  We quantify the constraints on the flavour separation between the quarks and
antiquarks in the proton provided by the recent forward weak gauge boson
production data from the LHCb experiment at $\sqrt{s}=7$ and 8 TeV. Performed
in the framework of the NNPDF3.1 global analysis, this study highlights the key
role that the LHCb W and Z data have in achieving a robust quark flavour
separation in the large-x region, including the strange and charm quarks. We
demonstrate how the LHCb measurements lead to improved determinations of the
the up and down quark PDFs in the region $x\ge 0.1$, with an uncertainty
reduction that can be as large as a factor 2. We also show how the LHCb forward
measurements severely restrict the size of the fitted charm PDF at large x,
imposing stringent constraints on non-perturbative models for the charm content
of the nucleon.
