  Motivated by the unusual temperature dependence of the specific heat in MnSi,
comprising a combination of a sharp first-order feature accompanied by a broad
hump, we study the extended Heisenberg model with competing exchange $J$ and
anisotropic Dzyaloshinskii-Moriya $D$ interactions in a broad range of ratio
$D/J$. Utilizing classical Monte Carlo simulations we find an evolution of the
temperature dependence of the specific heat and magnetic susceptibility with
variation of $D/J$. Combined with an analysis of the Bragg intensity patterns,
we clearly demonstrate that the observed puzzling hump in the specific heat of
MnSi originates from smearing out of the virtual ferromagnetic second order
phase transition by helical fluctuations, which manifest themselves in the
transient multiple spiral state. These fluctuations finally condense into the
helical ordered phase via a first order phase transition as is indicated by the
specific heat peak. Thus the model demonstrates a crossover from a second-order
to a first-order transition with increasing $D/J$. Upon further increasing
$D/J$ another crossover from a first-order to a second-order transition takes
place in the system. Moreover, the results of the calculations clearly indicate
that these competing interactions are the primary factor responsible for the
appearance of first order phase transitions in helical magnets with the
Dzyaloshinskii-Moriya (DM) interaction.
