Adaptation is the most fundamental trait in the perenniality of alfalfa, and perenniality requires persistence. Persistence has its own genetic base. The base is not necessarily the same for all materials with equivalent persistence. And, there is much interaction with specific traits. The genetic base is not fixed in most materials and requires selection to maintain it.
Persistence usually decreases over sexual generations for other traits.
The more intervening sexual generations for other traits, the greater the potential for loss in persistence. Winter hardy plant materials may be fall dormant or relatively non-fall dormant. Winter hardy plant materials are not always persistent. Hence, some materials may overwinter one or two winters just fine, and then crash due to lack of persistence. Persistent plant materials may be fall dormant or relatively non-fall dormant.