Part 158 Nontarget Plant Protection Data Requirements:
Guidance for Calculating Percent Survival in Seedling

Emergence Studies

January 4, 2018

Office of Pesticide Programs
Environmental Protection Agency

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I. Purpose

The purpose of this document is to clarify EFED's recommendations for calculating percent
survival in seedling emergence studies.

II. Issue

The Seedling Emergence guideline (OCSPP 850.4100, January 2012, EPA 712-C-012) specifies
that the percent survival endpoint should be calculated as the number of surviving plants divided
by the number of planted seeds that emerged. Accurately capturing seedling survival, as defined
in OCSPP 850.4100 (i.e., number of surviving plants divided by the number of planted seeds that
emerged), would require additional tracking of the fate of individual seeds and seedlings because
daily observations cease after 50% of the control seeds emerge. The 850.4100 guideline states:

"Controls are observed daily for number of seedlings emerged until
50% of seedlings have emerged. Number of seedlings emerged,
number of surviving and dead seedlings, and visual symptoms are
observed and recorded for treatment and controls at day 0, 7 and 14
post-emergence (and day 21 post-emergence if test is extended (see
paragraph (e)(2) of this guideline)) of 50% of control plants."

In practice, several seeds are typically planted in a single pot and are not individually tracked or
observed daily; therefore, there is a possibility of a seedling or seedlings emerging and then
dying during the same period that another seedling or seedlings emerge in the shared pot. In such
cases, mortality of a seedling or seedlings would likely be missed (because the dead seedling
would be 'replaced' by a seedling emerging later).

Therefore, it has been standard practice in the Environmental Fate and Effects Division for
several years to calculate survival based on the number of surviving plants at the end of the study
as a percentage of the number of seeds planted (Equation 1). This method more accurately
predicts potential impacts to non-target plants from the use of pesticides.

Calculating % survival based on Equation 1 captures effects to both seeds and seedlings. For
example, if 10 seeds were planted and only three emerged, all of which survived, the survival
rate would be considered 100% if based on the survival calculation as defined in the OCSPP
850.4100 guideline (Equation 2), even though only 30% of the original planted seeds survived.
In contrast, the same data would result in a survival rate of 30% based on Equation 1, a more
meaningful ecological endpoint to evaluate impacts on non-target plants. The calculation using
Equation 1 is consistent with documentation associated with the Comprehensive Environmental
Toxicity Information System, the statistical package currently being used in the Environmental
Fate and Effects Division.

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III. Resolution

For all seedling emergence studies, survival should continue to be calculated based on number of
surviving plants at the end of the study as a percentage of the number of seeds planted (Equation

1).

Equation 1:

%Survival = (# surviving plants at end of study / # planted seeds) * 100

If the study report includes sufficient information (i.e., emergence status of each individual seed
was tracked throughout the study), then an additional calculation of the percent survival of
emerged plants (Equation 2) may be compared to the control and reported in the Data
Evaluation Record (DER) along with results from Equation 1.

Equation 2:

%Survival of Emerged Plants = (# surviving plants at end of study/ # plants emerged) * 100

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