  Using hydrodynamic theory of superfluids and the Lee-Huang-Yang equation of
state for interacting Bose gases we derive the first correction to the
collective frequencies of a trapped gas, due to effects beyond mean field
approximation. The corresponding frequency shift, which is calculated at zero
temperature and for large N, is compared with other corrections due to finite
size, non-linearity and finite temperature. We show that for reasonable choices
of the relevant parameters of the system, the non-mean field correction is the
leading contribution and amounts to about 1%. The role of the deformation of
the trap is also discussed.
