Louisville Slugger Wood Bat XX Prime Birch Pro I13 Cupped. Birch, the fastest growing species in professional baseball, features the ideal combination of surface hardness and flexibility for increased durability. Hardness similar to maple provides great sound and feel at contact. Flexibility closer to ash allows for forgiveness on nonbarrel contact, decreasing the chance of fractured breakage. Lighter per pound than maple, birch also affords lighter swing weights for comparable turning models.