  We present detailed results for the amplitude and field dependence of the de
Haas-van Alphen (dHvA) signal arising from the electron-like $\pi$ sheet of
Fermi surface in MgB_2. Our data and analysis show that the dip in dHvA
amplitude when the field is close to the basal plane is caused by a beat
between two very similar dHvA frequencies and not a spin-zero effect as
previously assumed. Our results imply that the Stoner enhancement factors in
MgB_2 are small on both the Sigma and Pi sheets.
