Nutritional value of most plant materials is limited by the presence of numerous naturally occurring compounds which interfere with nutrient digestion and absorption. Although processing is employed widely in removal of these factors, selection of cultivars of soya beans with inherently low levels would have a considerable impact on efficiency of non-ruminant livestock production. The review considers the role of plant breeding in achieving this objective. The most abundant trypsin inhibitors are the Kunitz and the Bowman Birk inhibitors, containing 181 and 71 amino acids respectively. The Kunitz inhibitor is present at a concentration of 1.4g/kg of total seed contents and the Bowman Birk inhibitor 1.6g/kg. A large number of isoforms of the Bowman Birk inhibitor have been described in soya bean cultivars and it has been shown that the general properties of the inhibitor are, in fact, attributable to different isoforms. Nulls for both Bowman-Birk and Kunitz trypsin inhibitors have been identified, allowing new low trypsin inhibitor cultivars to be produced. However, research into breeding for low trypsin inhibitor cultivars currently has limited application as trypsin inhibitors contribute a major proportion of the methionine content of soya beans. Trypsin inhibitors are thought to be involved in the regulation of and protection against unwanted proteolysis in plant tissues and also act as a defense mechanism against attack from diseases, insects and animals. Hence, in breeding programes for low trypsin inhibitor cultivars, alternative protection for growing plants must be considered. Use of soya beans in non-ruminant animal feeds is limited by the flatulence associated with their consumption
