Introduction

Over 200 non-protein amino acids occur naturally in plants (Rosenthal, 1982). With a few exceptions, these amino acids exist in unconjugated forms and many are associated with toxic properties (D'Mello, 1991). Legumes contain higher concentrations and a more diverse range of non-protein amino acids than any other plant species, and the seed is generally the most concentrated source of these substances.

The toxic non-protein amino acids are a distinguishing feature of many tropical legumes (D'Mello, 1992), contributing significantly to the noxious effects of a number of grain and forage legumes, including Canavalia ensiformis, Indigofera spicata, Leucaena leucocephala and at least three Lathyrus species. However, there is evidence of considerable variation in the concentrations of non-protein amino acids in different species of the same genus (Aylward et al, 1987). Toxic non-protein amino acids also occur in non-leguminous plants, both tropical (e.g. Blighia sapida; Cycas circinalis) and temperate (e.g. Brassica species).

The toxicity of non-protein amino acids has been observed in insects, laboratory and farm animals and in humans, but there are striking differences among animal species in their sensitivity to these compounds. Factors such as diet, duration of feeding and geographical differences in microbial ecology in ruminants may also modulate the response to these amino acids.

The role of the non-protein amino acids in plants remains a matter of some debate. However, there is mounting evidence that these compounds form part of the chemical defence system against predation and disease (Rosenthal & Bell, 1979).