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 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 flavor 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 > 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.