The spin and orbital phase diagram for perovskite manganites La1−x(Ca,Sr)xMnO3is numerically calculated as a function of hole concentration by usingt‐Jspin‐orbital model and adopting the slave‐fermion formalism. The superexchange and double exchange interactions compete with each other and the system demonstrates the highly frustrated behavior. It is shown that the experimentally observed isotropic magnon dispersion in the metallic ferromagnetic phase cannot be reproduced within the slave‐fermion approach. © 2002 American Institute of Physics