A power transmission system for a mixer wagon for preparing mixtures for animals, wherein the mixer wagon comprises a mixing tank and a mixing assembly comprising at least one auger housed in said mixing tank, wherein the power transmission system is operatively connectable between wagon engine means and the mixing assembly and comprises: i) a reducer group comprising: - an inlet shaft configured for receiving power from the wagon engine means; - an outlet shaft configured for transmitting power to the mixing assembly; - a planetary gear comprising: a) a carrier rigidly connected in rotation to the inlet shaft; b) a crown operatively connected in rotation to the carrier; c) a sun shaft operatively connected in rotation to the carrier; ii) a pump assembly comprising a variable displacement pump and pump transmission members operatively connected with the crown; iii) an engine assembly comprising a variable displacement hydraulic engine operatively connected with the variable displacement pump, and engine transmission members operatively connected with the outlet shaft and with the sun shaft to command the speed variation of the solar shaft by means of the hydraulic engine and transmit power to the outlet shaft; wherein the power transmission system is configurable in: - an initial configuration in which the engine assembly prevents the rotation of the outlet shaft; - a plurality of operating configurations in which the outlet shaft transmits power to the mixing assembly; wherein the pump transmission members comprise a pump shaft operatively connected in rotation to the variable displacement pump, and a first gear wheel rigidly connected in rotation to the pump shaft, wherein the crown comprises a first outer toothing meshed with the first gear wheel; wherein the engine transmission members comprise an engine shaft operatively connected in rotation to the hydraulic engine and a third gear wheel rigidly connected in rotation to the engine shaft, wherein the sun shaft comprises a third outer toothing meshed with the third gear wheel; wherein the power transmission system is characterized in that the engine transmission members comprise a second gear wheel rigidly connected in rotation to the engine shaft, wherein the outlet shaft comprises a second outer toothing meshed with the second gear wheel. i) a reducer group comprising: - an inlet shaft configured for receiving power from the wagon engine means; - an outlet shaft configured for transmitting power to the mixing assembly; - a planetary gear comprising: a) a carrier rigidly connected in rotation to the inlet shaft; b) a crown operatively connected in rotation to the carrier; c) a sun shaft operatively connected in rotation to the carrier; - an inlet shaft configured for receiving power from the wagon engine means; - an outlet shaft configured for transmitting power to the mixing assembly; - a planetary gear comprising: a) a carrier rigidly connected in rotation to the inlet shaft; b) a crown operatively connected in rotation to the carrier; c) a sun shaft operatively connected in rotation to the carrier; a) a carrier rigidly connected in rotation to the inlet shaft; b) a crown operatively connected in rotation to the carrier; c) a sun shaft operatively connected in rotation to the carrier; ii) a pump assembly comprising a variable displacement pump and pump transmission members operatively connected with the crown; iii) an engine assembly comprising a variable displacement hydraulic engine operatively connected with the variable displacement pump, and engine transmission members operatively connected with the outlet shaft and with the sun shaft to command the speed variation of the solar shaft by means of the hydraulic engine and transmit power to the outlet shaft; wherein the power transmission system is configurable in: - an initial configuration in which the engine assembly prevents the rotation of the outlet shaft; - a plurality of operating configurations in which the outlet shaft transmits power to the mixing assembly; - an initial configuration in which the engine assembly prevents the rotation of the outlet shaft; - a plurality of operating configurations in which the outlet shaft transmits power to the mixing assembly; wherein the pump transmission members comprise a pump shaft operatively connected in rotation to the variable displacement pump, and a first gear wheel rigidly connected in rotation to the pump shaft, wherein the crown comprises a first outer toothing meshed with the first gear wheel; wherein the engine transmission members comprise an engine shaft operatively connected in rotation to the hydraulic engine and a third gear wheel rigidly connected in rotation to the engine shaft, wherein the sun shaft comprises a third outer toothing meshed with the third gear wheel; wherein the power transmission system is characterized in that the engine transmission members comprise a second gear wheel rigidly connected in rotation to the engine shaft, wherein the outlet shaft comprises a second outer toothing meshed with the second gear wheel.