Alfalfa is subjected to interspecific competition in two situations (i) intercropping with a forage grass, where the required long-lasting equilibrium between species is frequently threatened by excessive alfalfa competitive ability ; (ii) competition against weeds, which can be severe in monoculture and require shigh competitive ability to avoid any chemical weed control. In both cases, competition for light interception is the main mechanism that drives the species proportion. Vegetative vigour and plant architectural traits can affect the alfalfa competition dynamics across successive harvests. Once completely unveiled, these traits could be selected for new varieties with improved adaptation to competition.