Ring spinning of a pre-blended roving comprising Cotton (40%), Polyester (30%), and Modal (30%) by cross-sectional area (equivalent to 40.8/29.5/29.7 by weight) is analysed as a single composite-material system passing through a ring-spinning drafting zone. Unlike multi-roving siro-spinning, where each strand drafts independently, a pre-blended roving presents a unified cross-section whose effective physical properties are governed by blend-weighted composite laws. This article derives from first principles, using the supplied fibre data (E: Cotton 7 GPa, Polyester 14 GPa, Modal 11 GPa; blend ratio 40/30/30 by cross-sectional area (equivalent to 40.8/29.5/29.7 by weight; see §2.1); Ne 30/1 ring singles (final product Ne 30/2 after plying); draft ratio 27.5), four distinct and quantified mechanisms by which the blend alters drafting dynamics compared to a pure-cotton roving of the same linear density: (1) the effective friction coefficient falls to f_eff = 0.305, advancing the single slip front by 41% relative to cotton and reducing the drafting resistance to 27.45 N; (2) the composite bending stiffness EI_blend = 2.78× EI_cotton raises the machine calibration constant C_mach from 4.71 to 6.08, increasing the required spinning-triangle roving space d* by 29%; (3) the composite damping coefficient c_comp = 0.175 (30% below pure cotton) prolongs tension-fluctuation decay, directly elevating yarn CVm; (4) blend composition heterogeneity adds a structural CVm component of 5.86%, raising total CVm from a Martindale baseline of 7.20% to 9.29%. Each mechanism is independently quantified and associated with a specific corrective action applicable in production. The practical implication is that a pre-blended roving must not be optimised using cotton-only process settings; all four parameters — d*, roller weighting, twist multiplier, and draw-frame blend uniformity — require explicit recalibration for the blend composition.