Relative bioavailability of dl-methionine compared with l-methionine fed to nursery pigs

• l-Methionine (Met) is the form that is directly utilized by pigs for protein synthesis.
• Relative bioavailability of dl- to l-Met was determined based on nitrogen balance.
• Differences in bioavailability between dl-Met and l-Met were not significant.

The objective of the present study was to estimate the bioavailability of dl-methionine (Met) relative to l-Met for nursery pigs using the slope-ratio assay. A total of 35 crossbred barrows with an initial body weight of 13.4 kg (standard deviation = 0.5) were randomly allotted to 5 dietary treatments in 7 replicates for a nitrogen (N) balance study. The basal diet (BD) was formulated to contain 2.1 g/kg Met. Dietary treatments included (1) BD, (2) BD + 0.3 g/kg dl-Met, (3) BD + 0.6 g/kg dl-Met, (4) BD + 0.3 g/kg l-Met, and (5) BD + 0.6 g/kg l-Met. The experiment consisted of a 5-day adaptation period and 5 days of total but separate collection of feces and urine. The amounts of feed consumed by pigs were similar across treatments. A linear decrease was observed for the urinary N output with increasing concentration of dl- or l-Met (P < 0.001) and consequent linear increase in retained N was observed for both Met sources (P < 0.001). In addition, N retention (% of intake) also linearly increased (P < 0.001) as the concentration of both Met isomers increased. With the slope-ratio assay using retained N or retention of N (% of intake) as dependent variables and supplemental intake of Met isomers as independent variable, the estimates of relative bioavailability of dl-Met compared to l-Met were 87.9 or 89.3%, respectively. However, the differences in the bioavailability between 2 Met isomers were not significant. In conclusion, with the slope-ratio assay using N balance as the response, we did not find the difference between dl- and l-Met in bioefficacy.