  The coupled equations that describe the effect of large-scale magnetic and
velocity fields on forced high-diffusivity magnetohydrodynamic flows are
investigated through an extension of mean field electrodynamics. Our results
generalise those of R\"adler & Brandenburg (2010), who consider a similar
situation but assume that the effect of the Lorentz force on the momentum
equation can be neglected. New mean coupling terms are shown to appear, which
can lead to large-scale growth of magnetic and velocity fields even when the
usual $\alpha$-effects are absent.
