
After a review of original response to the ecological risk assessment, it appears that Syngenta's calculation of the mg/kg/day dose level may not have been clearly described, which has led to the Agency confusion.  The 50 mg a.i./kg  - bw/day value used in the preliminary ecological risk assessment appears to be a generic default value used in the T-REX model.  However, the study report used for this risk assessment contains data, which when used to calculate the actual exposures on a mg a.i./kg-bw/day basis, provides evidence that the actual time-weighted dose levels to these animals exceeded this default value.  Therefore, time-weighted average (TWA) dose levels were calculated using group mean values for body weight, food consumption, and dietary concentrations (ppm). These calculations were conducted as follows:
   1. The mean body weight recorded at the end of each dosing week was used in these calculations. This value was considered conservative, in that a mid-week body weight value is typically calculated for use in determining TWA doses.  Use of this higher body weight at the end of each measurement period results in lower TWA values than would be calculated using a mid-week value.  
   2. Average food consumption values (g food/animal/day) were extracted from the study report (Table 5)
   3. Duplicate analyses were conducted on a weekly basis on all diets (Appendix H).  Therefore, the mean analytical values were utilized for diet concentrations rather than the nominal targeted value of 1000 ppm.
   4. Weekly dose levels were then calculated by dividing the average food consumption (g/animal/day) by mean body weight (kg/rat), and multiplied by the analytically determined concentrations (ppm) divided by 1000.

             Food Consumption (g/day) x Concentration (ppm= mg/kg)
                        Body weight (kg) x 1000 (g/kg)
Based on the use of these values, the TWA dose levels for the F0 males and females were 70.0 and 84.5 mg/kg/day, respectively.  It is important to note that calculation of the dose for the F1 males using this same scenario resulted in a TWA dose of 54.6 mg/kg/day.  However, as the 70 mg/kg/day dose level for the F0 males was considered an NOAEL, the use of this lower value in the risk assessment is not considered appropriate.
