I don't think grooves are necessary. The string groove is AFAIK a 20th century invention, and they've been making recurves for something like 5000 years. Some, but not all, of the wood-horn-sinew Scythian bows from 2500-300 years ago had short string grooves, but they also had nearly 270 degree semi-circular recurves which you can't get easily with modern materials. Turkish crab- bows don't have string grooves. Some ancient bows with foot long or longer contact (string-follow) siyahs had string pads added to the limbs but not gouged into them.