  Transport models cannot simultaneously explain very recent data on pion
multiplicities and pion charged ratios of Sn+Sn in the reaction at 0.27 A GeV.
This stimulates further investigations on the pion dispersion relation,
in-medium $N\pi\to \Delta$ cross sections and $\Delta \to N \pi$ decay widths
near the threshold energy or at subthreshold energy of pion production in
isospin asymmetric nuclear matter. In this study, the pion dispersion relation,
in-medium $N\pi\to \Delta$ cross section and $\Delta \to N \pi$ decay width
near the threshold energy are investigated in isospin asymmetric nuclear matter
by using the one-boson-exchange model. With the consideration of the energy
conservation effect, the in-medium $N\pi\to\Delta$ cross sections are enhanced
at $s^{1/2}<1.11$ GeV in nuclear medium. The prediction of pion multiplicity
and $\pi^-/\pi^+$ ratios near the threshold energy can be modified if this
effect is considered in transport model simulations.
