Alfalfa (Medicago sativa) is the most widely grown forage legume crop in the United States. Alfalfa is an obligate outcrossing and tetraploid species; such features make its genetic and genomic studies difficult and M. truncatula has therefore been promoted as a model legume. Many tools and resources have been developed in M. truncatula, such as retrotransposon mutant populations, EST and genespace sequencing information and the Gene Expression Atlas. By screening a large population of Tnt1 retrotransposon-tagged mutants of M. truncatula, we identified several types of mutants that exhibited the defects in compound leaf development, leaf senescence and seed development. To apply the information gained from M. truncatula to improve hay quality of alfalfa, we focused on a leaf senescence related mutant that maintained green leaves during senescence. Genetic and molecular analyses revealed that the mutation was caused by Tnt1 insertion in a M. truncatula STAY-GREEN (MtSGR) gene, which is induced by senescence and responsible for chlorophyll breakdown. Transcript profiling revealed that large numbers of genes were either up-regulated or down-regulated in the mutant. Based on the MtSGR sequence, an alfalfa STAY-GREEN gene (MsSGR) was cloned, and transgenic alfalfa lines were produced by RNA interference of MsSGR. Silencing of MsSGR led to the production of stay-green transgenic alfalfa. This beneficial trait offers the opportunity to produce premium alfalfa hay with a more greenish appearance. Furthermore, most of the transgenic alfalfa lines retained more than 50% of chlorophylls during senescence and had increased crude protein content. This study illustrates the effective application of the knowledge from a model system for the genetic improvement of an important commercial crop.