We study dynamics and relaxation of elementary excitations (magnons) in the
spin nematic (quadrupole ordered) phase of S=1 magnets. We develop a general
phenomenological theory of spin dynamics and relaxation for spin-1 systems.
Results of the phenomenological approach are compared to those obtained by
microscopic calculations for the specific S=1 model with isotropic bilinear and
biquadratic exchange interactions. This model exhibits a rich behavior
depending on the ratio of bilinear and biquadratic exchange constants,
including several points with an enhanced symmetry. It is shown that symmetry
plays an important role in relaxation. Particularly, at the SU(3) ferromagnetic
point the magnon damping $\Gamma$ depends on its wavevector k as $\Gamma\propto
k^{4}$, while a deviation from the high-symmetry point changes the behavior of
the leading term to $\Gamma\propto k^{2}$. We point out a similarity between
the behavior of magnon relaxation in spin nematics to that in an isotropic
ferromagnet.Comment: the final published versio