WHEAT (TRITICUM-AESTIVUM) YIELD REDUCTION FROM COMMON MILKWEED (ASCLEPIAS-SYRIACA) COMPETITION

Yield loss of hard red spring wheat due to competition from common mil kweed was measured in grower fields in Minnesota. Wheat yield loss was measured using the area of influence and additive competitive methods . Simple linear regression of wheat yield and percentage wheat yield l oss against distance from a common milkweed shoot gave r(2) values of 0.013 and 0.015, respectively, indicating limited value of the area of influence model for common milkweed in spring wheat. In an additive c ompetition model, wheat yield was reduced 47% at the highest density o f 12 common milkweed shoots m(-2). Coefficients of determination for p ercentage yield loss regressed against common milkweed shoot density w ere 0.548, 0.547, and 0.529 for simple linear, nonlinear rectangular h yperbolic, and linear square root function models, respectively. Regre ssion of percentage yield loss against common milkweed biomass resulte d in r(2) values of 0.566, 0.645, and 0.658 for simple linear, nonline ar rectangular hyperbolic, and linear square root function models, res pectively Restrictions of common milkweed density due to factors other than competition limited yield loss response to the simple linear pha se of both the nonlinear rectangular hyperbolic and the linear square root function models previously described.