APPARENT INHIBITION TO DIGESTION BY LIGNIN IN NORMAL AND BROWN MIDRIB STEMS

Lignin, a cell wall component, limits digestibility of plant cell walls. Brown midrib (bmr) mutants of forages have lignin with altered chemical composition compared with their normal counterparts. The objectives of this study were to determine if bmr lignin is more inhibitory to digestion than is normal lignin and if bmr has a consistent effect on rate of digestion across species and environments. Extent and rate of in-vitro cell wall digestion of normal and bmr stems of sorghum (Sorghum bicolor (L) Moench, two comparisons), millet (Pennisetum americanum (L) Leeke) and maize (Zea mays L, two comparisons) were determined. Samples were incubated in rumen fluid, and data were fitted with a first-order, nonlinear model to estimate concentrations of potentially digestible neutral detergent fibre (PDNDF), digestion rate of PDNDF, concentration of indigestible residue (IR), and lag time before digestion. The NDF, acid-detergent fibre (ADF), and acid-detergent lignin (ADL) analyses were conducted sequentially on undigested samples. The IR:ADL ratio was 37 % greater for bmr than for normal plants, which indicates that bmr lignin inhibits digestion more than normal lignin per unit of lignin. Digestion rate of PDNDF was raster in bmr than in normal counterparts in one of the two sorghum comparisons (difference of 59 %) and in the millet comparison (difference of 27 %), but in neither maize comparison. The bmr mutants were lower than normal genotypes in NDF (9 %) and ADL (47 %) concentrations. The PDNDF concentration was 19 % greater for bmr than for normal lines. Thus, decreased lignin concentration in bmr mutants increased the extent of NDF digestion but did not consistently increase the rate of digestion.