Butynski (1996), Butynski and Koster (1994), Butynski et al. (1996), Eisentraut (1973), Exell (1973), FAO (1992), Fernández Casas and Morales (1995), García (1996), Pérez del Val (1996), Pérez del Val et al. (1994), Pérez del Val et al. (1995), Tomás et al. (1999).