  We investigate the influence of a uniform magnetic field ${\bf B_0}=B_0 e//$
on energy decay laws in incompressible magnetohydrodynamic (MHD) turbulence.
The nonlinear transfer reduction along $B_0$ is included in a model that
distinguishes parallel and perpendicular directions, following a phenomenology
{\it \`a la} Kraichnan. We predict a slowing down of the energy decay due to
anisotropy in the limit of strong $B_0$, with distinct power laws for energy
decay of shear- and pseudo-Alfv\'en waves. Numerical results from the kinetic
equations of Alfv\'en wave turbulence recover these predictions, and MHD
numerical results clearly tend to follow them in the lowest perpendicular
planes.
