Journal of the Science of Food and Agriculture J Sci Food Agric 86:2603–2608 (2006) Ileal amino acid digestibility of some novel dietary protein sources for growing chickens Velmurugu Ravindran ∗ and Patrick CH Morel Institute for Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand Abstract: The apparent ileal digestibility coefficients of amino acids in five high-protein ingredients (casein, isolated soyprotein, maize gluten meal, spray-dried bovine plasma (SBP) and spray-dried porcine plasma (SPP)) and soybean meal were determined using 3-week-old broiler chickens. The amino acids in casein were highly digestible. Isolated soyprotein was also well digested by the birds, but the digestibility coefficients of all amino acids in isolated soyprotein were lower (P < 0.05) than those in casein. The amino acid digestibility coefficients in isolated soyprotein were greater (P < 0.05) than those in soybean meal, which may be reflective of reduced concentrations of anti-nutritive factors, particularly indigestible oligosaccharides. The digestibilities of amino acids in maize gluten meal were higher (P < 0.05) than those in soybean meal, except for the digestibilities of arginine, histidine, threonine and aspartic acid, which were similar (P > 0.05) between the two protein sources. The ileal digestibilities of all indispensable amino acids in SBP and SPP were lower (P < 0.05) than those in casein and isolated soybean meal but higher (P < 0.05) than those in soybean meal. No differences (P > 0.05) were observed in the digestibility of amino acids between SBP and SPP. The overall mean amino acid digestibility coefficients of casein, isolated soyprotein, soybean meal, maize gluten meal, SBP and SPP were 0.955, 0.888, 0.823, 0.849, 0.860 and 0.856 respectively.  2006 Society of Chemical Industry Keywords: amino acids; broilers; protein sources; ileal digestibility INTRODUCTION Amino acid digestibility is not the same for all feed ingredients. Differences in digestibility can be effectively used as a tool to improve the precision of feed formulations. This benefit is recognised by the broiler industry, and digestible amino acid values are increasingly being used to formulate commercial diets. 1 Most of the available published data on amino acid digestibility coefficients in feed ingredients for poultry 2–7 have been determined in the excreta of adult roosters using the precision feeding assay of Sibbald 8 or modifications of this assay. Digestibility values based on excreta analysis have the advantage of simplicity over values based on ileal digesta analysis, because the birds need not be sacrificed and assays can be carried out on large numbers of birds. However, the variable and modifying effects of caecal microflora on protein digestion have now been recognised 9 and it is generally agreed that the analysis of ileal digesta rather than excreta is the preferred method for assessing amino acid digestibility in poultry. 1,10 While there is a wealth of published data, including several compilations based on large numbers of samples, on the excreta-based amino acid digestibility of feed ingredients, corresponding information at the ileal level for growing broiler chickens remains limited. 11 – 14 In particular, digestibility data for less commonly used, novel feed ingredients are scant. In the present study the apparent ileal amino acid digestibility coefficients of five high-protein ingredients and soybean meal for growing broiler chickens were determined. MATERIALS AND METHODS Ingredients and diet composition Five high-protein ingredients, namely casein, isolated soyprotein, maize gluten meal, spray-dried bovine plasma (SBP) and spray-dried porcine plasma (SPP), were assayed. Soybean meal, the meal remaining after the extraction of oil from soybeans, is the preferred source of high-quality plant protein in poultry feed manufacture worldwide. For this reason a sample of soybean meal was also assayed for comparison purposes. Assay diets were based on dextrose and contained the test feed ingredient as the only source of protein (Table 1). The proportions of dextrose and test ingredient were varied in each diet to obtain 200 g kg −1 crude protein. Oil was added to all diets except the soybean meal diet to improve their texture and palatability. Sodium bicarbonate was also included in all diets except the soybean meal diet, as previous studies in our laboratory had shown that this addition was necessary to maintain feed intake of diets containing purified proteins as the only source of protein. In previous studies, intake of assay diets based ∗ Correspondence to: Velmurugu Ravindran, Institute for Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand E-mail: v.ravindran@massey.ac.nz (Received 22 June 2005; revised version received 22 October 2005; accepted 14 July 2006) Published online 2 October 2006; DOI: 10.1002/jsfa.2657  2006 Society of Chemical Industry. J Sci Food Agric 0022–5142/2006/$30.00