Bioavailability of D-methionine relative to L-methionine for nursery pigs using the slope-ratio assay

This experiment was conducted to determine the bioavailability of _D-methionine (Met) relative to _L-Met for nursery pigs using the slope-ratio assay. A total of 50 crossbred barrows with an initial BW of 13.5 kg (SD = 1.0) were used in an N balance study. A Met-deficient basal diet (BD) was formulated to contain an adequate amount of all amino acids for 10 to 20 kg pigs except for Met. The two reference diets were prepared by supplementing the BD with 0.4 or 0.8 g _L-Met/kg at the expense of corn starch, and an equivalent concentration of _D-Met was added to the BD for the two test diets. The pigs were adapted to the experimental diets for 5 d and then total but separated collection of feces and urine was conducted for 4 d according to the marker-to-marker procedure. Nitrogen intakes were similar across the treatments. Fecal N output was not affected by Met supplementation regardless of source and consequently apparent N digestibility did not change. Conversely, there was a negative linear response (P < 0.01) to Met supplementation with both Met isomers in urinary N output, which resulted in increased retained N (g/4 d) and N retention (% of intake). No quadratic response was observed in any of the N balance criteria. The estimated bioavailability of _D-Met relative to _L-Met from urinary N output (g/4 d) and N retention (% of intake) as dependent variables using supplemental Met intake (g/4 d) as an independent variable were 87.6 and 89.6%, respectively, but approximate 95% fiducial limits for the relative bioavailability estimates included 100%. In conclusion, with an absence of statistical significance, the present study indicated that the mean relative bioequivalence of _D- to _L-Met was 87.6% based on urinary N output or 89.6% based on N retention.