Lignin concentration and its correlation with degradability of tropical grasses

Abstract Text: Lignin inhibits the degradation of structural carbohydrates in the plant cell wall, thus, a precise and accurate method to determine lignin concentration is desirable. The spectroscopic acetyl bromide lignin (ABL) method has recently been receiving more attention by researchers and in this study it was compared to ADL, KL and permanganate lignin (PL). Five species of grasses, Brachiaria brizantha cv. Marandú, Brachiaria brizantha cv. Xaraés, Panicum maximum cv. Mombaça, Pennisetum purpureum cv. Cameroon and Pennisetum purpureum cv. Napier, harvested at seven maturity stages were used. Three fibrous preparations: NDF, ADF and cell wall (CW) were used to determine lignin concentrations. Protein (N x 6.25) and ash content were determined in the NDF and CW residues. A completely randomized experimental design with duplicate analysis for the lignin assays was used. A randomized block design was used for the in vitro experiment, with rumen fluid blocked by week. Individual treatments were compared by Tukey´s test (P<0.05). Correlation coefficients between lignin methods and in vitro digestibility values were obtained using PROC CORR from SAS. The mean CW values were higher (P < 0.05) than NDF values, 768.1 g/kg versus 713 g/kg, reflecting solubilization of pectin and other neutral detergent soluble cell wall oligosaccharides. The ADL method yielded the lowest mean values (P < 0.05) of all methods, 95 g/kg versus 107.6, 116.2 and 199.1 g/kg for KL, PerL and ABL, respectively, which may indicate partial lignin solubilization by the acid detergent solution and/or by the 72% sulfuric acid solution. Results obtained by PerL were higher (P < 0.05) than those of ADL, possibly due to hemicellulose and pectin oxidation by potassium permanganate. The values for KL were higher (P < 0.05) than those of ADL, possibly due to protein contamination. The highest concentrations were obtained by the ABL method. In vitro dry matter degradability showed high negative correlation with lignin content, -0.8505, -0.9130 and -0.8883 when determined by ADL, PerL and ABL, respectively. A proposed correction factor (2.23) applied to the ADL values resulted in a degradability curve similar to the ABL curve. It is interesting to note that this value of 2.23 is very close to the 2.4 value used in the Cornell Net Carbohydrate and Protein System equations B₂and C, to estimate carbohydrate fractions, when adjusting lignin content. The ABL method is an easy, fast and convenient method to determine lignin content in forages.

Keywords: Lignin, ABL, Digestibility