Limiting factors in plant residues of low biodegradability

The resistances of low quality plant residues to anaerobic digestion are related to the high content of total structural matter and its intrinsic physicochemical characteristics. Natural factors include lignin, tannins and biogenic silica, while processing and storing further reduce quality through non-enzymatic browning reactions, and changes in particle size and physical surface properties. Delignified materials may exhibit greater lag and lower rates of digestion. Exchange capacity is much reduced in treated materials. Individually plant residues vary in the nature of their principal limiting factors. Silica is the primary factor in rice straw and lignin in most other crop residues. The effect of lignin depends on covalent linkage with structural carbohydrate and lignin obligately protects 1.4 times its own weight of carbohydrate in untreated materials. Lignin cleaved by alkali treatment is soluble in neutral detergent as evidence by ultraviolet absorption of neutral detergent extracts. It may lower digestibility of solubles through dilution and tanning reactions. The efficiency of alkali treatment of graminaceous straws can be assayed by the measurement of saponifiable groups within the residual structural matter.