Bioavailability of tryptophan in soybean meal for 10-kg pigs using slope-ratio assay

A basal diet formulated to contain 1.1 (analyzed to contain 1.03) g of tryptophan/kg diet as-fed, but adequate in all other amino acids, was supplemented with .2, .4, or .6 g tryptophan/kg from L-tryptophan or .2 or .4 g tryptophan/kg from soybean meal. Each of the six diets was fed for 28 d to six barrows and six gilts (crossbred; Hampshire x Duroc sires by Yorkshire x Landrace dams) in individual pens and with an average initial weight of 9.8 kg. Dietary supplemental tryptophan from either L-tryptophan or soybean meal increased (P < .05) daily weight gain, feed intake, and gain:feed ratio. Plasma concentrations of tryptophan increased and urea decreased with increasing dietary L-tryptophan supplementation (P < .05). Common-intercept, multiple linear regression in a slope-ratio methodology were performed using nonpartitioned and partitioned daily weight gain, gain:feed ratio, or plasma tryptophan: urea ratio as a dependent variable and supplemental tryptophan level (g/kg), daily total tryptophan intake (g/d), or daily supplemental tryptophan intake (g/d) as independent variables. For each of the three dependent variables, estimates of tryptophan bioavailability were > 100% when daily supplemental tryptophan intake was used as the independent variable. Estimates of TRP bioavailability based on nonpartitioned 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.