Approximate Modeling of the Uptake of Pesticides by Grass for Grazing Risk Assessment and Pasture Management

Pesticide application on grasslands is a major concern in pastoral farming. To evaluate the uptake of pesticides by grass in grasslands, we developed a three-compartment approximate model that simplified the simulation of pesticide residue levels in grass. In addition, we introduced default safety emission rates (ERSafe0) for pesticides based on simulated pesticide concentrations in grass (CG)on the surface of grass leaves (CLS) and in the soil (CS)to help protect against risks to livestock health. The simulated results for 744 pesticides show that, in general, the degradability of pesticides in soil and the octanol–water partition coefficient (KOW) exert a relatively strong influence on the model outputs (e.g., CG and ERSafe0); when the pesticide degradation rate in soil (kS) is greater than approximately 0.01 day–1, there is a positive correlation between degradability and ERSafe0. This indicates that livestock animals (e.g., cattle) have lower risks of exposure to lipophilic pesticides with a high degradability in soil. Other parameters of pesticides, such as their degradability in grass, can also affect their uptake (e.g., the time required to reach the maximum CG value); however, kS and KOW dominate their overall simulated fates. We validated the approximate model by comparing simulated bioconcentration factors (BCFs) with data from the current literature and models and found that the BCFs are consistent when comparing our model and other studies for 86% of the studied pesticides if the steady-state condition is considered. Thus, the simple and approximate model developed in this study can be a useful and practical tool for screening-level risk assessment and pasture management.