Commercially competitive forage yields, and in some cases high-parent heterosis, were previously reported for several alfalfa (Medicago sativa L.) diallel hybrids derived from nine unimproved germplasm sources that have been important contributors to North American cultivars.  This study evaluated biomass production of genetically unimproved and improved sets of nine of those hybrids, and their respective parents, over three years near Las Cruces, NM.  Our objectives were to determine if the performance of unimproved ancestral populations provided useful information to construct superior hybrids using more elite descendant materials, and if such improved hybrids exhibited high-parent heterosis.  All improved hybrids performed either similar to, or superior to, five check cultivars.  High-parent heterosis was detected among the top four yielding improved hybrids.  Unimproved hybrid performance and midparent value rankings provided reasonable indicators of potential hybrids that should be attempted among related elite populations.  The results indicated that opportunities to exploit hybrid vigor still appear to exist among some elite alfalfa populations that possess a predominating germplasm source.