Application of a method to determine ileal digestibility in broilers of amino acids in wheat

Growing male chickens (Ross) were fed diets of increasing wheat content (250, 500 and 750 g/kg) in which wheat was the only protein source. Other dietary ingredients were: oil (50 g/kg), a vitamin and mineral premix (50 g/kg) and a 50 : 50 mixture of pure maize starch and glucose to make diets up to 1000 g/kg. Titanium dioxide was included at 5 g/kg as an inert marker. Four wheat samples were evaluated: varieties B and H each with a low and high crude protein content (102 and 130, respectively ; 96 and 128 g CP/kg at 880 g DM/kg. Experimental diets were fed ad libitum for 3 days from 18 days of age to birds housed in pairs in cages in a fully randomised design comprising 6 replicates per diet. Samples of ileal digesta were obtained from the birds following slaughter at 21 days of age and analysed for titanium and amino acid content. For each amino acid, the amount of apparently digestible amino acid in the diet was regressed against the dietary wheat content and the linear regression thus derived extrapolated to 1000 and 0 g wheat/kg, allowing the estimation of the apparently digestible amino acid content of the wheat sample and the endogenous loss of amino acids, respectively. Dividing the former by the total amino acid content of the wheat allowed calculation of the coefficient of apparent amino acid digestibility (CAD). Addition of the intercept to the amount of apparently digestible amino acid in the wheat sample and dividing this by the total amount of amino acid in the wheat allowed the determination of the coefficient of true digestibility (CTD). Values for methionine for the four wheat samples ranged between 0.749 to 0.842 (CAD) and 0.719 to 0.892 (CTD), and for lysine from 0.531 to 0.697 (CAD) and 0.510 to 0.793 (CTD). Results revealed that an increase in crude protein in the wheat varieties resulted in a significant increase in apparently digestible amino acid content (P < 0.001), for example, the digestible cystine contents of wheat were 1.54 and 2.22 g/kg for wheat H at the low and high crude protein contents, respectively. Coefficients of digestibility also appeared to increase as the crude protein increased.