Observed changes in soyabean growth and seed yield from Abutilon theophrasti competition as a function of carbon dioxide concentration

Soyabean (Glycine max) was grown at ambient and projected levels of atmospheric carbon dioxide (+250molmol1 above ambient) over two field seasons with and without the presence of a weed, Abutilon theophrasti, to quantify the potential effect of rising atmospheric carbon dioxide concentration on weedcrop interactions and potential yield loss in soyabean. Under weed-free conditions, elevated CO2 resulted in stimulations in soyabean seed yield and associated components, including pod number. At an approximate density of 6 plants m2, A.theophrasti competition resulted in a significant reduction (40%) in soyabean seed yield. Although differences in seed yield reduction by A.theophrasti were observed as a function of year, the relative decrease in seed yield with A.theophrasti biomass did not differ in response to CO2. Although careful weed management will be necessary if CO2-induced increases in seed yield for soyabean are to be achieved, these data suggest that soyabean seed yield may be more resilient in competition with A.theophrasti as a function of rising atmospheric levels of carbon dioxide.