Alkali effects on agricultural wastes and their cell wall fraction

Abstract

Five experiments are reported which examine the effect of alkalis (NaOH, 0-1 4 g/100 g DM ; NH,OH, 0-2.9 g/l00 g DM), reaction time and temperature upon dry matter (DM) digestibility and related properties of mature forage waste materials. Digestion was assessed by microdigestion of test samples in the rumens of fistulated sheep fed lucerne base diets. Digestion of the DM of brewer's grains was increased from 50.2 per cent to 58.5 per cent by application of 2.9 per cent NaOH or NH4OH at room temperature for 24 hours, and to 81.9 per cent when heated to 80¦C for 24 hours. Heating at 150¦C for 1 hour produced no improvement over heating at 80¦C for 24 hours. Average DM digestion of a range of mature grasses was improved by 6.9 per cent by applying 1.5 g NH4OH/100 g DM at room temperature for 17 hours and by 13.7 per cent when heated to 80¦C for the same period. The DM digestion of mature legumes did not increase due to these treatments, but when heated with 1.5 g NH4OH/100 g DM to 130¦C for 4 hours increased DM digestibility by 16.6 per cent. The digestion of unashed cell wall (CW) and acid detergent (AD) residues in mature grasses was increased by a prior extraction with neutral detergent and acid detergent. In mature legumes these pretreatments had no effect upon digestibility. The unashed (KmnO4 extracted) cellulose residues of grasses and legumes were incompletely digested compared with pure cotton. Both NaOH and the chelating agent EDTA eventually remove the CW-ash fraction and this is associated with increases in CW digestion with increasing level of their application. These chemicals also initially increase and then decrease the organic fraction of the mature forage CW, its ADF and lignin. The digestibility of cell walls of lucerne hay is increased from 24.3 per cent to 43.3 per cent by application of NaOH (9 g /100 g DM) for 2 hours at room temperatures; the corresponding increase for cell walls of wheaten chaff is from 33.9 per cent to 72.3 per cent. It is shown that alkalis induce increases in DM digestion by degrading cell walls and that the cell walls of mature grasses are more easily degraded than are the cell walls of mature legumes.