Quantifying multispecies interactions between crops (soybean: Glycine max) and weeds: a new approach.
Gibson, David*,1, Andrew, Wood1, Bryan, Young1, John, Connolly2, Laura, Kirwan2, Kathy, Millar1, Joseph, Ely3, 1 Southern Illinois University, Carbondale, Carbondale, IL2 University College Dublin, Dublin, Ireland3 Central Missouri State University, Warrensburg, MO
ABSTRACT- Competitive interactions are important for understanding plant communities and reflect mechanisms involved in the dynamics of weeds and invasive plants both in agricultural and natural settings. Crops are rarely infested with a single weed species, and many different invasive plants colonize natural settings. An accepted design and analysis for quantifying these multispecies mixtures has not been developed and weed decision support models do not take the interactions among weed species into consideration. We test a new design and analysis based upon Simplex-Axial mixtures; a procedure routinely used in many commercial and industrial applications including the determination of fuel, cake, and fruit punch mixtures. The majority of experimental mixtures used in agriculture and ecology are based upon pairs of species established at various proportions and densities. The designs that we develop are based upon species mixtures represented as a point lying either on the boundaries or in the interior of a triangle (the simplex) for a 3-species mixture, a tetrahedron for a 4-species mixture, a pentagon for a 5-species mixture, etc. We conducted mesocosm and field experiments in two separate years in which multiple densities and proportions of three target weeds (Ambrosia trifida, Amaranthus rudis, Setaria faberi) were sown or allowed to seed naturally into each of three densities of soybean. Data are presented here from the first year experiment. A synergism among the weeds led to increased biomass of A. trifida which in turn led to decreased seed weight in soybean. For A. trifida, at most density combinations, output per unit input was higher when in the presence of other weeds (i.e. in mixture) than when in weed-monocultures. This was not the case for the other two weed species.
Key words: competition, simplex-axial mixtures, Glycine max
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