  Drell-Yan production in the forward region at the Large Hadron Collider is
sensitive to multiple radiation of QCD partons not collinearly ordered, emitted
over large rapidity intervals. We propose a method to take account of these
radiative contributions via a factorization formula which depends on the
unintegrated, or transverse momentum dependent, splitting function associated
with the evolution of the initial-state sea quark distribution. We analyze this
formula numerically, and point out kinematic effects from the initial-state
transverse momentum on the vector boson spectrum.
