Hydrodynamic exposure and time since application influence endothall amine potency against submersed aquatic plants.

Highlights • Endothall amine efficacy was greater when plants were exposed in moving water. • Endothall amine was less potent as it aged in water solutions. • Potency was generally reduced from 12 h after application. Abstract The amine formulation of endothall is a herbicide that may be applied to flowing irrigation canals to control submerged aquatic weeds and algae. We undertook a bioassay experiment to simulate the effects of water movement and increasing distance away from an application point on control of aquatic weeds. We exposed two submerged aquatic plants (Elodea canadensis and Vallisneria australis) to endothall amine solutions. This was done a) with solutions that were aged for 0, 12, 24 and 48 h prior to plant exposure, to simulate the effect of distance downstream of the application point, and b) in three hydrodynamic environments: quiescent, low-flow and high-flow. Endothall amine was less potent on both species as the solution aged, such that potency was generally reduced from 12 h after application. The concentration of endothall acid did not decrease in the exposure tanks during the experiment, suggesting that there is a change of the endothall amine molecule as it ages in water, which reduces the herbicide's potency. Endothall amine efficacy was greater for both species when plants were exposed in moving water compared to quiescent water. We propose that the increased efficacy in moving water is caused by a reduction in the thickness of the aqueous diffusive boundary layer in the hydrodynamic environment, thus increasing the rate of endothall amine diffusion into the plant tissue. This information can be used to improve management of submersed aquatic weeds with endothall amine and to understand its potential impact on non-target aquatic plants as herbicide-treated water moves away from the area of application. [ABSTRACT FROM AUTHOR]