  We present results on deep sub threshold hyperon production in nuclear
collisions, with the UrQMD transport model. Introducing anti-kaon+baryon and
hyperon+hyperon strangeness exchange reactions we obtain a good description of
experimental data on single strange hadron production in Ar+KCl reactions at
$E_{lab}=1.76$ A GeV. We find that the hyperon strangeness exchange is the
dominant process contributing to the $\Xi^-$ yield, however remains short of
explaining the $\Xi^-/\Lambda$ ratio measured with the HADES experiment. We
also discuss possible reasons for the discrepancy with previous studies and the
experimental results, finding that many details of the transport simulation may
have significant effects on the final $\Xi^-$ yield.
