Grass–Legume Mixtures Suppress Weeds during Establishment Better than Monocultures

Maintaining a diversity of plant species in pastures may reduce weed invasion. Knowledge of how the proportion of species in a mixture (i.e., species evenness) affects weed invasion would be useful in formulating seed mixtures. We hypothesized that forage mixtures with greater species evenness would reduce weed invasion at establishment better than mixtures dominated by a few species (low species evenness) or monocultures. Fifteen mixtures and monocultures of orchardgrass ( Dactylis glomerata L.), quackgrass ( Elytrigia repens L.), alfalfa ( Medicago sativa L.), and white clover ( Trifolium repens L.) were sown in autumn 2008 at four locations in Pennsylvania and Wisconsin. There were four monocultures, four mixtures dominated by one species (evenness = 0.64), six mixtures dominated by pairs of species (evenness = 0.88), and one equal mixture (evenness = 1). We measured the amount of naturally occurring weeds in harvested herbage at each location in 2009. At two locations, we added seed of plumeless thistle ( Carduus acanthoides L.) and canola ( Brassica napus L.; a surrogate weed) to each treatment during autumn of 2008 and measured their establishment and dry mass during 2009. Grass–legume mixtures resisted weed invasion better than monocultures. Within mixtures, however, species evenness did not influence weed invasion. Species evenness did not affect resource use (light interception or soil inorganic N levels). Individual forage species had a strong effect because weed proportions decreased curvilinearly as orchardgrass proportion of the seed mixture increased. Selecting appropriate species to use in mixtures is more important than the evenness of the species in the mixture.