Bioavailability of threonine in soybean meal for young pigs2

The bioavailability of threonine in solvent-extracted soybean meal for 10- to 20-kg pigs was determined using the slope-ratio method. In Exp. 1, the assay range was determined by feeding six diets to 144 pigs. The basal diet (.40% threonine) contained corn, soybean meal, and corn gluten meal. Five additional diets were formulated by supplementing the basal diet with .05 to .25% crystalline L-threonine in .05% increments. Weight gain, gain/feed, and plasma concentrations of threonine and urea responded quadratically (P < .05) to increasing dietary threonine. Breakpoints ranged from .51 to .54% dietary threonine. Experiment 2 consisted of seven trials in which a total of 239 pigs were used in a randomized complete block design. Pigs were penned individually and had ad libitum access to feed and water during the 21-d experiment. The same basal diet that was used in Exp. 1 was supplemented with .018, .053, or .070% threonine from either L-threonine or soybean meal. The weight gains of the pigs were partitioned to yield the response due to the supplemental threonine ingested. Multiple regression was performed on partitioned weight gain vs supplemental threonine intake, and the assay was tested for validity. The regression lines for L-threonine and soybean meal were linear (P < .05) and the intercepts were not significantly different (P > .10). The slope ratio for soybean meal:L-threonine was .80. Although the difference between the soybean meal and L-threonine slopes was not significant (P > .23), the best estimate of the bioavailability of threonine in soybean meal relative to that of L-threonine was 80%.