accounted for 68-84% of the forage gross energy compared to 85-97% of the gross energy using Klason lignin. These results indicate that while Klason lignin estimates are substantially higher than acid detergent lignin estimates, Klason lignin is the more accurate lignin method and does not overestimate lignin because gross energy recoveries were less than 100%.