Under-use may take the form of (i) dumping or burning products which still have value as fuel or as a source of plant nutrients; (ii) returning them to the land when they could be used for animal feeding; (iii) feeding them to animals when they could be eaten by the human consumer. A major under-use in the first category in the United Kingdom is that of cereal straw where it has been estimated that about 20% only of the 9 million tonnes produced is used for feeding. Apart from technical problems there are powerful economic reasons for this under-use. In the second category, animal excreta include energy, nitrogen and minerals, some of which may be further used directly or indirectly for the feeding of other animals usually of a different species. So far the main effort has been in the use of poultry manure as a source of nitrogen for ruminants. Since a high proportion of the diet of ruminants is obtained from grass and forage which contribute little directly to the human dietary, there is a stronger technical case for the use of cereals for very young ruminants and for cattle in late pregnancy and early lactation than is the case for non-ruminants where the diet consists mainly of cereals throughout the growing period.