ACID DETERGENT DISPERSIBLE LIGNIN IN TROPICAL GRASSES

The detergent system of analysis partitions lignin into two fractions: acid detergent lignin (ADL), usually equated with forage lignin, and 'acid detergent dispersible lignin' (ADDL), the existence of which has scarcely been recognised, but can be higher than ADL in tropical gras ses. For nine species ADL and ADDL as percentage of original dry matte r were, respectively: Aristida calcyna, 8.9 and 9.8; Astrebla squarros a, 8.6 and 14.7; Chloris gayana, 3.9 and 6.7; Eragrostis sp, 4.3 and 8 .8; Heteropogon contortus, 3.9 and 10.4; Iseilema membranaceum, 7.2 an d 8.2; Panicum maximum, 1.8 and 5.7; Themeda triandra, 7.3 and 8.2; So rghum sp, 3.4 and 13.8. Tropical legumes (four species) had much lower levels of ADDL. Ate-stained fibre showed that most of the free phenol ic functions and the alkali-soluble lignin were in the ADDL fraction. The difference between grasses and legumes was so marked that ate-stai ning may provide a method for distinguishing grass and legume particle s in feed or faecal samples. The apparent discrepancies between grasse s and legumes in the effect of lignin on digestibility may be explaine d by the grass lignin having been grossly underestimated as ADL. When Heteropogon contortus and Astrebla squarrosa were incubated in dacron bags in the rumen the ratio of ADDL to ADL in the residual fibre decre ased markedly in the first 48 h, indicating a higher apparent digestib ility for ADDL. From feeding experiments with sheep the apparent diges tibility of ADL and ADDL were 18 and 38% for Astrebla squarrosa and 14 and 26% for Stylosanthes hamata. As isolated lignin corresponding to ADDL was inhibitory in vitro to a mixed rumen microbial population at a concentration of 0.12 mg ml(-1). Measurement of ADDL could easily be included in fibre analysis by the sequential detergent method as it w ould require only UV absorbance measurement on the acid detergent filt rate. This would allow measurement of total lignin within the existing system.