Quantitative Evaluation of the Environmental Impact Quotient (EIQ) for Comparing Herbicides

<div><p>Various indicators of pesticide environmental risk have been proposed, and one of the most widely known and used is the environmental impact quotient (EIQ). The EIQ has been criticized by others in the past, but it continues to be used regularly in the weed science literature. The EIQ is typically considered an improvement over simply comparing the amount of herbicides applied by weight. Herbicides are treated differently compared to other pesticide groups when calculating the EIQ, and therefore, it is important to understand how different risk factors affect the EIQ for herbicides. The purpose of this work was to evaluate the suitability of the EIQ as an environmental indicator for herbicides. Simulation analysis was conducted to quantify relative sensitivity of the EIQ to changes in risk factors, and actual herbicide EIQ values were used to quantify the impact of herbicide application rate on the EIQ Field Use Rating. Herbicide use rate was highly correlated with the EIQ Field Use Rating (Spearman’s <i>rho</i> >0.96, P-value <0.001) for two herbicide datasets. Two important risk factors for herbicides, leaching and surface runoff potential, are included in the EIQ calculation but explain less than 1% of total variation in the EIQ. Plant surface half-life was the risk factor with the greatest relative influence on herbicide EIQ, explaining 26 to 28% of the total variation in EIQ for actual and simulated EIQ values, respectively. For herbicides, the plant surface half-life risk factor is assigned values without any supporting quantitative data, and can result in EIQ estimates that are contrary to quantitative risk estimates for some herbicides. In its current form, the EIQ is a poor measure of herbicide environmental impact.</p></div

The relationship between plant surface half-life risk factor (P) and EIQ value for 116 herbicides (Senseman data set).

<p>Filled circles with error bars represent means and standard errors, grey open circles represent individual herbicides (N = 116). For box-plots, dark bars represent median EIQ value, boxes enclose the first and third quartiles; preemergence (PRE) and postemergence (POST) boxplots represent the data for P = 1 and P = 3, respectively.</p

The influence of risk factors on the calculated environmental impact quotient (EIQ) as determined by simulation analysis (N = 100,000) for herbicides applied before crop emergence (PRE) and herbicides applied after crop emergence (POST).

<p>The influence of risk factors on the calculated environmental impact quotient (EIQ) as determined by simulation analysis (N = 100,000) for herbicides applied before crop emergence (PRE) and herbicides applied after crop emergence (POST).</p

Histogram of Monte Carlo simulation of 100,000 EIQ values calculated by random draws of values for risk factors included in the EIQ formula (Eq 1).

<p>For box-plots, dark bars represent median EIQ value, boxes enclose the first and third quartiles, and whiskers extend to minimum and maximum observed EIQ values.</p

Relationship between Field EIQ and herbicide use rate (A,B) and the EIQ (C,D) for two herbicide data sets.

<p>(A) Relationship between maximum herbicide use rate and Field EIQ for Senseman data set; (B) relationship between herbicide field use rate and Field EIQ for Beckie data set; (C) relationship between herbicide EIQ and Field EIQ for the Senseman data set; (D) relationship between herbicide EIQ and Field EIQ for the Beckie data set. Spearman’s rank correlation <i>rho</i> and P-value are provided in each panel.</p

The influence of risk factors on the calculated environmental impact quotient (EIQ) as determined by simulation analysis (N = 100,000).

<p>The influence of risk factors on the calculated environmental impact quotient (EIQ) as determined by simulation analysis (N = 100,000).</p

Simulated EIQ values for low, medium, and high risk categories for each risk factor used in the herbicide EIQ calculation (Eq 1).

<p>Dark bars represent median EIQ value, boxes enclose the first and third quartiles, and whiskers extend to minimum and maximum observed EIQ values.</p