Two experiments were conducted to investigate whether the choice of digestibility marker or marker concentration in corn-soybean meal diets influence apparent ileal AA digestibility (AIAAD) or the potential phytase-induced improvement in AIAAD in broiler chickens and pigs. One hundred ninety-two, 42-d-old, Ross 708 broilers were used in a 7-d study in Exp 1. The birds were allocated to 6 dietary treatments in a 2 × 3 factorial arrangement of treatments in a split-plot design. The factors were a combination of chromic oxide and titanium dioxide (0.3% or 0.5% of both markers, as-fed basis), and 3 levels of phytase inclusion [0, 500, or 1,000 phytase units (FTU)/kg]. In Exp. 2, 6 barrows fitted with a simple T-cannula at the distal ileum were allocated to 4 diets in a 6 × 4 Youden square design and 2 × 2 factorial arrangement of treatments. The factors were similar to Exp. 1, except the 500-FTU/kg phytase level was not used in Exp. 2. There were no marker type × marker concentration, phytase × marker type, or phytase × marker type × marker concentration interactions for any of the AA in either experiment. On average, AIAAD values calculated using Ti was greater (P < 0.05) than those calculated using Cr, regardless of the phytase inclusion level in both experiments. In Exp. 1, AIAAD values for His, Trp, Cys, and Pro were greater (P < 0.05) at the 0.3% than 0.5% marker concentration. The AIAAD values were consistently greater when calculated using Ti compared with Cr, irrespective of phytase level. It is concluded that the type of marker used does not influence whether a response to phytase supplementation, in terms of AIAAD, is observed.
