Description: Medicago sativa L. is a major forage legume grown extensively worldwide. This perennial species has a tetraploid (2n = 4x = 32) genome which displays tetrasomic inheritance and its populations are extremely polymorphic due to a high degree of outcrossing. Superior tolerance to environmental stress and decline in stem digestibility during plant development are major issues for its breeding programs. Insufficient tolerance to subfreezing temperatures reduces long term persistence of alfalfa in northern countries and is a source of major costs associated with stands reestablishment and the need to buy forage outside the farm.