Ratio between $\pt$-differential cross sections of prompt $\Dzero$ mesons at midrapidity ($|y|<0.5$) in pp collisions at $\s=5.02$ TeV  and $13$ TeV and those at forward rapidity in three rapidity intervals (left: $2< y< 2.5$, middle: $3< y< 3.5$, right: $4< y< 4.5$) measured by the LHCb Collaboration. The double ratios $\rho$, defined in Eq. 2, are shown in the middle and bottom rows together with FONLL calculations employing the CTEQ6.6 (middle row) and NNPDF30 (bottom row) PDF sets. The statistical (systematic) uncertainties are shown as vertical bars (boxes).