  We employ abundances from the Sloan Digital Sky Survey (SDSS) and the Sloan
Extension for Galactic Understanding and Exploration (SEGUE) to study the
alpha-element distribution of the stellar members of the Sagittarius stream. To
test the reliability of SDSS/SEGUE abundances for the study of Sagittarius, we
select high-likelihood samples tracing the different components of the Milky
Way, and recover known literature alpha-element distributions. Using selection
criteria based on the spatial position, radial velocity, distance and colours
of individual stars, we obtain a robust sample of Sagittarius-stream stars. The
alpha-element distribution of the Sagittarius stream forms a narrow sequence at
intermediate metallicities with a clear turn-down, consistent with the presence
of an alpha-element "knee". This is the first time that the alpha-element knee
of the Sagittarius dwarf galaxy has been detected. Fitting a toy model to our
data, we determine that the alpha-knee in Sagittarius takes place at
[Fe/H]=-1.27pm0.05, only slightly less metal-poor than the knee in the Milky
Way. This indicates that a small number of Sagittarius-like galaxies could have
contributed significantly to the build-up of the Milky Way's stellar halo
system at ancient times.
