The C₄ -plants are more efficient than C₃ -plants because

The C₄ plants are more efficient than the C₃ plants because they lack photorespiration.
C₄ plants including maize, sugarcane, and sorghum avoid photorespiration by using another enzyme called PEP during the first step of carbon fixation. This step takes place in the mesophyll cells that are located close to the stomata where carbon dioxide and oxygen enter the plant.
Photorespiration happens in C₃ plants when the CO₂ concentration drops to about 50 ppm. The key enzyme that accomplishes the fixing of carbon is rubisco, and at low concentrations of CO₂, it begins to fix oxygen.