The purpose of the experiment was to estimate the bioavailability of tryptophan (TRP) in soybean meal for young pigs in the live weight range of 10 to 20 kg and demonstrate that the estimates of bioavailability depend on the combination of dependent and independent variables used to statistically analyze the data. Seventy-two, 10-kg pigs (barrows:gilts, 1:1) were used in a slope-ratio experiment to determine the bioavailability of tryptophan in soybean meal. A basal diet formulated to contain 1.1 g of TRP/kg, but adequate in all other amino acids, was supplemented with 0, .2, .4, or .6 g TRP/kg from L-TRP; or .2 or .4 g TRP/kg from soybean meal. Each of the six diets was fed for 28 d to twelve individually-penned pigs (six barrows and six gilts) with an average initial weight of 9.8 kg. Dietary supplemental TRP from either L-TRP or soybean meal increased (P < .05) ADG, ADFI and gain:feed ratio. Plasma concentrations of TRP (increased) and urea (decreased) were affected (P <. 05) by dietary L-TRP supplementation. Common-intercept, multiple linear regressions in slope-ratio methodology were performed using non-partitioned and partitioned ADG, gain:feed ratio, or plasma TRP:urea ratio as dependent variables and supplemental TRP level (g/kg), daily total TRP intake (g/d), or daily supplemental TRP intake (g/d) as independent variables. Estimates of TRP bioavailability based on non-partitioned ADG were 90, 99 and 110% when supplemental TRP level, daily total TRP intake and daily supplemental TRP intake were used as the independent variables, respectively. For a given dependent variable, estimates of TRP bioavailability were higher than 100% when daily supplemental TRP intake was used as the independent variable. For a given dependent variable, estimates of TRP bioavailability ranged from 90 to 110% depending on the independent variable.
Key Words: Tryptophan, Bioavailability, Pigs