Bioavailability of threonine and tryptophan in peanut meal for starter pigs using slope-ratio assay

The objective of the two studies was to determine the bioavailability of threonine (thr) and tryptophan (trp) in peanut meal, relative to l-thr and l-trp, for starter pigs using the slope-ratio bioassay. Basal diets (BDs) contained surfeit amounts of all amino acids for 10 to 20 kg pigs, except for thr (Experiment 1) or trp (Experiment 2). In the first study, four reference diets were formulated by supplementing the BD with 0, 0.4, 0.8 or 1.2 g of l-thr/kg at the expense of cornstarch; two test diets were formulated by replacing cornstarch in the BD with peanut meal at 32 or 64 g/kg of diet to supply 0.4 or 0.8 g thr/kg, respectively. Four reference diets consisting of the BD supplemented with 0, 0.15, 0.3 or 0.45 g of l-trp/kg at the expense of cornstarch and two test diets in which cornstarch in the BD was replaced with peanut meal at 30 or 60 g/kg of diet to supply 0.14 or 0.28 g trp/kg, respectively were used in the second study. Body weight gain responded in a linear way to supplemental l-thr or thr from peanut meal (P < 0.001). There was a linear response (P < 0.001) to thr supplementation from l-thr or peanut meal in gain-to-feed ratio. The addition of trp to the BD linearly increased (P < 0.05) body weight gain, feed intake and gain-to-feed ratio regardless of the trp source. Common-intercept, multiple linear regression in slope-ratio methodology using weight gain or gain-to-feed ratio as dependent variables and supplemental thr intake as independent variable gave relative bioavailability estimates of 71.9% or 75.7%, respectively. Corresponding values for trp were 92% and 75.7%. The fiducial limits for none of the relative bioavailability estimates included 100%. The data from these studies suggest that the bioavailabilities of thr and trp in peanut meal are less than those of l-thr and l-trp, and that the bioavailabilities of thr and trp in peanut meal are 72% to 76% and 76% to 92%, respectively.