  The Universe evolution during the radiation-dominated epoch in the
R^2-extended gravity theory is considered. The equations of motion for R and H
are solved analytically and numerically. The particle production rate by the
oscillating curvature is calculated in one-loop approximation and the back
reaction of particle production on the evolution of R is taken into account.
Possible implications of the model for cosmological creation of non-thermal
dark matter is discussed.
