Volume

45, 2007

Issue

1

Page

47

Title

Author(s)

Keywords

aquatic herbicides, penoxsulam, imazamox, bispyribac, chemical control, selectivity.

Abstract

Three acetolactate synthase (ALS) inhibiting herbicides are currently being evaluated for the control of hydrilla (Hydrilla verticillata L.f. Royle) and other submersed aquatic weeds. The impacts of long-term aqueous exposures of these herbicides on non-target native emergent plant species are unknown. Therefore, trials were conducted to determine the effect of aqueous applications of the ALS-inhibitors penoxsulam [2-(2,2-difluoroethoxy)-N-(5,8-dimethoxy [1,2,4]triazolo[1,5- c] pyrimidin-2-yl)-6-(trifluoromethyl) benzenesulfonamide], imazamox [2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo- 1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid], and bispyribac sodium [2,6-bis(4,6-dimethoxypyrimidin- 2-yloxy)benzoic acid] on established soft-stem bulrush (Scirpus validus Vahl.), Egyptian panicgrass (Paspalidium geminatum (Forssk.) Stapf), maidencane (Panicum hemitomon Schult.), pickerelweed (Pontederia cordata L.) and sagittaria (Sagittaria lancifolia L.). Plants were exposed to initial aqueous concentrations of each herbicide at 25, 75, 150 and 300 μg/L for 66 d. At harvest, shoot and root biomass of untreated controls by species increased from 100 to >1500%. While both grass species increased in biomass following exposure to ALS-inhibitors at rates up to 300 μg/L, the shoot biomass of panicgrass was inhibited by bispyribac and penoxsulam at concentrations of 75 μg/L and higher when compared to untreated controls. Maidencane was generally more tolerant than panicgrass to all three herbicides. Bulrush shoot growth was inhibited at all concentrations, with differential responses to the 3 herbicides. Pickerelweed shoot growth was reduced below pre-treatment weights at 25 μg/L and higher following treatment with penoxsulam and bispyribac. Sagittaria was more sensitive to penoxsulam at rates of 25 and 75ug/L compared to imazamox and bispyribac. There was inhibition of root growth for all species, but whether inhibition resulted from direct toxicity to roots or as an indirect result of shoot inhibition was not determined. The spectrum of herbicidal activity for all three herbicides was similar (in order of increasing tolerance: pickerelweed<sagittaria<bulrush<panicgrass< maidencane). Use patterns for control of submersed plants need to be established in order to determine the likelihood of impacting non-target emergent plants.