Nutritional quality of grain of Australian cultivars of lablab bead (Lablab purpureus)

Abstract

Nutritional studies using rats have shown that the grains of Rongai and Highworth, the current commercial cultivars of lablab bean in Australia, and of CPI41222, a white-seeded accession, have low nutritive value. Diets containing raw beans of the three accessions at levels up to 12% crude protein led to severe weight losses of up to 0.89 g/day compared with weight gains of up to 2.8 g/day with autoclaved soybean meal. Autoclaving for 20 min or boiling the grain improved growth rates only marginally. Increasing the period of autoclaving reduced the feeding value; extraction of the grain with saline solution or treatment with formaldehyde failed to improve it. These results are consistent with previous reports of the presence in lablab beans of a heat-labile phytohaemagglutinin. Blood haemoglobin levels were signifycantly depressed on diets which contained Highworth beans. The proteins of all accessions contained low levels of the sulfur-containing amino acids; a deficiency of methionine was confirmed by feeding trials. The level of histidine in the lablab protein was about 50% higher than in soybean and, in supplementation trials, additions of histidine reduced growth rates. In two feeding trials, there were good correlations (R2=0.85 and 0.92) between weight gains and digestible dry matter intake, indicating that the differences in growth rates were primarily the result of differences in intake. Intakes of digestible dry matter and digestible crude protein and the digestibilities of dry matter and crude protein were all significantly lower with Rongai than with the other two accessions. These differences do not appear to be associated with the quality of the protein as shown by amino acid analyses or by the responses to supplementation with essential amino acids. They also do not appear to be associated with seed coat colour, seed size or crude protein content of the grain.