Formulating grass-legume mixtures requires knowledge of how the proportion of species in a seed mixture (i.e., species evenness) affects productivity and weed abundance. We hypothesized that mixtures with more equal proportions of species in the seed mixture (i.e., greater species evenness) would have greater productivity and fewer weeds than mixtures dominated by one or two species or monocultures. Two experiments with 15 mixtures and monocultures of orchardgrass (Dactylis glomerata L.), quackgrass (Elytrigia repens L.), alfalfa (Medicago sativa L.), and white clover (Trifolium repens L.) (Exp. 1) or 15 mixtures and monocultures of meadow fescue [Schedonorus pratensis (Huds.) P. Beauv.], reed canarygrass (Phalaris arundinacea L.), red clover (T. pratense L.), and kura clover (T. ambiguum L.) (Exp. 2) were sown in autumn 2008 at four locations in Pennsylvania and Wisconsin. In each experiment, there were four monocultures, four mixtures dominated by one species, six mixtures dominated by pairs of species, and one equal mixture. Mixtures and monocultures were harvested four to five times each year from 2009 to 2011. Mixtures often had more biomass than the average of legume or N-fertilized grass monocultures. Mixtures with more equal proportions of species in the seed mixture, however, did not have more biomass or fewer weeds than other mixtures. Rather, differences in yield were related to the dominant species in the mixture. Optimal legume percentages (30-40%) in the harvested biomass were achieved with a wide range of grass and legume seed proportions, which suggested that farmers have wide flexibility in formulating seed mixtures for pastures.

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) aff ects 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. Fift een 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. Th ere 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 infl uence weed invasion. Species evenness did not aff ect resource use (light interception or soil inorganic N levels). Individual forage species had a strong eff ect 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.