  The frustrated magnet Nd$_2$Ir$_2$O$_7$, where strong correlations together
with spin-orbit coupling play a crucial role, is predicted to be a Weyl
semimetal and to host topological pairs of bulk Dirac-like valleys. Here we use
an external magnetic field to manipulate the localized rare earth 4f moments
coupled to the 5d electronic bands. Low energy optical spectroscopy reveals
that a field of only a few teslas suffices to create charge compensating
pockets of holes and electrons in different regions of momentum space, thus
introducing a valley population shift that can be tuned with the field.
