Development of Dose-Response Functions for Individual Species

2 Development of Dose-Response Functions for Individual Species 

A number of issues influenced the selection of the dose-response model form and the treatment of the data prior to fitting the model. First, shoot weight and shoot length are continuous response measurements therefore, use of a standardized logistic model form is not appropriate. Second, the natural variation in plant growth often resulted in apparent increased shoot weight and shoot length measurements relative to the control at low herbicide application rates. A dose-response model needs to perform well even when some measurements in treatment levels exceed the controls. 

Application of Uncertainty Analysis to Ecological Risk of Pesticides 

A standard approach was used to model the data after normalization relative to the appropriate control. The raw shoot weight or shoot length data were normalized by dividing by the control mean for each test to transform the endpoint to the fraction of the control (which was used as the response endpoint in the model). This transformation is presented as  

i is the rescaled response at the control, Cj ], i is the dose or rate, and 8 is the error term, and a and P are regression coefficients. Wq i is the expected lowest shoot weight (or length) value, and effectively rescales the model predictions over the dose-response range. Both 7 o,i and Wq, were calculated for each species-specific dose-response curve. 

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