ARS | Publication request: Grass-legume mixtures suppress weeds better than monocultures during establishment

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 16, 2011
Publication Date: February 1, 2012
Citation: Sanderson, M.A., Brink, G.E., Stout, R., Ruth, L. 2012. Grass-legume mixtures suppress weeds better than monocultures during establishment. Agronomy Journal. 104(1):36-42.

Interpretive Summary: Controlling weeds in mixed-species pastures is difficult, especially at establishment, because many broadleaf herbicides also damage or kill legumes. Our objective was to test the hypothesis that mixed plant communities with greater species evenness are more resistant to weed invasion at establishment than mixtures with lower evenness or monocultures. Field studies were conducted at two sites in Pennsylvania and two sites in Wisconsin. Fifteen forage mixture and monoculture treatments were constructed to include grass and legume species that contrast in establishment rate, growth habit, and environmental adaptation. We tested resistance to weed invasion by determining the proportion of unsown species in each plot at all four sites and by hand planting seeds of plumeless thistle and canola at the two Pennsylvania sites. On average, grass-legume mixtures resisted weed invasion better than monocultures. Increasing the species evenness of grass-legume mixtures reduced the proportion of naturally occurring weeds in harvested herbage at two of four locations. When mixtures were challenged with the addition of specific weed species (canola and plumeless thistle) there were no effects of species evenness or mixture composition on the establishment of the sown weeds. Our data suggest that mixtures of multiple species of grasses and legumes should be formulated to have nearly equal proportions of each species (i.e., have high species evenness) to improve resistance to weed invasion during forage establishment.

Technical Abstract: 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 ecosystem functions of pastures will be useful in formulating seed mixtures. We hypothesized that forage mixtures with nearly equal composition (high species evenness) would reduce weed invasion at establishment better than mixtures dominated by a few species (low species evenness) or monocultures. Fifteen treatments were sown in autumn 2008 at four locations in Pennsylvania and Wisconsin in a modified simplex design that systematically varied the species evenness of mixtures. The mixtures and monocultures included orchardgrass (Dactylis glomerata L.), quackgrass (Elytrigia repens L.), alfalfa (Medicago sativa L.), and white clover (Trifolium repens L.). There were four monocultures (evenness = 0), four mixtures dominated by one species (evenness = 0.64), six mixtures dominated by pairs of each 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. Increasing the species evenness of mixtures reduced the proportion of naturally occurring weeds in harvested herbage. Individual forage species also had a strong effect because weed proportions decreased curvilinearly as orchardgrass proportion of the seed mixture increased. When mixtures were challenged with the addition of specific weeds there were no effects of species evenness or mixture composition on the establishment of the sown weeds.