Residues are not used as foods because they are inedible without some form of bioconversion. Table 3 shows that the chemical composition of most residues is not well balanced. Straw contains 48 per cent crude fibre and 3 per cent crude protein. It is hardly a good product for human consumption. Grass has a better composition, but if it were to be used for monogastric consumers like man, there would still be severe problems because of the relatively high crude fibre content.
Poor digestibility is another reason for rejecting unprocessed residues.
Table 4 gives the digestibility coefficients of some residues for ruminants. The organic matter of straw is only 38 per cent digestible even for ruminants. For monogastric organisms like man, poultry, and pigs, the coefficients are even lower. Grass is more digestible but less suitable for monogastrics. Digestibility of citrus and animal wastes is reasonable, but not particularly good.